Green Manure Toolkit

Module 3: Managing Green Manures

1. Rotational Strategies

Green manures are legumes that are grown without the intent to harvest, but to improve soil fertility. Green manures are an important part of the organic grain rotation: they supply the nitrogen that is needed to fuel the following crops in the rotation. It is important to consider how and when green manures will be grown when designing your crop rotation.

Generally, green manures are grown every two to five years in the rotation, depending on the farm, the soils and the climate. There are numerous ways to integrate green manures into the crop rotation, with options that will fit most any Prairie organic grain production system.

The various strategies are illustrated below, with additional details provided.

Source: Natural Systems Agriculture Lab

Also consider the crops that will follow green manures in the rotation. Green manures are typically best immediately followed by crops that have high nitrogen needs, such as hemp or high protein wheat. Later phases of the rotation can include crops with reduced fertility needs, as the nitrogen available from the green manure is used up.  In rotations that also include grain legumes or pulse crops, it is wise to make sure that a legume green manure is not immediately followed by a harvested legume. This can reduce disease and insect pest pressures.

Definitions:

Cover crop: A crop that is grown primarily for the benefits it can provide to the soil and to the environment, rather than for harvest.  Cover crops are grown to reduce soil erosion, improve soils, smother weeds, capture or supply nutrients, and break pest cycles.

Green manure: A legume cover crop that is grown specifically to improve soil fertility.

Underseeding: A green manure, often small seeded perennials or biennials, planted into or with a grain cash crop.

Relay crop: A green manure that is planted into an established cash crop, and can continue growing after cash crop harvest. Often also referred to as underseeding. Examples include planting sweetclover, red clover or alfalfa into an established cereal crop.

Mixed-crop green manure or intercrop: Growing two or more species, often a legume and a cereal, in the same field at the same time. Intercrops can include cereals and legumes that are planted together, such as peas and oats, or cereals and legumes that are relay cropped or undersown.

Catch crop: A cover crop planted to take up available soil nutrients, so that they are not lost from the soil through leaching. When the catch crop is terminated, these nutrients are then released back into the soil for the following crop.

Double cropping: Planting a green manure after the harvest of a cash crop, often a winter cereal.

1.1. Dedicating a Full-season to Green Manures

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Field pea blossom. Photo by Brenda Frick.

Annual green manure species are well suited to occupying a full season in the crop rotation. Most annual green manures in use on the Prairies tend to be grown for only two to three months. In this time, the green manures accumulate biomass and nitrogen. By the time the green manure legume flowers, the plants have achieved their maximum biomass and their tissues are rich in nitrogen. In general, annual green manures should be terminated at flowering for optimal nitrogen, or earlier (at the bud stage) when moisture conservation is required. While the annual green manures typically require only part of the growing season to provide their maximum nitrogen contributions, dedicating a year-long spot in the rotation to a green manure provides flexibility.

This flexibility can be of great value in the rotation. It provides an opportunity for additional weed management operations before planting or after termination. Having time between the termination of a green manure and the planting of the next crop can allow soil moisture to replenish. This is particularly important in dry soil zones or dry years. In some Prairie climates, there can even be sufficient growing season remaining to sow a winter cereal or another cover crop to further boost soil organic matter and to capture any nitrogen that might leach from decomposing legumes.

Full-season annual green manures also open an opportunity for a period of reduced tillage in the rotation. The annual green manure can be terminated with a roller-crimper to form a thick mulch that can be no-till planted the following spring. Green manure choice is important for ensuring a thick mulch can be obtained – for more information, see the Termination section. The green manure mulch provides soil cover, weed control and the benefits of reducing the number of tillage operations. Research has demonstrated successful integration of no-till green manure systems into Prairie organic grain systems, with spring-planted hairy vetch showing the most promise, especially in southern Manitoba. Termination with a mower and delaying incorporation until the following spring can also reduce tillage operations while protecting the soil and retaining moisture. Mowing can also be done in strips, leaving some standing green manure that can capture and retain snow.

While biennial or perennial legumes can be planted and terminated in the same season, they are slow to establish and will not likely generate a lot of biomass or nitrogen in a single season of growth. So, this is not generally a recommended practice. Nonetheless, longer-lived green manures can be planted so that there is one full season of the rotation dedicated solely to a green manure. This strategy, known as relay cropping or underseeding, involves seeding a green manure into (or with) a grain cash crop. This method is commonly used with alfalfa, red clover and sweetclover on the Prairies.  For more information, see the Reducing Green Manure Commitment in the Rotation: Relay and Double Cropping section. When managed with this method, as is common practice, only one year in the crop rotation is dedicated solely to a biennial sweetclover green manure.

1.2. Reducing Green Manure Commitment in the Rotation and Finding Synergies: Relay and Double Cropping

 

1.2.1. Double cropping

Annual green manures can grow quite quickly, and many flower in just over two months in Prairie conditions. In areas where the climate allows, annual green manures could be planted after the harvest of a cash crop, especially a winter crop. This is also known as double cropping. Double cropping can be an attractive option if you do not want to dedicate a full year of your rotation to a green manure.

Double cropping of green manures is often done following the harvest of a winter cereal. The earlier harvest of a winter vs. a spring cereal affords more time for the green manure to produce biomass and take up the nitrogen fixed by its associated Rhizobia. If growing seasons lengthen with climate change, the opportunity for double cropping may expand, and could be explored even following early-planted, quick maturing crops like barley. In double cropping, the green manure is often killed by frost, which reduces the management required for the green manure.  In addition, the terminated green manure residues are likely to break down much more slowly in cold weather. This could provide an opportunity to better match the spring nitrogen needs of the following crop, while also reducing opportunities for leaching.

Double cropping green manures also comes with challenges. Establishment of the green manure can be difficult, especially if the preceding cash crop is later to mature than planned, or if the environment is cold or dry. Dry falls can be particularly challenging for double cropping, and limits this management option to higher moisture areas. The use of soil moisture by the green manure should also be considered carefully, especially in the drier Brown and Dark Brown soil zones. Fall water uptake by the green manure may limit soil moisture the following spring, restricting crop growth. On the other hand, the presence of the green manure can help to retain soil moisture and trap snow to counter this moisture use.

Another challenge can be slow crop growth and the potential for the crop to be winterkilled before producing optimal biomass and reaching peak nitrogen content. A growing season that extends beyond normal can also be a risk, should the growing season be long enough that the green manure sets seeds that can volunteer the following year. In this case, mechanical termination, rather than winterkilling, may be required.

Researchers have successfully used green manure double crops in the Black soil zone in south-central Manitoba, and with some success in the Dark Brown soil zone around Lethbridge, Alberta. Studies of climate patterns suggest that there is potential for double cropping in southwestern and south-central Manitoba and southern Saskatchewan, so long as moisture levels permit. Growing season length and temperatures likely would not support a successful double crop in areas further to the north and west, although this could vary by season. Even in those areas where the growing season could support double cropping, variability in late season moisture means that decisions to double crop should be based on the on-the-ground conditions at the time. Double cropping is best attempted in fine-textured soils with a high water holding capacity, or soils that may not be dependent on late-season rains to have sufficient moisture to support a second crop.

Annual green manures, such as chickling vetch, lentil, hairy vetch and pea are suitable for double cropping. These annual green manures reach peak nitrogen quickly, and can produce lots of biomass in a short window of time when conditions are favourable. Research suggests that peas are a particularly good option, as they produce lots of biomass and are readily available. Chickling vetch is also a good option, but seed costs can be high.

1.2.2. Relay cropping

Relay cropping, or underseeding, involves seeding a green manure into a grain cash crop. Relay cropping is generally employed with the smaller-seeded and longer-lived perennial and biennial green manures. Relay crops of alfalfa, red clover and sweetclover planted into winter or spring cereals have all been successful in Prairie conditions. This method provides the benefit of a multi-season green manure while still producing a harvestable cash crop in the first year of the green manure stand. The perennial green manure can be terminated the following spring or allowed to continue to grow later into the second season.

Relay cropped green manures are typically broadcast seeded into grain crops early in their growth. A pass of a harrow following the broadcast seeding can both aid establishment of the green manure and provide weed control. The small-seeded relay cropped legumes do not necessarily require immediately available soil moisture, so long as rains will come some time before fall so that the green manure can establish. Because they are planted shallowly, even a small rain may be enough to trigger germination.
Oats, alfalfa mix

Alfalfa underseeded into oats. Photo by Laura Telford.

With relay seeding, a cash crop is still harvested in the establishment year of the green manure. The green manure can then be terminated later in the fall, or can be allowed to overwinter after which can be allowed to revive and continue growing for a second season, or can be terminated to allow another crop to be planted. Relay seeded green manures provide late- and early-season weed control, and protect soils from erosion. At the same time, the small-seeded, slow-growing perennial and biennial green manures benefit from the weed competition provided by the grain crop.

Use this strategy with caution in the drier soil zones, particularly the Brown soil zone. The perennial and biennial green manures that lend themselves well to relay cropping are also high water users.  Terminate the green manure early, at the bud stage in the second year of growth, in dry areas to reduce water use and allow a period of water recharge. Also note that the ability of a green manure crop to establish and grow well may be hindered by the existing grain canopy. Where growing season permits, the green manure can build biomass quickly after the cash crop is harvested. When your objective is the successful establishment of a green manure rather than a high yielding cereal stand, it may be wise to reduce the seeding rate for the cereal to half of the recommended rate. A smaller grain harvest while establishing a healthy green manure crop may still be better than no cash crop harvest at all. Alternatively, the green manure may compete with the grain crop or interfere with harvest, and result in lower yields. This is more likely to be the case in wet years when the legume green manure can grow rapidly and abundantly. To counter this, consider delaying the planting of the legume relay crop in wet years.

1.3. Multiple-Year Green Manures

Multiple-season green manures of perennial legumes can also be included in the rotation. While this means fields are taken out of cash crop production for multiple years, there are many benefits. These long-lived stands provide not only nitrogen, but also improve soils. Perennial green manures tend to have extensive rooting systems and taproots that can bring up nutrients from deep in the soil while improving soil structure and reducing compaction. Multiple-season green manures provide continuous ground cover, protecting soils from erosion even in the late fall, winter and early spring. They also support soil biology, with their long-lived and extensive rooting systems. Perennial green manures also promote biodiversity and are often excellent food and refuge sources for pollinators and other beneficial organisms. These longer-lived green manures can also reduce weed pressures by breaking weed cycles that have adapted to annual cropping. This is especially true when the green manure is mowed or grazed.
Alfalfa flower. Photo by Andy Hammermeister.

Alfalfa flower. Photo by Andy Hammermeister.

The costs of including longer-lived green manure stands must be carefully weighed against the benefits. For this reason, perennial legumes are often favoured on farms that include livestock. Here, the perennial green manure can be grazed or harvested as hay, and will regrow. This allows income from the sales of milk or meat while the green manure is in place. A word of caution: haying reduces overall green manure biomass and reduces the amount of nitrogen that the green manure can add to the soil. If the green manure is able to regrow before termination, hay can be harvested without sacrificing much soil nitrogen. However, the removal of hay also removes other important nutrients, most notably phosphorus, from the field. Returning animal manure can help to offset these losses. The ruminant gut speeds up the release of nitrogen from plant tissues, so be aware that returning manure to fields, or grazing in place, results in readily and quickly available nitrogen that can be easily lost through leaching. Consider planting a catch crop after the green manure is terminated to retain these nutrients. See the Termination section for more details.

Perennial legumes, whether used strictly as a green manure or also used for livestock feed, should be considered with caution in the drier soil zones. Perennial green manures tend to require more water than shorter-lived species, and should be considered with extreme caution in drier areas, such as the Brown and Dark Brown soil zones. Their high water needs can have a negative impact on yields of following cash crops. Water use can be mitigated by terminating early to allow a period of moisture recharge. Leaving strips of standing green manure, or leaving 30% of the terminated green manure residue on the soil surface when tilling, can help to capture snow to replenish water before the following cash crop is planted.

1.4. Mixed-Crop Green Manures

Legume green manures are well known for the fertility benefits they can bring to a rotation. They can also bring many other benefits to the farm and rotation (see Choosing a Green Manure for more details). But, what if you are looking to maximize the return on the investment of removing a field from cash crop production for a year?  Mixed-crop green manures may be your answer.

Pairing a legume green manure with another cover crop provides both the fertility benefits of the legume with the benefits that come with other cover crops. There are cover crops that excel at providing weed or pest suppression, improving soils, capturing or cycling nutrients, and protecting soils from erosion. Growing multiple crops together also brings diversity to the system, which can support a range of beneficial organisms both above- and below-ground.
A mixed-crop green manure of yellow sweetclover and red clover can provide a form of insurance that one component of the mix will grow well if moisture conditions are variable. Photo by Joanne Thiessen Martens.

A mixed-crop green manure of yellow sweetclover and red clover can provide a form of insurance that one component of the mix will grow well if moisture conditions are variable. Photo by Joanne Thiessen Martens.

There are seemingly endless potential combinations of crops that can be planted together as a green manure. It is important to choose species that complement each other well, both in terms of their growth habits, lifecycles, and the benefits that they bring to the system. The most common green manure mixtures include two components: a legume and a cereal. A mix of peas and oats is one of the most common green manure mixtures on the Prairies, and has been shown to produce abundant biomass while effectively suppressing weeds.

Choose green manure mixtures that include plants with differing growth habits, both above- and below- ground. This can allow the green manure mixture to exploit all opportunities while limiting competition between the components of the mix. For instance, pairing deep-rooted and shallow-rooted plants allows access to the nutrients and moisture available in different portions of the soil. Likewise, pairing upright with sprawling plants, or slender grasses with wider-leaved legumes, allows the mix to capture all available sunlight. This limits opportunities for weeds while producing abundant biomass. For guidelines in how to best choose a green manure mixture, see the COG Field Crop Handbook.
Consider how the mixed green manure will be seeded, and how and when it will be terminated. It is important to consider whether the mixed crop can be seeded in one operation, or whether multiple operations may be required. It is also important to think about when and how the green manure will be terminated. Mixed-crop green manures can produce abundant biomass that may require multiple operations to fully terminate. Alternatively, some green manure mixes may not terminate all at once; a pass with the roller-crimper after the cereal in a hairy vetch-cereal mix has headed out will terminate the cereal, but allow the hairy vetch to keep growing until freeze-up. For more information, please see the Green Manure Profiles.

Consider the competitive nature of all components of the mixture. It can be beneficial to include a highly competitive cereal alongside a slower growing and less competitive legume. However, the cereal can easily come to dominate the stand. Reducing the seeding rate of the competitive crop to only ¼ to ½ of its recommended rate while planting the legume at its full seeding rate can help to achieve the right balance. If using very highly competitive fall rye in a green manure mixture, its seeding rate should be reduced to less than ¼ of the recommended rate. For more information on seeding rates for mixed green manures, see the Green Manure Profiles.

Green manures are generally planted for the primary purpose of supplying nitrogen. So, it is important to make sure that the legume in the mixture is able to perform this role. It is generally accepted that, so long as the legume makes up at least 50% of the mixed green manure stand, nitrogen contributions can be considered equivalent to a legume monoculture. Some suggest that even stands composed of as little as 25% legume can still contribute the same amount of nitrogen as a pure legume stand.

The most common green manure mixes are two-way mixtures that include a legume and a cereal. Legumes and cereals complement each other well. The cereal emerges quickly and vigourously, providing early-season ground cover and weed control for the slower growing legume. The cereal will also take up any nitrogen in the soil, nudging the green manure toward forming an association with Rhizobia for nitrogen fixation. As the crop grows, the cereal acts like scaffolding and gives vining legumes a structure to climb. This improves light penetration and airflow, and can reduce disease. The cereal also provides additional biomass, and can contribute a more carbon-rich component to the green manure residue that slows decomposition and spreads out the mineralization of nitrogen to align more closely with crop demand. Cereal-legume mixes are also good options for grazing, should livestock be available. For more details on legume-cereal mixes, see the Green Manure Profiles.

Mixes that include more than one legume, like sweetclover and red clover can be a form of crop insurance, as each performs best in different conditions. For instance, sweetclover will perform well under drier conditions and on higher and drier parts of a field, while red clover will perform best in wetter years, or in areas of the field that tend to be wetter.

There is increased interest in multi-species green manure mixes, often referred to as cocktail mixes. These mixes often contain up to twelve different species, each occupying a different niche. They tend to include cool season grasses and legumes, warm season grasses and legumes, brassicas, and sunflowers or other broadleaf plants. Watch for more information, coming soon.

With the many possible combinations that can be used in a mixed-crop green manure, it can be wise to experiment on your farm. Try different crop combinations, different seeding rates, etc. on small parcels of land where you can observe their growth and performance. For more information, please see the Intercropping factsheet.

2. Planting Strategies

Green manures are an investment in the fertility and health of your soils that will provide the best return on investment when managed with care.  Proper seeding and the establishment of a full and healthy green manure stand will optimize the benefits that you receive from the green manure. Consider the seed that you use, the timing of planting, seed bed preparation, seeding rate and pattern, and planting method that will let you establish a vigourous and productive green manure stand.

2.1. Seed

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Field pea (4010 variety) seeds. Photo by Brenda Frick.

The availability of seed is perhaps the first thing to consider. Seed produced and stored on-farm can be used. Otherwise, green manures are becoming more prevalent, and the supply of organic or untreated seed has followed. It is important to source seed early to ensure that the green manure that you want to use is available, preferably from a reliable supplier that can provide high quality seed.

It can be wise to test the germination rate of your green manure seed before planting, especially when using older or stored seed. To perform a simple germination test, place either 50 or 100 representative or randomly chosen seeds on a double-layered piece of damp paper towel. Fold the towel over the seeds to cover, or add a second layer of damp paper towels. Roll up the paper towel and place in a sealable plastic bag to retain moisture, and keep it in a warm place.  After about a week, open the towel and count the germinated seeds. Repeat after another three to four days, repeating until you no longer see new seeds germinating. The percent germination can then be tallied. For approximate germination timings for the common green manure species, see the seeding information table below. Should the germination rate be low, the seeding rate can then be increased (see the Seeding Rate section for more information), to ensure that enough viable seeds are planted to produce a healthy, dense green manure stand. Also note the vigour of the seed – do seedlings look strong and sturdy, or pale, brown or twisted?  Seed lots with very low germination or low vigour should be replaced.

It is essential that legume seeds are inoculated to optimize nitrogen fixation. Please refer to the Inoculation section for more details.

Be sure that seeds are certified organic. Should organic seed not be available, complete the organic seed search documentation required by your certifying body, and be sure to use seed that has not been genetically modified or treated with any prohibited substances. Some certifying bodies may require testing of alfalfa seed to ensure that the seed lot does not contain any seed from genetically modified plants.

2.2. Timing of Planting

Green manures can provide flexibility in their timing of planting that can be used to the advantage of your crop rotation. A perennial or biennial can be planted as a relay crop into a cash crop, or can be planted in the spring for a full season or multiple seasons of coverage. Annual green manures lend themselves well to green manure mixtures, and often provide some flexibility in planting timing. Annuals can be planted in the spring as a full-season green manure, in mid-summer, or can be planted in the late summer as a double crop with a winter cereal cash crop in some environments.

Consider the temperature and soil moisture that are likely to be available when you plant. Most legume green manures are quite cold hardy early in their lifecycle, and so can be planted early in the spring. That being said, warmer soils will allow legumes to germinate more rapidly than cooler soils. Seeds also require soil moisture to germinate well. Planting at a time when soil moisture is available at the proper depth for your seeds can promote good and even germination that will get your green manure off to a good start.
In dry soil zones, especially the Brown soil zone, early season planting can be beneficial. There is the potential for early season moisture to be available from snow melt to give the legume green manure a good start. As well, getting an early start also means that the green manure will reach the flowering stage earlier in the season, when termination will result in maximum nitrogen contributions. This earlier termination then can allow a period of moisture recharge before the next cash crop.

Late season green manures can be an option in some regions of the Prairies. In this case, it is important to make sure that enough growing season remains to allow the green manure to produce lots of biomass. For optimal nitrogen, the green manure should be provided with enough time to flower before being terminated either by winterkill or another termination method. Most annual green manures flower in about two to three months after planting (see the Green Manures Profiles). Even if they do not flower, short windows for green manures can still prove beneficial and can supply the system with at least some nitrogen. See the Rotation Strategies double cropping section for additional information.

Green manures can also be planted into established cereal crops. This is most typically done with the longer-lasting and smaller-seeded perennial and biennial green manure species. This is known as relay cropping or underseeding – for more information, please see the Rotation Strategies section. When underseeding or relay planting, be mindful that there is sufficient soil moisture available, or that this moisture will later be present, so that the legume seed can germinate. It is often best to plant the legume green manure quite soon after the cereal has been planted and emerged. In many cases, the small seeded legume can be broadcast seeded into the emerged cereal stand and harrowed in. This provides both good seed-to-soil contact for the legume and weed control for the cereal. If this is your plan, be sure to seed at a time when the cereal can withstand a pass with the harrow. If the cereal is very well established and tall, it will shade and outcompete the legume green manure, resulting in poor establishment. Alternatively, if planted too soon after cereal planting, the green manure could overgrow the cereal, especially in years or areas with abundant soil moisture.

Warm-season legumes, typically grown in more southern climates, can also be used as green manures. Examples include cowpea, sunn hemp, and lablab. As a green manure, there is no need for the warm-season legume to reach maturity. These warm-season legumes can be seeded late, into mid-July, allowing them to take advantage of the warmest part of the summer before being winterkilled. Research on these crops in Prairie rotations and conditions to date is limited.

It is also important to think about how the timing of planting for a green manure aligns with other planting and management operations on your farm. Be sure that you have the equipment and labour resources required to dedicate the time and attention required to both your cash crops and your green manure. The flexibility that can come with green manures can allow alternative planting timings that can help to spread the workload.

2.3. Seed Beds and Fertility

Just like any other crop in your rotation, providing green manures with a good start by seeding into a well prepared seed bed will give them a leg up. It is important to manage your green manures with the same care as you would give any cash crop.

Work your soils before planting a green manure, as you would any other crop. Some, when aiming to reduce tillage, have success with seeding early without first working the soil. Seeding into an even and firm seed bed will help to ensure uniform seed placement and good seed-to-soil contact. Packing after planting, especially on the seed row, can also be effective. This in turn will lead to a uniform and healthy plant stand. To preserve soil moisture near the surface, especially for small-seeded crops that are planted shallowly, till to only a shallow depth when preparing the seed bed.

Seed bed preparation can also provide an opportunity for early season weed control. Legumes are often slow to germinate and establish, as they divert some of their energy to their nitrogen-fixing bacteria partners instead of an initial rapid burst of growth. So, legumes tend to be poor competitors against weeds early in their lifecycle. The spot that green manures occupy in the crop rotation is often one where weed pressures can be expected. Managing weeds well before planting the green manure can provide a window with minimal competition while the legume makes its slow start. There are also measures that can be taken after green manure establishment to minimize weed pressures (see Green Manures and Weed Management for more information), and some weed pressure can often be tolerated in the green manure, where the economic impacts of weeds are less of a concern than in a cash crop.

It is also important to have a ready supply of the essential nutrients required for plant growth, aside from nitrogen. Legumes require potassium, sulfur, zinc, iron and molybdenum for proper nodulation and nitrogen fixation. Phosphorus is also a limiting nutrient that is required for healthy plant growth. Aside from phosphorus, these nutrients are typically available in Prairie soils, but soils should be tested if legumes routinely perform poorly. The new Green Manure Plant Nutrient Bioassay, developed by the Entz research group at the University of Manitoba, uses plant sampling to determine when nutrients are limiting, and can be very informative.  Also consider the pH of your soil, as the nitrogen-fixing Rhizobia will not thrive in acidic soils with a pH below 5.

2.4. Planting Methods & Equipment

 

2.4.1. Frost seeding

Frost seeding can be a good option for smaller-seeded perennial green manures, particularly red clover. In frost seeding, inoculated legume seed is broadcast over a field before the ground thaws in the spring. As the ground thaws and freezes, it opens and closes, drawing the seed into the soil. The seed will then germinate when soil moisture and temperature allow.

Seed after most snow has melted, but early enough to allow for several freeze-thaw cycles. This window can, admittedly, be narrow in the Prairie climate. Many legume green manures have some frost tolerance, so frost seeding can be done quite early in the season. Frost seeding can also be done in the late fall or early winter, after plant growth has stopped, but before heavy snowfalls. If frost seeding in the late fall or winter, be aware that prolonged winter thaws may trigger early germination.

A broadcast seed spreader is required for frost seeding. Be sure to evenly cover the ground with seed when broadcasting. See the Broadcast Seeding section below for more details.

2.4.2 Broadcast seeding

Broadcast seeding scatters green manure seed over a relatively large area. Broadcast seeding can give good ground coverage, as plants are not grown in neat and tidy rows, but are instead uniformly distributed across the whole field. Broadcast seeding can be done mechanically with a broadcast seeder or even a fertilizer spreader. These implements can be tractor-mounted or mounted on an ATV.

To ensure even ground coverage, first test the spread of your broadcast seeder with the green manure seed you intend to plant. The placement of seed can vary with their size and weight. When seeding, allow for enough overlap to make sure there are no gaps between passes, which could provide an excellent opportunity for weeds.

It is typically recommended that green manure seeding rates are increased when broadcast seeding in comparison to the rates used when drilling seed (see the Table below for details). This compensates for the possibility of poorer stands due to inadequate seed-to-soil contact, losses to birds and other animals, and lower soil moisture on the surface. Smaller-seeded perennial and biennial green manures, such as alfalfa, red clover and sweetclover tend to be broadcast seeded more frequently than larger, more expensive annual seeds that are more successful when planted at a greater depth. In general, larger-seeded legumes need to be seeded into moisture, especially where soil surfaces dry quickly, as they cannot always pull enough moisture from the air for germination.

It is generally wise to work broadcast seeds into the soil in some way to improve seed-to-soil contact and promote the establishment of a good stand. When broadcast seeding into an early established cereal stand, a shallow harrowing or harrow-packing can help to establish the seed while also providing some in-crop, post-emergence weed control.  Rolling or packing after planting can also help seed-to-soil contact.

Broadcast seed can be slower to emerge than drilled seed, especially if soil moisture at the surface is low. Try to time broadcast seeding to periods with enough surface soil moisture, or just before a light rain is forecast. Do not plant into excessively moist soils, where equipment traffic can result in soil compaction.

Broadcast seeding can also be done aerially with a broadcast seeder attached to an airplane hopper. Aerial seeding can work well for seeding into an established cash crop. It can be difficult to accurately cover all of field and/or not also seed some on other fields when applying from a fast-moving airplane, but aerial seeding can allow the rapid planting of large acreages. Winds must be minimal for aerial seeding, especially with light seeds. But, aerial seeding can happen on wetter fields where seeding equipment traffic could otherwise damage soils. As most agricultural planes also broadcast insecticide, be sure that the equipment is thoroughly cleaned to prevent contamination.

2.4.3 Drilling

Seed into a well prepared seed bed, and check seeding depth and rate to ensure good green manure stand establishment. Photo by Andy Hammermeister.

Seed into a well prepared seed bed, and check seeding depth and rate to ensure good green manure stand establishment. Photo by Andy Hammermeister.

 

Green manures, especially the annual legumes that tend to have a larger seed size than perennial or biennial legumes, do well when seeded with a seed drill. Drilling allows the accurate placement of seeds at a desired depth and spacing. In contrast to broadcast seeding, drilled seeds are placed in equally spaced rows, with even spacing between seeds within a row. Seeding equipment with shovel openers also work well for green manures, although plant and row spacing will be less precise. Drilling also ensures good seed-to-soil contact, which can promote and improve germination, especially when seeds are planted at a depth where soil moisture is present.

An advantage to seed drilling is that most grain producers will already have this type of planter on hand.  Seeding equipment with shovel openers also work well for green manures. Most seeding equipment has a seeding chart that will provide information on the proper settings for various crops. For larger-seeded green manures, particularly faba bean, be sure that the drill openings are large enough to accommodate the seed without damaging the seed or clogging the equipment. Handle legume seed gently, as the larger-seeded legume green manures tend to have seed coats that are easily cracked and damaged. Consider using ladder conveyors rather than augers for filling the seeder to reduce seed damage.

Seed drills may not be ideal for the smaller-seeded perennial and biennial legumes that are planted at lower seeding rates. Most seed drills are not well equipped to deal with the very small seeds and low seeding rates that are typical of red clover, sweetclover and alfalfa. But, drills may be successful with smaller-seeded legumes if there is a setting or adjustment that can be made for small seeds and if planting depth can be adjusted to be shallow. A small-seed attachment, such as a grass box or even a fertilizer boot, can also allow seed drills to accommodate smaller-seeded legumes.

Seed drills should be calibrated and tested before seeding fields, to ensure that seed is being planted at the desired rate and depth. The seeding chart provided by the drill manufacturer will provide you with a good starting point.

2.4.4. Air seeding

Air seeders can also be used for green manure seeding, especially when this is the seeding equipment available on-farm. Do, however, be cautious, as air seeders can easily damage seed, which can reduce germination and result in uneven stands. Seed damage typically occurs where seed is metered and at the seed openers. Damage is more apt to occur when fan speeds and airstream velocities are high. To limit damage, adjust air velocity so it is just high enough to prevent lines from clogging. Be sure all openings are large enough to accommodate the legume seed.

2.5. Seeding Rates, Depths and Patterns

Seeding rates are generally a reflection of seed size. Annual green manures tend to be larger-seeded and so require a high seeding rate to ensure good coverage, which can lead to higher seed costs. When using annual legume green manures, consider choosing varieties that have been bred for a smaller seed size, such as Indianhead lentil and 4010 field peas. This smaller seed size can reduce the overall seeding rate required to establish a dense and healthy stand – the same number of seeds of a small variety weigh less than the same number of seeds of a larger-seeded variety.

In general, seeding rates should be such that plant stands are established at a density of approximately 50 plants per m2 for larger-seeded legumes, such as peas, lentils, faba beans and vetches. For the smaller-seeded biennial and perennial green manures, such as red clover, alfalfa and sweetclover, aim for approximately 110 plants per m2.

It is important to follow recommended seeding rates to allow the establishment of a green manure stand that is dense enough to outcompete weeds without being so dense as to have strong competition within the stand or create conditions for disease. For seeding rate recommendations, follow the instructions provided by your seed supplier, consult the table below, or refer to the Green Manure Profiles.

There are times when recommended seeding rates should not be followed. If planting into a weedy field, increase the seeding rate (1.5-2x) to better outcompete weeds and compensate for any plant loss that may occur with post-emergence harrowing. When intercropping, relay seeding or underseeding a green manure, seeding rates may need to be manipulated to make sure that all parts of the mix can perform as required. In intercrops, this often means planting the legume at the recommended rate to ensure that they form at least 50% of the stand for nitrogen fixation, while the seeding rate for other species in the mix are often reduced (see the Mixed-Crop Green Manures section for more details). When underseeding, the cereal crop is typically planted at half of the recommended rate, while the perennial or biennial legume is seeded at its full rate a few days or weeks later.

While it is always preferable to use high quality seed, seeding rates can be adjusted to account for reduced germination that may occur in older or lower quality seeds. If germination rates or vigour are very low, replace the seed. For information on how to conduct a simple on-farm germination test, please seet the Seed section. Once you have determined the germination rate of a batch of seeds, an adjusted seeding rate can be calculated. Simply divide the recommended seeding rate by the germination rate (expressed as a decimal, e.g. 90%=0.9).  For example, for a green manure legume with a recommended seeding rate of 35 lb/ac and an 90% germination rate, the adjusted seeding rate would be 35/0.9 = 39 lb/ac).

Like seeding rate, seeding depth is also generally a reflection of the size of the green manure seed: larger-seeded green manure legumes should be planted at a greater depth than smaller-seeded options. For more information on seeding depth recommendations, see the table below and the Green Manure Profiles. Within the seeding depth ranges provided, seed no deeper than is needed to reach soil moisture. However, plant at the deeper end of the range if you are planning pre- or post-emergence tillage for weed control.

Table 1. Seeding information for common legume green manure crops on the Prairies.

Species Seeding Depth (inches) Seeding Rate: Drilled (lb/ac) Seeding Rate: Broadcast (lb/ac) Inoculant Time to Germinate (~Days)
Alfalfa 1/4 – 1/2 8 – 10 10 -12 Rhizobium meliloti 7
Red Clover 1/4 – 1/2 6-10 10-12 Rhizobium leguminosarum var trifoli 7
Sweetclover 1/4 – 1 6-10 10-15 Rhizobium meliloti 7
Faba Bean 1 1/2 – 3 170-300 Not recommended Rhizobium leguminosarum var viceae 10
Field Pea 1 – 3 120 – 175 Not recommended Rhizobium leguminosarum var viceae 8
Lentil 1-2 35-40 Not recommended Rhizobium leguminosarum var viceae 8
Chickling vetch 1/2 – 1 1/2 60-80 Not recommended Rhizobium leguminosarum var viceae 14+
Hairy vetch 1/2 – 1 1/2 15-20 25-40 Rhizobium leguminosarum var viceae 14

3. Inoculation

 

3.1. Importance of Inoculation

The nitrogen-fixing ability of green manures is dependent on a successful partnership with Rhizobia soil bacteria. These bacteria are able to convert abundant atmospheric nitrogen into plant-available forms through a process known as nitrogen fixation (see the Choosing Green Manures Module for more details). When this partnership is entered, Rhizobia infect the roots of their legume host, forming nodules. In exchange for energy in the form of carbohydrates from the legume, the Rhizobia supply the legume with plant-available nitrogen. This nitrogen is then used by the plant to build tissues needed for growth.

Nitrogen cycling with a legume green manure. Figure modified slightly from that kindly provided by Dr. Julie Grossman and Sharon Perrone, University of Minnesota.

When legumes are inoculated, it means that they are planted with an amendment that contains living cultures of Rhizobia bacteria. Each legume species associates best with a particular strain or species of Rhizobia. Inoculation with the properly matched strain helps to populate the soil and plant roots with the best-suited Rhizobia. When correctly coupled, this allows a strong and effective partnership between the legume and bacteria to form, which can boost nitrogen fixation and improve green manure growth.

Rhizobia can survive in the soil for many years. When growing the same legume, this soil reserve can allow nitrogen fixation to occur. But, the adapted strain may no longer be the most abundant in the soil, especially if other pulses have been grown in the rotation. Inoculating the legume at planting also places the bacteria near the seed, potentially speeding the connection between the legume and the Rhizobia. Inoculating legumes each time that they are grown is well worth the cost for the biomass produced and nitrogen that is fixed.

3.2. Choosing an Inoculant

Each green manure legume is suited to a particular strain of Rhizobia. While there are often Rhizobia present in soils, they may not be the best strain for the legume green manure being planted. Properly pairing a green manure legume with its matched Rhizobia strain will help to improve nodulation and optimize nitrogen fixation. Inoculating green manures with the proper Rhizobia strain ensures that this well-adapted strain is present in the soil.  Most inoculants are labelled with which legumes they should be paired. You can also refer to the table in the Planting section for inoculant Rhizobia strains appropriate for the major green manure legumes.

For organic producers, it is important to check with your certification body each year to make sure your chosen inoculant formulation is permitted. This can change from year to year, depending on any formulation changes, so be sure to check yearly.  Some inoculants have been bioengineered or contain other ingredients, such as sticking agents, which may not be permitted for organic production.

Inoculants come in a variety of formulations, including liquid preparations, granular preparations, and peat-based formulations. Use the format that works best for your operation, when choices for organically allowed appropriate strains are available. Liquid and peat-based inoculants can be applied to the seed before planting. Granular and peat-based forms can be soil-applied while seeding.

3.3. Inoculant Use & Handling

Remember that legume inoculants are preparations of living bacteria. Check expiry dates on your inoculants before use to make sure that the product is fresh and active. Follow the directions for inoculation on the package of your purchased inoculant.

It is important to handle inoculants with care to keep them alive and healthy. Inoculants should be kept cool. This means refrigerating before use, and planting soon after inoculation. Leaving inoculated seed in the seeder on a hot summer afternoon can kill the living Rhizobia. When possible, plant inoculated seed in the morning or evening, and be sure that the seed is covered after planting before it heats up or dries out. If you are concerned that inoculated seed has sat for too long at warm temperatures, apply additional inoculant.

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Pea roots with nitrogen-fixing nodules.

If you are curious to see whether the inoculation of your green manure has been effective, there is an easy way to do so. This is best done when the green manure is well established, but not yet mature. Gently dig up a plant from the soil. Brush or wash soil off of the root to first check for nodules, or swellings along the root. Active nodules that are working to fix nitrogen will be pink or red inside when sliced open.

4. Pest Management

There are two aspects of pest management to consider when green manuring: pests of the green manure, and how a green manure may impact the pests of other crops in the rotation.

Cereal diseases, such as ergot, can be reduced when there is a break between cereal crops in the rotation. The same can be said for insect pests, such as the wheat midge. In this way, legume green manures can provide a break in rotations that may otherwise be dominated by cereals and oilseeds, helping to reduce pest and insect pressures in other phases of the rotation.

When cash crop legumes (pulses) are part of a rotation that also includes legume green manures, it is important to be aware of any diseases or insect pests that may be common to both. Ascochyta blight in lentils can proliferate when lentil is included as both a cash crop and a green manure. Root rots common in legumes can infect both green manure legumes and pulses, while Sclerotinia and Fusarium diseases can infect multiple legume species. To manage these diseases, it is best if a legume green manure is not immediately preceded or followed by a legume cash crop. However, in many rotations, it makes sense to follow a low-nitrogen using pulse crop (planted when the nitrogen has been used up by previous crops in the rotation) with a legume green manure that will replenish nitrogen supplies. In this case, try to use different legumes for the cash crop and green manure component. Including other grains, such as cereals or oilseeds, in the rotation can help to break these disease cycles. Rotating green manures, so that the same green manure legume is not always used, can also help to break disease cycles.

There are also disease and insect pests that can infect or infest green manures. Information on these is provided below. Managing your green manures well can create healthy and resilient plant stands that can withstand some insect and disease pressure. The economic impact of crop pests also tends to be lower in green manures, as the green manure is not brought to maturity. Nonetheless, pests can be a concern if they reduce green manure biomass, impact nitrogen fixation, or may spread to adjacent fields or into following crops in the rotation.

4.1. Diseases of Green Manures

Legume green manures, like most crops, are susceptible to bacterial, viral and fungal diseases. Managing your green manure and rotation with care can help to reduce the occurrence or severity of disease infections. There are a few key strategies that can reduce disease pressure in organic green manure stands.

Crop rotation is important: try to arrange your rotation so that green manure legumes and cash crop pulses are not planted back-to-back. Including unsuitable host crops between legumes in the rotation can reduce the build-up of disease inoculum. When possible, use varieties that have disease resistance. In the case of seed-borne diseases and diseases with spores that can contaminate seed, plant seeds that are disease-free.

Establishing a good green manure stand, through the use of good seed planted at the proper seeding rate into a well prepared seed bed can also improve the resilience of the green manure to disease pressures.

Green manures support healthy soils, which can in turn contribute to plant health. Healthy plants are more resilient to attacks by insects and disease, while healthy soils may have the potential to suppress soil-borne diseases.

Interplanting cereals with dense, sprawling legumes, such as peas and hairy vetch, can help to reduce disease pressures in the green manure. The cereal provides a structural support that the vining legume can climb, rather than being prostrate on the ground. This improves light penetration and air flow, which can limit disease formation and spread.
Consider altering planting timing to reduce disease pressures. For instance, planting field peas early can reduce the severity of powdery mildew infections. Delaying lentil planting until soils warm can reduce ascochyta infection. Planting seeds deeply in cold soils can create optimal conditions for seedling blights and damping off.

Disease-infected plant residues from a terminated green manure stand can be a source of inoculum for future infections. Tilling infected residues into the soil can aid in the breakdown of the pathogens and limit wind-blown movement of spores to help limit disease spread.

In spite of your best efforts, diseases may sometimes arise in your green manures. Monitor your green manures for signs of disease infection, and take action when required. Early stand termination may help to reduce disease spread and can allow time for disease inoculum to die before the next crop. Diseases that may be found in the main green manure crops of the Prairies are outlined briefly below, with links to identification and management guides provided, where available.

Field peas can be attacked by a number of diseases, including blights, mildews, root and stem rots, mosaic viruses, leaf spots and blotches.

Lentils can be susceptible to blights, root and stem rots, molds, and anthracnose. For more information and tips on how to identify these diseases, the Alberta and Saskatchewan governments have wonderful online resources.

Faba beans are also susceptible to disease, and may become infected with spot diseases, mildew, rust, blights, and rots.

Vetches can be vulnerable to stem rot, black stem, gray mold, leaf spot and root-knot nematodes.
Alfalfa and sweetclover may be vulnerable to wilts, crown and root rots, and foliar diseases. Red clover stands can be infected with seedling blight soon after planting, as well as black stem and anthracnose.

4.2. Insect Pests of Green Manures

There are few insect pests that will infest green manures on the Canadian Prairies. Most insects that do infect green manures tend to be crop specific, so management of these insect pests is best performed through good crop rotations, and working to establish healthy stands that can withstand some insect pressure.

Green manures can also provide habitat and food for beneficial organisms. They can support insects, birds and mammals that eat or parasitize insect pests and limit pest insect populations. Green manures also support healthy soils, which can in turn contribute to plant health; healthy plants are more resilient to attacks by insects.

That being said, legume green manures can still be vulnerable to insect pests. The impact of these pests on plant material that is destined for termination and incorporation should be carefully considered, as their economic impact may not be so great as if the crop was to be harvested. So long as insect infestations will not spread to adjacent crops or continue into following crops, many of the insects that afflict green manures may be tolerable.

Sweetclover can be impacted by infestations of the sweetclover weevil. The weevil can cause severe damage to sweetclover stands, with stand failure a possibility. To limit the possibility of sweetclover weevil infestation, be sure that there is a break between sweetclover plantings.

Field peas and faba beans can be vulnerable to the pea leaf weevil. This insect pest feeds on the leaves and growing points of these legumes. The larval form also feeds on nodules on the legume root where nitrogen fixation occurs, and can result in a reduction in the nitrogen fixed by the legume green manure.

Pea aphids can populate stands of field peas, alfalfa, clovers and most other green manure legumes. These aphids can impact seed production, but may not be as problematic for green manure biomass.

Alfalfa can also be impacted by infestations with the alfalfa weevil. This weevil tends to do most of its damage around the time of early bloom. Early termination or haying of the alfalfa stand can minimize impacts. Lygus bugs may also infest alfalfa green manures, but tend to damage seed production more than biomass production.

Red clover can be damaged by red clover thrips. Thrips tend to worsen as the stand ages, and are often worse in dry years. The impacts of this insect tends to be greater on stands grown for seed than those turned under as a green manure. Red clover may also be host to lygus bugs, pea aphids, and weevils.

Grasshoppers are a concern for Prairie grain growers, especially in dry years. While grasshoppers tend to favour grain stands, green manures can also be susceptible to grasshopper damage. However, Prairie research suggests that a pea green manure can deter and even harm the grasshopper. Seeding early can allow the green manure stand to grow before grasshopper attack, improving its resilience. For more information about grasshoppers, see your local Department or Ministry of Agriculture.

For more information about weed management in green manures, please see the Green Manures and Weeds.

5. Termination Strategies

Green manures are legumes grown without intent to harvest, but rather for the fertility benefits that they can provide to the crop rotation. To get the nitrogen accumulated in the green manure’s tissues into soils in plant-available forms, the green manure must be terminated and allowed to decompose.

The choice of when and how to terminate a green manure stand will depend on the nature of the green manure, the soil and climate conditions, the equipment available, whether there is a need to reduce tillage or conserve soil moisture, and how the nitrogen released from the terminated green manure can be best synchronized with crop needs.

5.1. Termination Timing

Timing green manure termination can be a delicate balancing act. You may have to juggle green manure termination with other operations on your farm. The need for water conservation, the want for maximizing nitrogen contributions, and the desire to reduce weeds from contributing to the seedbank will all need to be weighed and balanced.

Green manures, like this field of peas, reach peak nitrogen around the time that they flower. Photo by Laura Telford.

Green manures, like this field of peas, reach peak nitrogen around the time that they flower. Photo by Laura Telford.

 

The primary task of green manures is to supply nitrogen. To maximize nitrogen contributions, green manures should be terminated at the flowering stage. This is because green manures reach their peak in biomass and nitrogen content at the flowering or early pod formation stage. Once seeds begin to form, the legume stops growing and begins moving nitrogen from its tissues into the seed. This seed-stored nitrogen is not as readily available as nitrogen in the plant’s other tissues. As plant growth stops, nitrogen-fixation in the nodules also stops. Terminating green manures at flowering means that the plant will have taken up all of the nitrogen that it will receive from its Rhizobia partner, and will have not yet begun to sequester that nitrogen away in its seeds. Terminating at flowering also limits the chance of volunteer green manure plants acting as weeds in the following crop. There are, however, situations when termination should occur before the green manure flowers.

In drier areas or droughty years, there is often a tradeoff between nitrogen contributions and soil water supply. Here, it is often wise to terminate the green manure stand before it fully flowers. Instead, it is recommended that green manures be terminated at the bud stage, or by the end of June. Early termination reduces the amount of water that is taken up by the green manure, leaving more moisture in the soil for the following crop. Early termination also prolongs the time between the green manure and the next crop, providing an opportunity for rains and snows to replenish soil water. This does, however, come at a cost. Early termination reduces the amount of nitrogen supplied by the green manure, although this may still be considerable.

Green manures can be weedy. If weed pressures are high, or if a particularly problematic weed is present, making sure that these weeds do not set seed can help to reduce weed pressure in future crops. When weeds are about to set seed, early termination can make sure they do not. Terminating the stand early means that less nitrogen will be made available. This will have to be weighed against the possible weed issues that may arise from allowing the green manure, and the weeds, to grow for longer. This is a case-by-case decision that needs to be made with careful consideration of the nitrogen needs and sensitivity to weeds of the following crop in the rotation.

With annual green manures, especially peas, lentils or faba beans, there is also the option to let the green manure go to seed. The legume seeds can be allowed to mature, they will then drop from the plant, and germinate to establish a second green manure crop. This can provide on-going ground cover and save seed costs for establishing a second green manure or cover crop.  The self-reseeded green manure can then be left to winterkill or can be terminated at flowering if the climate allows. While this is a strategy that has been used by some Prairie farmers, there is to date little research to illuminate the nitrogen implications of this tactic.

5.2. Termination Methods & Equipment

There are a number of possible methods for green manure termination, each with its benefits and drawbacks. For a summary, see the table below.

Strengths and weaknesses of green manure termination methods. Adapted from Podolsky 2013.

Ease of Use Erosion Protection Weed Control Nitrogen Availability Energy Savings
 Tillage **** * ***** ***** *
 Winterkill ***** ** * *** *****
 Mow *** *** ** *** **
 Roller Crimper *** ***** *** *** ****
 Wide blade cultivator ** **** *** *** ***
 Grazing ** *** **** ***** ***

* = poor  *** = moderate, ***** = excellent

5.2.1. Winterkill

Winterkilling can be the easiest termination method, and does not require management time or equipment. If planning to let your green manure winterkill, be sure that it will succumb to winter conditions in your region. The cool season annuals, such as lentil, pea, faba bean and chickling vetch will not overwinter in Prairie conditions. Winter annuals, such as hairy vetch, are likely to winterkill if they have flowered before the cold season hits, so be sure to plant in the spring if your plan is to winterkill a hairy vetch stand. Perennials and biennials will winterkill if not hardy in your growing zone. See the Green Manure Profiles for more information.

If planning to winterkill your annual green manure, try to plant late enough that the green manure will not set seed before a killing frost, but will reach the flowering stage. This will maximize your nitrogen returns, while limiting volunteers in the following crop. See the Green Manure Profiles for estimates of flowering timings.

The crop residue from winterkilled green manures can provide some soil protection over the winter, and can help to retain soil moisture. In the spring, the crop residue can suppress weed growth, but may also slow soil warming that can delay the planting of the following cash crop. Winterkilled green manure residues can be slimy, and may benefit from spring tillage before planting the next cash crop.

5.2.2. Tillage

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Green manure terminated by tillage with discs. Photo by Brenda Frick.

Tillage is perhaps the most traditional and common method for green manure termination on organic operations. Tillage implements commonly used include tandem or offset discs, plows and cultivators.

When a green manure is terminated with tillage, its tissues are broken up and incorporated into the soil. This puts the plant material in direct contact with the soil, and the soil organisms that will break it down and liberate plant-available nitrogen. By breaking up the green manure tissues, the surface area is increased, allowing the soil microorganisms better access to the plant tissues, and again speeding up decomposition.

The downside of tillage is the soil disturbance that it causes, as well as the lack of residue left on the soil surface that leaves soils vulnerable to erosion. This can be countered somewhat by tilling in a way that leaves 30% of the plant residue on the soil surface.

Green manures can be terminated and incorporated with moldboard plows. This implement turns over the soil and buries the vast majority of green manure residue with a single pass. While effective in terminating even dense green manures, the moldboard plow also leaves soils exposed and prone to erosion. In addition, the plow creates a dense mat of buried material that can be slower to decompose than more broken-up residues.

Green manure termination with tillage is more routinely accomplished with discs. Discs allow operators to leave some residues on the soil surface to reduce erosion potential. In general, shallow tillage with discs that turns 70% of the plant material below-ground is recommended. This leaves some residue on the surface to protect the soil and capture snow. Discs also chop and mix the green manure residues into the soil. This can allow for even decomposition. Green manures with abundant biomass, such as sweetclover, alfalfa and hairy vetch, may not be completely terminated with a single disc operation. Multiple passes, or mowing before discing, can alleviate these difficulties.

For a short video demonstration of green manure termination with a tandem disk, see the Natural Systems Agriculture YouTube video below.

5.2.3. Mowing

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Green manure terminated by mowing. Photo by Joanne Thiessen Martens.

Terminating a green manure by mowing allows plant residues to be left on the soil surface. This can provide ground cover into the fall that can limit potential erosion, suppress weeds, reduce water loss through evaporation from the soil surface, and trap snow. Mowing can also be used before tillage on high biomass or tough green manures to ease incorporation and ensure complete termination.

Most mowers chop plant tissues as the green manure is terminated. These smaller pieces of tissue decompose more quickly than whole plants, as there is more surface area for contact with soil and microorganisms. Mowing can also scatter plant material and reduce the overall bulk, which can provide opportunities for weed growth. Timely mowing can be used to prevent seed set for most weeds, though some weeds may continue to grow from lower branches.

There are various types of mowers that can be used to terminate green manures. Flail mowers tend to evenly distribute the chopped material across a field. Rotary mowers tend to leave piled rows of plant residue, rather than evenly distributing material across the field. Sickle bar mowers tends to largely leave cut plants where they grow. Sickle bar and rotary mowers should be used with caution on viney green manures, which can easily tangle and bind the blades.

Green manures terminated by mowing are often later incorporated into the soil with tillage. Mowing provides the ability to terminate the green manure while delaying incorporation to provide soil cover and slow green manure decomposition and nitrogen mineralization. Mowing can also be useful in preventing weed seed set if weeds flower before it is time to till.

For a short video demonstration of green manure termination with a flail mower, see the Natural Systems Agriculture YouTube video below.

5.2.4. Wide blade cultivators

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Green manure terminated with a wide blade cultivator. Photo by Joanne Thiessen Martens.

Wide blade cultivators, also known as the Noble blade or undercutters, can also be used to terminate green manure stands while leaving residue on the soil surface. These cultivators work by undercutting, or severing the green manure from its roots. The above-ground portion of the green manure is left standing or may be flattened, but otherwise is left intact. Because the plants are left largely intact and soil is disturbed only at a shallow depth, green manures terminated with the wide blade cultivator tend to break down more slowly than those terminated with mowing or tillage. Termination with the wide-blade cultivator also can help to retain soil moisture and capture snow.

This termination method reduces soil disturbance, but can be difficult to implement. Prairie studies suggest that terminating sweetclover stands at 70% bloom with a wide blade cultivator can suppress weeds into the fall. However, proper conditions at termination are important when using a wide blade cultivator, and can greatly impact the ease with which this piece of equipment can be used. High moisture, very dense plant stands, rocky fields, or hard and dry soils can impede termination. An additional tillage operation is often required to later incorporate the plant material before planting the next crop.

For a short video demonstration of green manure termination with a wide blade cultivator, see the Natural Systems Agriculture YouTube video below.

5.2.5. Roller-crimper

The roller-crimper, also known as the blade-roller, is a non-tillage based method for termination of a green manure. The roller-crimper is a large weighted drum with lengthwise blades that crushes the vascular system of plants. This effectively starves the plant tissues of water, carbohydrates and nutrients. The stems are also flattened to form a mat of plants on the soil surface. As the plants are essentially intact when terminated, they break down much more slowly than the shredded plant material that results from mowing.

Termination timing is very important when using the roller-crimper. Young plant tissues can rebound or regrow when crimped. It is only when the green manure crop flowers that termination with the roller-crimper is effective. Rolling a younger stand will result in incomplete termination. Roll the green manure in the same direction as you will plant the following no-till crop.

rolling-pea-oat-IV

Green manure terminated with a roller-crimper. Photo by Iris Vaisman.

Interest in incorporating no-till phases into organic rotations has been on the rise. Dr. Martin Entz and his research team at the University of Manitoba have examined the feasibility of no-till organic within typical Prairie grain rotations and climate. While the need to incorporate green manures for nitrogen release was perhaps previously seen as a limitation for no-till organic production, their work suggests that no-till phases in organic grain rotations are indeed possible, and hinge on green manures. The most successful system sees hairy vetch planted (often as a mixed-stand green manure with a cereal intercrop) in the spring as a green manure. The hairy vetch stand is then rolled in the mid- to late-summer. The roller-crimper successfully terminates weeds and the cereal portion of the intercrop. The hairy vetch, however, is quite resilient to the roller-crimper at this time, and will continue to actively grow, building biomass and accumulating nitrogen until it is winterkilled. A grain crop can then be no-till planted into the resulting mulch the following spring. With this system, the need for tillage can be eliminated for a 1.5-2 year period in the organic crop rotation. Extending this period can result in yield reductions, largely due to the introduction of perennial weeds and the need for additional nitrogen.

If planning to no-till plant into the rolled mulch the following spring, be sure that you have sufficient biomass to provide weed suppression without tillage. Prairie research suggests that 6-8 tonnes of biomass per hectare is required at the time of seeding the following cash crop in the spring to meet this need. This level of biomass has been obtained in Prairie studies of hairy vetch or hairy vetch-barley green manures in southern Manitoba. Otherwise, weeds can quickly grow through the mulch and impact the cash crop. Should your mulch biomass be below this level, it may be wise to reconsider tillage. Likewise, it is important that the mulch decompose slowly, to provide ongoing ground coverage until the cash crop is well-established and competitive. This can be accomplished with a hairy vetch stand that continues growth until winterkilled.

No-till organic production with green manures requires specialized equipment: the roller-crimper and a no-till planter that can seed into the thick mulch. Seeding when the mulch is dry using a disk-drill seeder has worked well for even thick hairy vetch-barley mulches. Other no-till seeders can also work well. Using these methods, up to three tillage operations can be eliminated over 1.5 to 2 production years. Stands terminated by the roller-crimper can also later be incorporated into the soil with tillage equipment.

Introducing a reduced-tillage sequence in the rotation can also impact weed pressures, with key weed populations often shifting from summer annuals to winter annuals and perennials. This is especially true when low residue amounts are left on the soil surface that cannot adequately suppress weeds. Surface residues can also impact the soil climate. Residues shade soils, which can slow soil warming in the spring, and can also retain more soil moisture than bare soils. These lowered soil temperatures in the spring can slow the germination and establishment of the cash crop.
For a short video demonstration of green manure termination with a roller-crimper, see the Natural Systems Agriculture YouTube video below.

5.2.6. Grazing

Grazing has recently been explored as an option for terminating annual green manure stands. Grazing provides an opportunity for financial gains, through the sale of meat or milk, while still achieving most (~80%) of the nitrogen benefits of the green manure. Perennial green manures are generally resilient when grazed, so long as they are not grazed too aggressively too early in their growth or grazed below the regrowth point. Annual green manures, on the other hand, are generally terminated when grazed.
Termination of annual green manures with grazing requires some investment in infrastructure. This includes access to ruminants, whether already a part of the farm system or borrowed from a neighbor. Knowledge of livestock handling and management is also essential.  Fencing is required to assure that only the green manure is grazed, and not nearby cash crops. Lots of water must also be readily available, as should shelters or shade from the summer heat.
Grazing can allow the termination of a green manure with minimal soil disturbance, save for the disturbance and compaction that occurs when livestock roam the field. Grazing can also provide weed control, as many weeds will be grazed along with the green manure. Depending on how the animals graze the stand, and how their manure is distributed throughout field, a harrowing or even more aggressive tillage may be required between animal-terminated green manures and seeding the next crop.
rolling-pea-oat-IV

Green manure terminated by grazing sheep. Photo by Laura Telford.

It is important to consider the attractiveness, nutrition, palatability and potential toxicity of a green manure that you plan to terminate with grazing. For crop-specific information, see the Green Manure Profiles or the Green Manure Selection Tools. Hairy vetch and chickling vetch in particular should be approached with caution. Annual green manures are often highly nutritive for grazing. As they are generally young plants when grazed, they contain higher levels of proteins and minerals than typical pastures. In some cases, the nitrogen levels in annual green manures may be too high for optimal grazing, so consider including a cereal or grass in the green manure mix. This can also offset potential bloat issues associated with some green manures, such as hairy vetch and field peas. Grazing the green manure at a more mature stage can also reduce the risk of bloat, as can grazing management practices such as moving animals to lush new growth later in the day when they have already filled up.

A mix of peas and oats has been found to be a good green manure option when considering termination with grazing. This mix produced abundant biomass, suppressed weeds, and was happily grazed by sheep in research trials. Other annual green manures can also be grazed, but some, such as Indianhead lentil or hairy vetch, may require some training for the animals to accept.
Grazing alters the nutrient dynamics of the green manure stand. The ruminant gut, whether a cow, sheep or goat, will break down the green manure tissues and mineralize nitrogen. This is very similar to what is otherwise done by soil organisms when green manures are incorporated into the soil, but happens at a much quicker rate in the ruminant gut. The mineralized nitrogen (and other nutrients) is returned to the soil through the excretions of the livestock, and is readily available for plant uptake. This means that plant-available nitrogen is supplied more quickly when green manures are terminated by grazing as opposed to incorporated.
For uniform soil fertility, livestock are more likely to distribute their excrement throughout the field when they are moved through smaller paddocks, and when congregation areas (such as water sources), are spaced out across the field. An intensive “mob-grazing” approach is popular among farmers who are grazing green manures with livestock.
With the speedy nitrogen cycling that results from grazing, consider including a catch crop in the rotation following a grazed green manure to limit nitrogen losses from leaching. Catch crops take up available nutrients, securing them in their plant tissues until they are terminated closer to the date of cash crop planting. But, make sure that there is enough time available for the breakdown and mineralization of the catch crop before planting a cash crop.

5.2.7. Challenges

High biomass cover crops may require multiple operations for termination. This is particularly true for multiple season green manures, such as perennial alfalfa and red clover and biennial sweetclover. Hairy vetch is an annual green manure that often requires multiple operations.

For viney green manures, in particular hairy vetch, be cautious of equipment failure due to the wrapping of vines around moving parts, cogs and chains. Hairy vetch in particular can damage equipment or drastically slow operations. To mitigate this, consider first terminating the green manure by mowing with a flail mower (rotary mowers can become quickly tangled with vetch) before tilling. Winter killing, potentially preceded by a pass with a roller-crimper in the mid- to late-summer, is also an option.

5.3. Nitrogen Implications of Termination Strategies

Legume green manures are primarily grown for their ability to contribute nitrogen to the crop rotation. So, it is important to think about how much nitrogen a green manure can contribute, when this nitrogen will be made available, and the potential effects of how and when the green manure is terminated. Management of green manure residues involves a balance between protecting soils, providing and protecting nitrogen.

Once a legume green manure has been terminated, the plant tissues will begin to be decomposed by microorganisms in the soil. As the plant tissues are digested, nitrogen is released, or mineralized. The rate of decomposition and nitrogen mineralization depends on many factors, including soil temperature, soil moisture, soil pH, nutrient availability, where and how the green manure residues have been placed, and the makeup of the plant tissues. Any conditions that are favourable for soil biology will speed decomposition, while factors that are unfavourable or that separate the green manure from the soil will slow decomposition. Highly lignified tissues, or tissues with a high ratio of carbon to nitrogen will break down much more slowly than more lush tissues with a higher proportion of nitrogen.

Nitrogen cycling with a legume green manure. Figure modified slightly from that kindly provided by Dr. Julie Grossman and Sharon Perrone, University of Minnesota.

Nitrogen cycling with a legume green manure. Figure modified slightly from that kindly provided by Dr. Julie Grossman and Sharon Perrone, University of Minnesota.

 

It is important to note that not all of the nitrogen that was contributed to the green manure by its Rhizobia will be available for the following crop in the rotation. In general, you can expect about 40-60% of the nitrogen in the green manure at termination to be available to the following crop. Some of the remaining nitrogen may be lost to the system, through leaching or to the atmosphere. The rest will become available in later years, as the green manure tissues and the bacteria that broke down those tissues continue to decompose. Please see the COG Field Crop Handbook for more information about nitrogen availability and nutrient budgeting.

5.3.1. Effect of environmental conditions

Mineralization will occur more quickly in moist soils, soils that are warm, and soils that are well aerated. These conditions are favourable for the soil microorganisms that decompose plant tissues and mineralize nitrogen.  When conditions are favourable, a large flush of plant-available nitrogen can be released within the first week or two after green manure termination. Studies outside of the Prairies suggest that the optimal temperature for soil biological activity is generally in the range of 20-40°C, with activity slowing in colder soils. Soils that are extremely dry or, at the other extreme, waterlogged will also slow microbial activity and nitrogen mineralization. Soil microbial activity peaks when soil moisture is at field capacity. The soil microbes responsible for decomposition and nitrogen mineralization also perform best in fertile soils that are not too acidic or alkaline (pH in the 6-8 range).

5.3.2. Effect of termination timing

The timing of green manure termination can have a large impact on nitrogen availability. Termination timing will have a bearing on the environmental factors that influence nitrogen mineralization. It will also impact how much nitrogen has been taken up by the legume, and can impact how the release of plant-available nitrogen will coincide with the nitrogen needs of the following cash crop.

For optimal nitrogen contributions, green manures should be terminated when they reach the bloom to early pod stage. At this time, the green manure begins to turn its efforts from growth and biomass accumulation to seed production. As seeds are produced, nitrogen is reallocated from a plant’s tissues (stems, leaves, roots) to its seeds. If terminating earlier to conserve soil moisture, often recommended at the bud stage, nitrogen contributions may be reduced as the plant will not have yet reached its peak biomass. While there are no concrete numbers on how greatly nitrogen contributions are reduced when terminating early, Prairie studies have demonstrated that even early-terminated green manures can often still supply enough nitrogen to support a following crop. You can do a quick estimate of the nitrogen available when you terminate by measuring the biomass of your green manure. Expect about 2.5% of the dry weight to be nitrogen. A sacrifice of some biomass, and hence nitrogen, may be well worth it to ensure that subsequent crop yields are not limited by moisture availability.

Terminating and soil-incorporating a green manure when there is moisture and warmth in the soil can result in a rapid breakdown of green manure tissues and nitrogen mineralization. If the optimal termination timing that you have decided for your green manure (see Termination Timing section) happens to fall in the mid- to late-summer in a region with available soil moisture, you should think about how you can capitalize on this rapid nitrogen availability, or how you could delay its release. Otherwise, you risk losing this rapidly mineralized nitrogen through leaching. Where climate allows, planting a winter cereal following the green manure crop, or planting a cover crop that can take up and hold this soil nitrogen temporarily are two options. Alternatively, terminating the green manure using a method that allows the residue to sit on the soil surface can slow nitrogen mineralization.

Terminating a green manure when soils are very dry or cold limits the activity of soil microorganisms and slows decomposition and nitrogen mineralization. When terminated in the fall, nitrogen mineralization may be slow, but will rapidly begin once soils warm in the spring. Planting a grain cash crop in the spring could then take advantage of the newly mineralized nitrogen.

It is also important to remember that the rate of green manure breakdown and nitrogen mineralization is also impacted by the plants themselves. Young, succulent annual green manures terminated at the flowering stage will break down quickly when tilled into the soil, resulting in an increase in plant available nitrogen soon after their incorporation (if the environmental conditions allow). In contrast, green manures that are terminated when they are relatively older or green manure mixes that include cereals or grasses that tend to have tougher tissues with more carbon (a higher C:N ratio) will slow the rate of decomposition and nitrogen mineralization. As a general guide, expect quick mineralization if the nitrogen concentration of the green manure is higher than 2.5% and slow mineralization if it is less than 1.5%.

5.3.3. Effect of termination method

Each termination strategy has implications on nitrogen availability. Decomposition and nitrogen mineralization occur more quickly when the terminated green manure is in contact with the soil, where the microorganisms involved in the decomposition process reside. This is sped up even more when green manures are grazed. Green manures that are terminated and left on the soil surface are not as accessible to microorganisms, and so tend to decompose more slowly.

When green manures are incorporated into the soil when terminated, plant-available nitrogen can be quickly released, especially when environmental conditions are favourable (see the Effect on environment conditions section above). This can be beneficial when the green manure is quickly followed by another crop that can use this nitrogen. But, if there are no actively growing plants to take up this nitrogen, it can be lost through leaching. In leaching, nitrogen mineralized from the green manure residues is carried down through the soil by water. This is of particular concern when soils are wet. The amount of nitrogen lost to leaching when green manures are tilled into soils is highly variable. When conditions are cold or dry, or when another crop is quickly planted following green manure termination, losses may be negligible. In years with lots of moisture, leaching can be a concern.

In contrast, leaving residues on the soil surface can slow nitrogen mineralization and reduce nitrogen losses through leaching. Remember, however, that there is also nitrogen in the roots of the legume. This root-bound nitrogen, generally estimated at 15-50% of the total nitrogen content of the plant, will be equally available no matter the termination method. While leaching losses may be reduced when leaving residues on the soil surface, losses of nitrogen to the atmosphere can occur through the process of ammonia volatilization. Most nitrogen losses through volatilization occur in an initial, large flush soon after termination (generally within one to two weeks), followed by a slower trickling loss. Losses can range from 5-16% of the nitrogen content of the terminated green manure. On the other hand, losses may be only nominal if conditions remain dry after termination, so that the green manure surface residue dries without much decomposition. Incorporating green manures virtually eliminates nitrogen loss to the atmosphere.

When green manure residues are left on the soil surface after termination, there are two options for their future. They can later be tilled into the soil, or can be left as a mulch on the soil with the next crop planted through the mulch.

If the surface residues are eventually tilled into the soil, soil microorganisms will quickly begin to work to decompose the tissues and mineralize nitrogen. For instance, a green manure could be terminated by rolling in the summer, left as a surface residue through the winter, and then tilled into the soil the following spring before planting a cash crop. This can reduce energy use, and provide erosion protection, while still providing weed control and sufficient nitrogen for the cash crop at a time when nitrogen is needed. In studies comparing mowed to tilled green manures, mowed green manures that were later tilled provided just as much nitrogen to the following crop as did green manures terminated by tilling.

If green manures terminated with a mower or roller-crimper are left on the soil surface for no-till planting the following crop, nitrogen supply to the next crop can be expected to be reduced. Studies with the roller-crimper suggest that approximately 44% less nitrogen may be available to the first crop after the green manure when rolled residues are left on the soil surface. This nitrogen will, however, become available once the green manure mulch is later tilled into the soil. Adequate nitrogen can be supplied to the cash crop, even in spite of this reduction in availability, when green manures with abundant biomass and nitrogen content are used.

Termination by grazing tends to speed the release of plant-available nitrogen, which is mineralized from plant tissues more rapidly in the rumen than in the soil. In general, livestock excrete about 80% of the nutrients that they ingest. Prairie studies suggest that while the nitrogen uptake of following plants does not differ between green manures terminated by incorporation or grazing, nitrogen is available at higher levels in the fall following grazing. Grazed green manures are vulnerable to volatilization losses if excrement is not quickly incorporated into the soil, and are also vulnerable to leaching.

6. Managing Challenges

Like the production of any crop, there are bound to be challenges that you will encounter when growing a green manure. You will ultimately need to make a decision based on what is best for your operation, weighing the pros and cons of the possible courses of action. Overall, green manure stands can bring diversity, flexibility and resilience to your rotation that can help you respond to challenging conditions.

Below, we discuss various challenging scenarios that may arise when green manuring, and suggest some of the possible management decisions that could be made. Your decision will ultimately be based on what works best for your unique circumstance.

6.1. High Weed Pressure

When a green manure crop is heavily infested with weeds, it can be difficult to know what to do. There is a tradeoff to consider:  nitrogen supply to fuel your rotation versus building weed populations that may negatively impact later stages of the rotation.

Begin by giving your green manure a healthy start that will allow it to compete at its best against weeds. Ensure that the green manure is properly inoculated, and that all other nutrients are readily available. Use high quality seed with a good germination rate, and plant into a well prepared seed bed at the recommended seeding rate.

When you anticipate weedy conditions, increase the seeding rate of the green manure by one and a half times, or even double. This will increase the density and competitiveness of the stand, and can also ensure a dense stand even if there are some losses from after-planting weed control. Plant the green manure densely, in close rows or even at rows that run at 45 or 90 degree angles. Choose your green manure carefully, leaning towards the high biomass, competitive green manures such as hairy vetch, field pea, sweetclover or alfalfa.

When possible, perform weed control operations before planting. A pre-emergent harrowing or rod-weeding after planting can also be considered. In green manures that will tolerate it, such as peas, a post-emergence harrowing can also be a valuable weed control measure. See Green Manures and Weeds for more information.

Weeds can also be controlled within the green manure. Many green manure legumes can withstand post-emergence harrowing (see Green Manures and Weeds for more information). Weeds can also be clipped above the green manure canopy if the green manure is short and the weeds are tall. This can prevent weed seed set and limit weed issues in following crops.

If weed levels are still uncomfortably high, consider terminating early. A lower nitrogen contribution from a green manure that has not yet flowered is likely preferable to allowing the weeds to set seeds and create problems for future crops. So long as the green manure legume makes up 50% of the field, you can still expect it to contribute the same nitrogen as a full green manure stand. If the green manure has not yet flowered before weeds flower or begin to set seed, nitrogen contributions from the green manure will be reduced as the legume has yet to reach peak biomass. But, the weeds will also have taken up nitrogen and other nutrients that can be returned to the field when they are incorporated.

When legumes make up only a small portion of the plant stand in your field (i.e. less than 25-50%), it may be wise to terminate very early, as nitrogen contributions will be limited. This can afford you time for additional weed control operations, to re-plant a green manure, or even to plant a vigourous cover crop that can help to remedy the weed problem.

For more information, see Green Manures and Weeds.

6.2. Drought

When starting off the spring in a drought condition, it can be difficult to establish a healthy green manure stand. Favour green manures with low water use, such as Indianhead lentil and chickling vetch, when you anticipate that conditions will be dry. However, even green manures that use water thriftily will need some moisture for germination.

Summerfallowing may be tempting for water conservation in dry years. But, summerfallows leave soils vulnerable to erosion and moisture losses from the soil surface through evaporation. When planted and terminated early, annual green manures with low water use, such as Indianhead lentil, will not deplete soil moisture more than fallow.  Using green manures with high water use efficiency limits water withdraw, protects the soil from surface evaporation, and, if residues are left on the surface, can help to capture snow to replenish moisture.

It is still important to include green manures in the crop rotation, even in areas that experience periodic drought. Green manures can boost soil organic matter, which in turn can improve the water holding capacity of the soil, building a more resilient system. Green manures also provide the nitrogen needed to fuel later phases of the crop rotation.

Green manures can be a good option in dry years. They can be planted very early in the spring, when soil moisture may still be available following snow melt. Alternatively, with the short time that is required for green manures to accumulate peak nitrogen, planting can be delayed in the hopes of later season rains.  Some green manures are also quite drought resistant, such as sweetclover and chickling vetch. In dry years in regions that typically have higher moisture, follow recommendations for the drier Brown and Dry Brown soil zones. For soil zone recommendations, please see Choosing a Green Manure and Green Manure Profiles.

Just as green manures tend to produce less biomass in the dry Brown soil zone than in the wetter Black soil zone, biomass production and hence nitrogen contributions can be expected to be lower in dry years. You may have to adjust your expectations, and perhaps your rotation, to this reality.

When drought is encountered after planting, there is little that can be done but hope for rains. When the drought lifts, green manures require a short time to reach peak nitrogen, and so may fit well in a brief window of water availability if enough growing season allows. Leaving green manure residues on the soil surface over the winter can help to capture winter snows that will return moisture to the field.

6.3. Flooding

Green manures can help to improve the structure and organic matter content of soils. This can improve the porosity of soils, allowing soils to better absorb water. Including green manures in the crop rotation can help to improve the resilience of your farm. Green manures can also boost soil organic matter, which in turn can improve the soil’s water holding capacity. The longer-lived and more extensively rooted perennial green manures are especially good at these tasks.

In years that get off to a wet start, choose green manures that don’t mind having their feet wet. Green manures such as alfalfa, sweetclover, lentil, peas and chickling vetch do not do well when there is excessive soil water. On the other hand, red clover can do very well in wet conditions, while hairy vetch and faba bean have some flood tolerance.

When early season floods interfere with normal planting, green manures can be a good option. With the relatively short time required for annual legumes to flower and reach peak nitrogen, green manures can be planted later in the season, once flooding has eased. Planting red clover can help to use up any excessive soil moisture, making conditions more favourable for following cash crops. Red clovers and sweetclovers also have the advantage in that they can be planted in the previous year, allowing plants to grow on fields that may otherwise be inaccessible in wet years. Aerial seeding can also be considered for planting in wet years.

6.4. Climate Uncertainty

Green manures can bring diversity and flexibility to your rotation that can be advantageous when facing a changing and highly variable climate.  Green manures lend flexibility to the rotation, and can be adapted to a wide range of conditions. Green manures can also improve soils, adding organic matter and improving porosity and water holding capacity, which can provide resilience when floods or droughts are faced.

In years with conditions that are not typical for your farm, refer to the recommendations for soil zones where the “normal” conditions most closely mirror the conditions that you are experiencing. Wet years can allow growers in the typically dry Brown soil zone to grow more water-demanding green manures, such as sweetclover or alfalfa. Dry years in typically moist soil zones may require a shift from the high biomass green manures that are heavy water users, such as alfalfa, sweetclover or faba bean, to more water-conserving green manures, such as lentil, chickling vetch or pea. See Choosing a Green Manure and Green Manure Profiles for details.

Green manures provide flexibility in the rotation that can be invaluable in an uncertain climate. Green manures require less time to reach optimal termination times than typical grain cash crops require for harvest. Legumes also tend to be hardy to cold early in life. This means green manures can be planted early when conditions permit, or can just as well be planted later in the summer should early spring conditions be unfavourable. Likewise, green manure termination can be flexible, both in timing and method. Early termination may sacrifice some nitrogen, but allow a period of water recharge in dry years.

Mixed-crop green manures can also be valuable in uncertain climates and conditions. Planting legume species with differing climate or moisture preferences together in one green manure can provide some insurance that at least one component of the mix will perform well. For example, planting water-loving red clover with drought-tolerant sweetclover can allow one of the green manure species to perform well if the year turns droughty or wet. Planting a green manure of faba bean, which does not do well in hot conditions, with heat-tolerant soybean is an example of an annual green manure mixture that also adds resilience.

6.5. Stand Failure

Failure of a cash crop, or inability to plant a cash crop in a timely fashion because of moisture, weather conditions, or other disruptions, can be an opportunity to use a green manure.  Green manures need only reach flowering for peak nitrogen contributions. Many of the annual green manure options typical on the Prairies flower in only two to three months. So, green manures can be successfully grown even when planted late or into a narrow growing window.

Green manure stands may also fail. Replanting is always an option when the climate, budget, and applicable crop insurance rules allow. Planting a winter cereal crop or another cover crop following the failure of a green manure stand is also an option. To limit the likelihood of green manure failure, be sure to manage your green manure with the same care that you afford your cash crop. They are, after all, an equally important part of your crop rotation. That being said, the flexibility that comes with green manures, and the fact that they need not be brought to maturity and that seed yields need not be optimized, makes them good options in years where stands may be expected to fail.

6.6. A Really Great Green Manure Stand

While it may seem odd to include this as a challenge, the lure of harvesting a really good green manure stand for seed or livestock forage can be very tempting. It can be difficult to get up on the tractor and plow under what could otherwise be an abundant harvest. But, try to hold fast to your decision to green manure. Will the price that you might get for that crop really outweigh all of the benefits that turning that green manure under will give to your farm?  Think of the abundant fertility that a really great green manure stand will provide to fuel the next phases of your crop rotation. Think also of the soil improvements, biodiversity, and pest management benefits that the green manure provides. And if need be, hire someone else to terminate the stand so you don’t have to watch the crop go down.

7. Want to Dig Deeper? Supporting Resources

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Bowren, K.E., D.A. Cooke and R.K. Downey. 1969. Yields of dry matter and nitrogen from tops and roots of sweetclover, alfalfa and red clover at five stages of growth. Canadian Journal of Plant Science 49: 61-68.

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Cicek, H., J.R. Thiessen Martens, K.C. Bamford and M.H. Entz. 2015. Late-season catch crops reduce nitrate leaching risk after grazed green manures but release N slower than wheat demand. Agriculture, Ecosystems & Environment 202: 31-41.

Cicek, H., J.R. Thiessen Martens, K.C. Bamford and M.H. Entz. 2014. Effects of grazing two green manure crop types in organic farming systems: N supply and productivity of following grain crops. Agriculture, Ecosystems & Environment 190: 27-36.

Cicek, H., M.H. Entz, J.R. Thiessen Martens and P.R. Bullock. 2014. Productivity and nitrogen benefits of late-season legume cover crops in organic wheat production. Canadian Journal of Plant Science 94: 771-783.

Clark, A. [Editor]. 2012. Managing Cover Crops Profitably – Third Edition. Sustainable Agriculture Research and Education. Available at http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition [Link verified 14 April 2016].

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Engel, R., C. Jones and R. Wallander. 2013. Ammonia volatilization losses were small after mowing field peas in dry conditions. Canadian Journal of Plant Science 93: 239-242.

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