Historically farmers considered crop rotation a valuable part of their farm management. Crop rotations can be complicated or straightforward. In any given situation there are many factors that each farmer will need to weigh and consider as they find the right crop rotation for them. There are a few guidelines that can help producers with their planning.
Climate is an important consideration in crop rotation. Producers need to match their crop rotation with the water cycle characteristics. If a crop rotation is not sufficiently intense there can be too much water left in the system. This can lead to water logging, saline seep formation, nutrient loss and soil compaction. On the other hand, a crop rotation that is too intense can lead to excessive water stress and poor yields.
In an effort to simplify things crops are often classified according to "crop types." There are four annual crop types; cool-season grasses (C3 grasses), cool-season broadleaves, warm-season grasses (C4 grasses) and warm-season broadleaves. Broadleaf crops can be subdivided into legume and oilseed categories within each type.
It is important to include as many different crop types (and crops within type) in a rotation as practical. Increasing diversity can decrease problems with weeds, diseases and insects. It can also increase the performance of the positive biology in the system such as soil microorganisms and beneficial insects.
To be most successful at this there needs to be forethought in how sequences and intervals in a rotation are constructed. They need to be designed such that they take advantage of beneficial natural cycles or at least do not hinder their operation or inadvertently enhance pest species or lead to the development of biotype resistance in pest species.
Biotype resistance occurs when rotations are too predictable in either crop sequence or crop interval. An example of this occurs with corn rootworm. If corn always follows soybeans or wheat, or if it occurs every two years, corn rootworms with variant behavior are selected and become a pest. Two types of CRW variants have developed in the Corn Belt because of the predictability of the corn-soybean rotation. This can be prevented by using sorghum as a substitute for corn in parts of the rotation or by diversifying the rotation (C-Sb-C-Sb-Wheat) or both.
The result will be a crop rotation that is not consistent in either interval or sequence. Herbicide resistance in weeds develop because the same herbicide is used at the same time on the same weeds year after year. This herbicide use pattern occurs when rotations are too predictable.
When planning rotations it is important to be aware of instances where sequences that appear to be acceptable have substantial risk. For example, it is not recommended to follow corn or sorghum with wheat or barley because this can lead to higher fusarium head blight in the wheat and barley. It can also lead to an increased incidence of root and crown disease. Both of these diseases, if accompanied with the right environmental conditions, can seriously reduce yields. Head blight can make grain unsuitable for human consumption.
It is important for producers to maintain residue on their soil to prevent erosion, increase infiltration, maintain soil structure and conserve soil water. This will mean they will need to include the proper proportion of crops such as corn, wheat and sorghum that have high carbon content in their stalks. The carbon in the residue of these crops is a good source of food for soil microbes and helps to increase soil microbial activity. It may be necessary to have up to 80% of the crops with high residue characteristics in order to maintain long-term productivity. Rotations that include perennial sequences and/or use high residue cover-crops may need less.
A good crop rotation can reduce producer risk. Each crop will have a different critical growth period. Environmental stresses that severely affect one crop may be negligible to another due to different life cycles. In addition, equipment and labor costs can be spread over more acres when crops are grown that are seeded, sprayed and harvested at different times.
There are numerous benefits to a good crop rotation that uses diverse crop types. Increased crop yield is only one of those benefits. More information on crop rotation can be found on the Dakota Lakes Research Farm website.