Try to consider the following principles when you’re thinking about a new rotation:
- Follow a legume forage crop, such as clover or alfalfa, with a high-nitrogen-demanding crop, such as corn, to take advantage of the nitrogen supply.
- Grow less of nitrogen-demanding crops, such as oats, barley, and wheat, in the second or third year after a legume sod.
- Grow the same annual crop for only one year, if possible, to decrease the likelihood of insects, diseases, and nematodes becoming a problem. (Note: For many years, the western corn rootworm was effectively controlled by alternating between corn and soybeans. Recently, populations of the rootworm with a longer resting period have developed in isolated regions in the Midwest, and they are able to survive the very simple two-year rotation.)
- Don’t follow a crop with a closely related species, since insect, disease, and nematode problems are frequently shared by members of closely related crops.
- If specific nematodes are known problems, consider planting non-host plants (such as grain crops for root-knot nematode) for a few years to decrease populations before planting a very susceptible crop such as carrots or lettuce. High populations of plant parasitic nematodes will also affect the choice of cover crops (see chapter 10 for a discussion of cover crops).
- Use crop sequences that promote healthier crops. Some crops seem to do well following a particular crop (for example, cabbage family crops following onions, or potatoes following corn). Other crop sequences may have adverse effects, as when potatoes have more scab following peas or oats.
- Use crop sequences that aid in controlling weeds. Small grains compete strongly against weeds and may inhibit germination of weed seeds, row crops permit mid-season cultivation, and sod crops that are mowed regularly or intensively grazed help control annual weeds.
- Use longer periods of perennial crops, such as a forage legume, on sloping land and on highly erosive soils. Using sound conservation practices, such as no-till planting, extensive cover cropping, or strip cropping (a practice that combines the benefits of rotations and erosion control), may lessen the need to follow this guideline.
- Try to grow a deep-rooted crop, such as alfalfa, safflower, or sunflower, as part of the rotation. These crops scavenge the subsoil for nutrients and water, and channels left from decayed roots can promote water infiltration.
- Grow some crops that will leave a significant amount of residue, like sorghum or corn harvested for grain, to help maintain organic matter levels.
- When growing a wide mix of crops—as is done on many direct-marketing vegetable farms—try grouping into blocks according to plant family, timing of crops (all early-season crops together, for example), type of crop (root vs. fruit vs. leaf), or cultural practices (irrigated, plastic mulch used).
- In regions with limited rainfall, the amount of water used by a crop may be a critically important issue— usually one of the most important issues. Without plentiful irrigation, growing high-water-use crops such as hay, as well as sunflower and safflower, may not leave sufficient moisture in the soil for the next crop in the rotation.
- Be flexible enough to adapt to annual climate and crop price variations, as well as development of soil pathogens and plant parasitic nematodes. For example, dry land rotations have been introduced in the Great Plains to replace the wheat-fallow system, resulting in better use of water and less soil erosion. (It is estimated that less than 25% of the rainfall that falls during the fourteen-month fallow period in the Central High Plains is made available to a following crop of winter wheat.) (See box “Flexible Cropping Systems” and table 11.2 for discussion and information on flexible, or dynamic, cropping systems.) As discussed above (see point 5), reconsider your crop sequence and cover crop use if nematodes become a problem.
CROP ROTATIONS AND PLANT DISEASES
Carefully selected rotations, especially when alternating between grains and broadleaf plants, can greatly assist control of plant diseases and nematodes. Sometimes a one-year break is sufficient for disease control, while for other diseases a number of years of growing a non-host crop is needed to sufficiently reduce inoculum levels. Inclusion of pulse crops in a rotation seems to stimulate beneficial organisms and reduce the severity of cereal root diseases. Severity of common root rot of wheat and barley is reduced by a multiyear break of growing broadleaf plants. Rotations can be relatively easy to develop for control of diseases and nematodes that have a fairly narrow host range. However, some diseases or nematodes have a wider host range, and more care is needed in developing or changing rotations if these are present. In addition, some diseases enter the field on contaminated seed, while others, like wheat leaf rust, can travel with the wind for long distances. Other tactics, aside from rotations, are needed to deal with such diseases.
—KRUPINSKY ET AL. (2002).