Crop Rotations for Increased Productivity

EB-48 (Revised), January 1998

Dr. Michael D. Peel, NDSU Small Grains Extension Agronomist

The combined efforts of many individuals were required to complete this publication, the contributions of each is duly acknowledged. The suggested rotations for each region of the state were provided by area and state specialist. Their names and region of the state represented are listed below.

Eastern North Dakota (Red River Valley), Michael Peel, Duane Berglund
Sugarbeet Rotations, Allan Cattanach, Alan Dexter
Northeast Region, Terry Gregoire
South-Central Region, Greg Endres
Southwest Region, Roger Ashley
Northwest, Kent McKay

Introduction
General Effects of Rotations
Results of Crop Rotation Experiments
Economic Considerations with Crop Rotations
Selecting a Rotation
Suggested Rotations
Bibliography

Crop rotation is a planned order of specific crops planted on the same field. Crop rotation also means that succeeding crops are of a different genus, species, subspecies, or variety than the previous crop. Examples would be barley after wheat, row crops after small grains, grain crops after legumes, etc. The planned rotation sequence may be for a two- or three-year or longer period. Some of the general purposes of rotations are to improve or maintain soil fertility, reduce erosion, reduce the build-up of pests, spread the workload, reduce risk of weather damage, reduce reliance on agricultural chemicals, and increase net profits.

Crop rotations have fallen somewhat into disfavor because they require additional planning and management skills, increasing the complexity of farming. A shift away from livestock programs in most parts of North Dakota has also reduced the need for pasture and hay crops and eliminated some rotational crops such as alfalfa from many farms. Solid-seeded crops such as small grains and flax have predominated in the past, but row crops such as sunflower, pinto bean, corn and soybean provide additional planting options and more reasons for crop rotations.

General Effects of Rotations

One immediate economic benefit of crop rotations is improved yields. For example, sunflower yields over eight years at Crookston, Minnesota (24) were often significantly greater in rotation with other crops than when continuous sunflower was grown (Table 1). Wheat yields were also greater with rotation than continuous wheat in an eight-year study (22) conducted with different crops at Fargo (Table 2). A study at the Agriculture Research Service at Mandan has shown that increased hard red spring wheat yields can be expected when an alternative crop is included in the rotation (27).

Table 1. Yields of sunflower following sunflower and in rotation with other crops at Crookston, MN.

Source: Miscellaneous Report 166 - 1979, AES, University of Minnesota.

Table 2. Effect of previous crop on wheat yields, Fargo, ND (22).

Rotating to a different crop such as wheat on barley ground usually results in higher grain yields when compared to continuous cropping of wheat. Even greater benefits are usually obtained by rotating two distinctly unrelated crops, such as a small grain seeded into land where the previous crop was a legume or other herbaceous dicot such as flax or sunflower (Table 2). Many of the reasons for the beneficial effects of rotations are not completely understood.

Some of the more important beneficial effects that can be obtained from a well planned crop rotation are:

• Reduced insect and disease problems
• Beneficial residual herbicide carryover
• Improved soil fertility
• Improvements in soil tilth and aggregate stability
• Soil water management
• Reduction of soil erosion
• Reduction of allelopathic or phytotoxic effects

Pest control is often an important reason for crop rotation. Rotations can be used to prevent or partially control several pests and reduce the reliance on chemical and mechanical control. A combination of crop rotation and pesticides is often more effective in reducing pest populations to economic levels than pesticides alone. Pesticides that provide economical control are not available for pests such as white mold in potato, dry bean, and sunflower, and crop rotations are the only feasible control method for reducing the impact of sclerotinia.

Disease control

Crop rotation has tremendous potential for reducing and often preventing the transmission of disease. Disease pressures change with changing environmental conditions. Table 3 lists examples of disease that can be controlled with rotation. Crop rotation, in combination with cultural practices plus necessary fungicides, is the most desirable method of disease control.

Table 3. Common disease controlled entirely or in part by rotation.

¹Wheat includes durum.

Insect control

Insects which can be controlled entirely or in part by rotations are listed in Table 4. In addition, insect populations may become greater within a region where only one or two crops are continuously grown in contrast to a region where several crops are grown in rotation. Insects such as corn borer, sunflower seed and stem weevil, and many others readily migrate to nearby or distant fields. Therefore, only partial control can be obtained by rotation. Increasing field isolation from fields seeded to the same crop the previous year will often increase the effectiveness of crop rotation as an insect control method.

Table 4. Insects controlled partially or entirely by rotation.

Weed control

Rotations can be used to cause shifts in weed populations. Populations of certain weed species can be suppressed by competition from the crop raised or by the selective use of herbicides. Wild mustard populations can be reduced by selective treatment of small grain grown in rotation with row crops. Grass weed populations, often a problem in small grains, can be reduced by the use of the appropriate herbicide in the previous row crop.

Herbicides can have both beneficial and harmful residual effects on the next crop. Therefore, planning the correct sequence of herbicide use together with crop selection has become a necessary part of rotation management. See Circular W-253, North Dakota Weed Control Guide, for specific herbicides and their potential for carryover injury on a subsequent crop.

Soil nitrogen

Legumes in the rotation can be used to increase the available soil nitrogen. Symbiotic nitrogen-fixing bacteria called rhizobia form nodules on the roots of legume plants and convert or fix atmospheric nitrogen to organic nitrogen. The amount of nitrogen fixed varies with species, available soil nitrogen, and many other factors. Fixed nitrogen not removed from the land by harvest becomes available to succeeding crops as the legume tissues undergo microbial decomposition. When the legume crop is seeded, rhizobia inoculum should always be applied to the seed to ensure the most productive commercial strains are available to form nodules and that inoculating bacteria are always present. Even though indigenous bacteria may be present in the soil, research shows improved commercial strains of rhizobia have more capacity to fix nitrogen (28).

Legumes have the capacity to fix large amounts of nitrogen. Research in Minnesota (16) indicated that alfalfa fixed an average of 172 pounds of nitrogen annually during the first two years of production. However, only a portion of the nitrogen fixed is available to the next crop because much is removed in the harvested alfalfa. Nitrogen credits of various legume crops are listed in Table 5.

Table 5. Nitrogen credited to a subsequent crop of various legume crops, NDSU soil testing lab.

Soil tilth and structure

Many farmers who rotate crops comment on the improvement in tilth or friability of soil following soybean or other row crops. In a Colorado study (25) involving corn, sugarbeet, and barley planted on succeeding years, soil aggregate stability was increased from 67 to 76 percent when three years of alfalfa were added to the rotation. Increased aggregate stability reduces the tendency of the soil to puddle or crust, increases rate of water infiltration under certain conditions, and may also reduce wind erosion.

Soil moisture

Crop rotation can lead to greater overall efficiency in soil water utilization. Spring seeded small grains usually deplete soil water 3 to 4 feet deep. In contrast, sunflower, safflower, corn and sugarbeet are deep-rooted crops which can deplete soil water to depths of 5 to 6 feet. Therefore, deep-rooted crops such as sunflower following small grains can take advantage of the extra reserve of deep moisture and also any nitrogen which was positionally unavailable to a shallow-rooted crop.

Alfalfa and sweetclover, also deep-rooted crops, can be used to dry up saline seeps and other wet areas. Depletion of soil water in saline areas prevents the accumulation of salts on the surface, permits movement of the salts downward by leaching, and allows recropping to a cash crop such as wheat. Alfalfa or sweetclover should be seeded on the upslope recharge area to use and remove soil water as deeply as possible from the soil profile, reducing lateral flow of water and salts into the saline seep discharge area (7).

Reduction of soil erosion

Crop rotations combined with recommended tillage practices can play an important role in reducing wind and water erosion. Solid seeded crops such as small grains provide more protection against water erosion than row crops. Permanent crops such as hay or pasture provide even more protection against erosion. Management of crops to provide sufficient residue throughout the year is essential for satisfactory control of both wind and water erosion. No-till or minimum till farming is highly desirable as a conservation practice, but crop rotation must be used to reduce the buildup of insect, disease and weed pests. Common pest problems associated with continuous no-till wheat have been Fusarium head blight (scab), wheat streak mosaic, root rots, tan spot, wheat stem sawfly, increases in winter annual grass species, and serious infestation of perennial noxious weeds such as quackgrass and Canada thistle. Other crops grown continuously have had similar types of problems.

The effect of various crop rotations (29) on soil erosion is vividly illustrated in Table 6. The addition of barley and hay or pasture to the rotation was estimated to reduced the degree of soil erosion by more than 50 percent when compared to continuous corn. The expected reduction in soil erosion would be even greater on steeper slopes.

Table 6. Expected soil losses in tons per acre per year from croping systems on moderately eroded slopes, Wisconsin Agri. Expt. Sta., Bulletin 452, 1941.

*Underseeded with sweetclover.

Allelopathy-phytotoxicity

The reasons for improved yields due to crop rotations are not completely understood. Research has attempted to reveal some of the unknown factors. Terms such as phytotoxicity, allelopathy and autotoxicity are coming into common usage. Phytotoxicity, a general term, is defined as chemical that is toxic to plant growth whether it is derived from plant products or synthetic (herbicide or other pesticide residues). Allelopathy refers to plant material or chemicals derived from these materials which inhibit the germination, growth or development of another species. Autotoxicity refers to plant material which inhibits the germination, growth and development of the same species.

Legumes such as alfalfa, while often beneficial to non-related species, exhibit autotoxic effects to alfalfa seedlings. Research in Illinois indicated older stands caused greater inhibition of new seedlings (Table 7). This study (21) indicated that one year out of alfalfa was sufficient to nullify the detrimental effects of the alfalfa.

Table 7. Alfalfa yields reestablished annually in Illinois.

[NEXT]

[Results of Crop Rotation Experiments]
[Economic Considerations with Crop Rotations]
[Selecting a Rotation] [Suggested Rotations] [Bibliography]

EB-48 (Revised), January 1998