Proper Rotations Require Careful Crop Management

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A sound rotation is the single most effective means of providing higher and more stable yields, soil benefits such as fertility and improved weed, pest and disease management.

Planning a rotation means managing a great many factors:

It must work agronomically and economically. It must meet soil fertility needs and provide for weed management. It must leave enough time between similar crops so that diseases and insect pests won’t persist from one susceptible crop to another.

In other words, a sound rotation is the key to managing the cropping system.

For an organic grain producer, a sound rotation usually starts with a legume green manure. Legumes partner with rhizobium bacteria to supply nitrogen, which would otherwise be the limiting nutrient.

On average, legumes supply nitrogen at about 2.5 percent of the above ground biomass. For example, an alfalfa field yielding 1.5 round bales per acre at approximately one tonne per bale would produce 3,306 pounds per acre, or 83 lb. per acre of nitrogen.

Only about 60 percent of the nitrogen is available in the first year. An additional 20 percent becomes available in the second year. These amounts can be balanced against subsequent crop needs. Cover crops can be used to prevent the loss of nitrogen in areas where leaching can be a problem.

A pulse crop can be ideal when the nitrogen level is nearly depleted. Pulses grown with rhizobium can access their own nitrogen supply, giving them a distinct advantage over weeds that cannot.

Nitrogen is not the only nutrient that can be limiting, but it is the only one that can be added naturally so easily.

It may be necessary to import the nutrients if other nutrients are limiting, especially phosphorus or sulfur. Mineral supplements are available, but they can be challenging to source and costly to transport. A more local solution might be to bring in alfalfa hay bales. These can be relatively rich sources of phosphorus and sulphur.

Bale grazing can add cash value to the bales to offset the cost. If there are no cattle on the grain farm, perhaps a neighbour’s cattle can bale graze in the field. Few nutrients are lost to the cattle, and they leave the resource in a very plant available form. Of course, an alternative would be to simply import manure.

A variable rotation can be the key to successful weed management. When the same crop management is applied year after year, those weeds that do well with this management increase, perhaps to overwhelming numbers. When the crop management is varied, it is hard for the weeds to adjust, and none receives a significant advantage or an opportunity to expand out of control.

Changing crop timing can reduce weed populations. Ideally, a rotation will alternate annual crops with winter crops with perennials. Perennial rotations can be especially useful in reducing annuals and eliminating specific perennial weeds.

For example, alfalfa greatly reduces Canada thistle in a rotation. Winter crops can be useful in reducing quackgrass, which often has uncontested reign over all in annual cropping systems.

Alternative management of crops can also reduce weeds. For instance, using a pea-oat intercrop as green feed or green manure can prevent annual weeds from going to seed.

Even with annual weeds, alternating between early seeding and late seeding can suppress outbreaks. With plant competition, it is often first come, first served. Early seeding establishes the crop before late emerging weeds can stake a claim. Late seeding gives the operator a chance to eliminate any early emerging weeds before the crop is seeding. Alternating the methods prevents either type from getting a consistent advantage.

Prevention can be the best form of disease control.

When susceptible crops are grown back-to-back, the disease inoculum can build up with devastating results. Tillage can reduce disease spread by incorporating residues in the soil, which helps them to decompose more rapidly.

Cereals share a number of diseases. Sclerotinia can be shared by legumes and oilseeds. Alternating cereals with broadleaf crops can disrupt disease cycles.

Rotations can also be used to reduce the insect pests that are crop specific, such as flax bollworm and wheat midge.

The ideal rotation is site specific and may change over time, but the basic principle is diversity.

Many sequences are possible within that theme, and producers are in the best position to determine what works for their farm. There are a number of markers of success, such as healthy crops, healthy economics and good whole farm functionality.

A rotation doesn’t need to be written in stone. It needs to be revised when it doesn’t work and be flexible to changes in weather and markets.

However, for best results, it needs to hold to the core principle of diversity for functionality.

Brenda Frick, Ph.D., P.Ag. is an extension agrologist and researcher in organic agriculture. She welcomes your comments at 306-260-0663 or email organic@usask.ca.