It’s impossible to recommend specific rotations for a wide variety of situations. Every farm has its own unique combination of soil and climate and of human, animal, and machine resources. The economic conditions and needs are also different on each farm. You may get useful ideas by considering a number of rotations with historical or current importance.
A five to seven-year rotation was common in the mixed livestock-crop farms of the northern Midwest and the Northeast during the first half of the 20th century. An example of this rotation is the following:
Year 1. Corn
Year 2. Oats (mixed legume–grass hay seeded)
Years 3, 4, and 5. Mixed grass–legume hay
Years 6 and 7. Pasture
The most nitrogen-demanding crop, corn, followed the pasture, and grain was harvested only two of every five to seven years. A less nitrogen-demanding crop, oats, was planted in the second year as a “nurse crop” when the grass-legume hay was seeded. The grain was harvested as animal feed, and oat straw was harvested to be used as cattle bedding; both eventually were returned to the soil as animal manure. This rotation maintained soil organic matter in many situations, or at least didn’t cause it to decrease too much. On prairie soils, with their very high original contents of organic matter, levels still probably decreased with this rotation.
FLEXIBLE CROPPING SYSTEMS
As discussed in point 13 under “General Principles,” it may be best for many farmers to adapt more “dynamic” crop sequences rather than strictly adhere to a particular sequence. Many things change from year to year, including prices paid for crops, pest pressures, and climate. And many farmers do deviate from plans and change what they plant in a particular field—for example, in a wetter than normal field a dry spring opens the opportunity for a vegetable farmer to plant an early-season crop, thus potentially enhancing the diversity of crops grown in that field. However, this issue is especially important for dryland farmers in water-limiting regions such as the Great Plains. In dryland agriculture low water availability is usually the greatest limitation to crop growth. In such regions, where much of the water needed for a crop is stored in the soil at planting time, growing of two heavy water users in a row may work out well if rainfall was plentiful the first year. However, if rainfall has been low, following a heavy-water-using crop (such as sunflowers or corn) with one that needs less water (such as dry pea or lentil) means that water stored in the soil may be enough, along with rainfall during the growing season, to result in a reasonable yield.
|Table 11.2: Comparison of Monoculture, Fixed-Sequence Rotations, and Dynamic Cropping Systems|
|Monoculture||Fixed-Sequence Rotations||Dynamic Cropping Systems|
|Numbers and types of crops||Single crop||Multiple crops: number depends on regionally adapted species, economics, farmer knowledge, infrastructure.|
Multiple crops: number depends on regionally adapted species, economics, farmer knowledge, infrastructure.
|Crop diversity||N/A||Diversity depends on length of fixed sequence.||Diversity high due to annual variation in growing conditions and marketing opportunities, as well as change in producer goals.|
|Crop-sequencing flexibility||N/A||None, although fixed-sequence cropping systems that incorporate opportunity crops increase flexibility.||High. All crops, in essence, are opportunity crops.|
|Biological and ecological knowledge||Basic knowledge of agronomy||Some knowledge of crop interactions is necessary.||Extended knowledge of complex, multiyear crop and crop-environment interactions.|
|Management complexity||Generally low, though variable depending on crop type||Complexity variable depending on length of fixed sequence and diversity of crops grown.||Complexity inherently high due to annual variation in growing conditions, markets, and producer goals.|
In the corn belt region of the Midwest, a change in rotations occurred as pesticides and fertilizers became readily available and animals were fed in large feedlots instead of on crop-producing farms. Once the mixed livestock farms became grain-crop farms or crop-hog farms, there was little reason to grow sod crops. In addition, government commodity price support programs unintentionally encouraged farmers to narrow production to just two feed grains. The two-year corn-soybean rotation is better than monoculture, but it has a number of problems, including erosion, groundwater pollution with nitrates and herbicides, depletion of soil organic matter, and in some situations increased insect problems. Research indicates that with high yields of corn grain there may be sufficient residues to maintain organic matter. With soybeans, residues are minimal.
The Thompson mixed crop-livestock (hogs and beef) farm in Iowa practices an alternative five-year corn belt rotation similar to the first rotation we described—corn/ soybeans/corn/oats (mixed/grass hay seeded)/hay. For fields that are convenient for pasturing beef cows, the Thompson eight-year rotation is as follows:
Year 1. Corn
Year 2. Oats (mixed/grass hay seeded)
Years 3 to 8. Pasture
Organic matter is maintained through a combination of practices that include the use of manures and municipal sewage sludge, green manure crops (oats and rye following soybeans and hairy vetch between corn and soybeans), crop residues, and sod crops. These practices have resulted in a porous soil that has significantly lower erosion, higher organic matter content, and more earthworms than neighbors’ fields.
A four-year rotation researched in Virginia used mainly no-till practices as follows:
Year 1. Corn, winter wheat no-till planted into corn stubble
Year 2. Winter wheat grazed by cattle after harvest, foxtail millet no-till planted into wheat stubble and hayed or grazed, alfalfa no-till planted in fall
Year 3. Alfalfa harvested and/or grazed
Year 4. Alfalfa harvested and/or grazed as usual until fall, then heavily stocked with animals to weaken it so that corn can be planted the next year
This rotation follows many of the principles discussed earlier in this chapter. It was designed by researchers, extension specialists, and farmers and is similar to the older rotation described earlier. A few differences exist: This rotation is shorter; alfalfa is used instead of clover or clover-grass mixtures; and there is a special effort to minimize pesticide use under no-till practices. Weed-control problems occurred when going from alfalfa (fourth year) back to corn. This caused the investigators to use fall tillage followed by a cover crop mixture of winter rye and hairy vetch. Some success was achieved suppressing the cover crop in the spring by just rolling over it with a disk harrow and planting corn through the surface residues with a modified no-till planter. The heavy cover crop residues on the surface provided excellent weed control for the corn.
Traditional wheat-cropping patterns for the semiarid regions of the Great Plains and the Northwest commonly include a fallow year to allow storage of water and more mineralization of nitrogen from organic matter for use by the next wheat crop. However, the wheat-fallow system has several problems. Because no crop residues are returned during the fallow year, soil organic matter decreases unless manure or other organic materials are provided from off the field. Water infiltrating below the root zone during the fallow year moves salts through the soil to the low parts of fields. Shallow groundwater can come to the surface in these low spots and create “saline seeps,” where yields will be decreased. Increased soil erosion, caused by either wind or water, commonly occurs during fallow years, and organic matter decreases (at a rate of about 2% per year, in one experiment).
In this wheat monoculture system, the buildup of grassy weed populations, such as jointed goat grass and downy brome, also indicates that crop diversification is essential.
Farmers in these regions who are trying to develop more sustainable cropping systems should consider using a number of species, including deeper-rooted crops, in a more diversified rotation. This would increase the amount of residues returned to the soil, reduce tillage, and lessen or eliminate the fallow period. (See box “Flexible Cropping Systems.”)
A four-year wheat-corn-millet-fallow rotation under evaluation in Colorado was found to be better than the traditional wheat-fallow system. Wheat yields have been higher in this rotation than wheat grown in monoculture. The extra residues from the corn and millet also are helping to increase soil organic matter. Many producers are including sunflower, a deep-rooting crop, in a wheat-corn-sunflower-fallow rotation. Sunflower is also being evaluated in Oregon as part of a wheat cropping sequence.
CROP ROTATION ON ORGANIC FARMS
Crop rotation is always a good idea, but on organic farms a sound crop rotation is essential. Options for rescuing crops from disease are limited on organic farms, making disease prevention through good crop rotation more important. Similarly, weed management requires a multiyear approach. Since nutrients for organic crop production come largely through release from organic matter in soil, manure, compost, and cover crops, a crop rotation that maintains regular organic matter inputs and large amounts of active soil organic matter is critical.
To obtain the benefits of a diverse crop rotation and take advantage of specialty markets, organic farmers usually grow a high diversity of crops. Thus, organic field crop producers commonly grow five to ten crop species, and fresh market vegetable growers may grow thirty or more. However, because of the large variation in acreage among crops and frequent changes in the crop mix due to weather and shifting market demands, planning crop rotations on highly diversified farms is difficult. Therefore, many organic farmers do not follow any regular rotation plan, but instead place crops on individual fields (or parts of fields) based on the cropping history of the location and its physical and biological characteristics (e.g., drainage, recent organic matter inputs, weed pressure). Skilled organic growers usually have next year’s cash crops and any intervening cover crops in mind as they make their placement decisions but find that planning further ahead is usually pointless because longer term plans are so frequently derailed.
Although precise long-term rotation plans can rarely be followed on farms growing a diverse mix of crops, some experienced organic farmers follow a general repeating scheme in which particular crops are placed by the ad hoc approach described above. For example, some vegetable operations plant cash crops every other year and grow a succession of cover crops in alternate years. Many field crop producers alternate some sequence of corn, soybeans, and small grains with several years of hay on a regular basis, and some vegetable growers similarly alternate a few years in vegetables with two to three years in hay. These rest periods in hay or cover crops build soil structure, allow time for soilborne diseases and weed seeds to die off, and provide nitrogen for subsequent heavy-feeding crops. Some vegetable growers alternate groups of plant families in a relatively regular sequence, but this generally requires growing cover crops on part of the field in years when groups that require less acreage appear in the sequence. Within all of these generalized rotation schemes, the particular crop occupying a specific location is chosen by the ad hoc process described above. Organizing the choices with a general rotation scheme greatly simplifies the decision-making process.
Dividing the farm into many small, permanently located management units also greatly facilitates effective ad hoc placement of crops onto fields each year. By this means, a precise cropping history of every part of each field is easy to maintain. Moreover, problem spots and particularly productive locations can be easily located for planting with appropriate crops.
—CHARLES MOHLER, CORNELL UNIVERSITY
Vegetable farmers who grow a large selection of crops find it best to rotate in large blocks, each containing crops from the same families or having similar production schedules or cultural practices. Many farmers are now using cover crops to help “grow their own nitrogen,” utilize extra nitrogen that might be there at the end of the season, and add organic matter to the soil. A four to fiveyear vegetable rotation might be as follows:
Year 1. Sweet corn followed by a hairy vetch/winter rye cover crop
Year 2. Pumpkins, winter squash, summer squash followed by a rye or oats cover crop
Year 3. Tomatoes, potatoes, peppers followed by a vetch/rye cover crop
Year 4. Crucifers, greens, legumes, carrots, onions, and miscellaneous vegetables followed by a rye cover crop
Year 5. (If land is available) oats and red clover or buckwheat followed by a vetch/rye cover crop
Another rotation for vegetable growers uses a two to three-year alfalfa sod as part of a six to eight-year cycle. In this case, the crops following the alfalfa are high nitrogen-demanding crops, such as corn or squash, followed by cabbage or tomatoes, and, in the last two years, crops needing a fine seedbed, such as lettuce, onions, or carrots. Annual weeds in this rotation are controlled by the harvesting of alfalfa a number of times each year. Perennial weed populations can be decreased by cultivation during the row-crop phase of the rotation.
Most vegetable farmers do not have enough land—or the markets—to have a multiyear hay crop on a significant portion of their land. Aggressive use of cover crops will help to maintain organic matter in this situation. Manures, composts, or other sources of organic materials, such as leaves, should also be applied every year or two to help maintain soil organic matter.
Cotton alternating with peanut is a common simple rotation in the Southeast coastal region. The soils in this area tend to be sandy, low in fertility and water-holding capacity, and have a subsoil compact layer. As with the corn-soybean alternation of the Midwest, a more complex system is very desirable from many viewpoints. A rotation including a perennial forage, for at least a few years, may provide many advantages to the cotton-peanut system. Research with two years of Bahia grass in a cotton-peanut system indicates greater cotton root growth, more soil organic matter and earthworms, and better water infiltration and storage.