Dr. Duane R. Bergiund

Extension Agronomist

North Dakota State University

Fargo, ND

Crop rotation is a planned order of crops sown in regular sequence 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 canola after wheat, barley after sunflower, and corn after alfalfa. The planned rotation may be two years, three years or longer. Rotations of continuous small grains such as spring wheat, barley, winter wheat, and oats would not be sustainable since it would be a monoculture of continuous cool season cereal crops.

Greatest benefits of rotations are usually obtained by rotating two distinctly unrelated crops such as small grains to legumes/or pulse crops or oilseed crops with small grains. There are four distinct types of crop growth types. These include: (a) cool season grasses (wheat, oats, barley, rye); (b) cool season broadleaf (canola, potato, field pea, lentil, mustard); (c) warm season grasses (corn, sorghum, millet, sudangrass); and (d) warm season broadleaf (sunflower, soybean, dry edible bean, buckwheat). For the most benefit of crop rotations, one must consider rotations between the various crop growth types, to maximize disease prevention, cultural weed control, and water use effectiveness.

Some of the more important beneficial effects of crop rotation which have been demonstrated are:

- reduced insect and disease problems

- changes in the weed spectrum

- beneficial residual herbicide carryover

- improved soil fertility

- improvements in soil tilth and aggregate stability

- increases in available stored soil water

- reduction of soil erosion

- reduction of alleopathic or phytotoxic effects

Crop rotation can lead to greater overall efficiency in soil water utilization. Spring seeded small grains usually only deplete soil water to 3 to 4 feet. In contrast, sunflower, safflower, alfalfa, 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 being deep-rooted crops can be used to dry up saline seeps and other wet areas.

Crop rotations combined with recommended reduced tillage practices can play an important role in the reduction of 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 farming is highly desirable as a conservation practice, but it has become increasingly obvious that some type of crop rotation must be used in no-till systems to reduce the build-up of insect, disease, and certain weed pests. Some of the more common pest problems associated with continuous no-till wheat have been wheat streak mosaic, root rot, tan spot, scab, wheat stem sawfly, increases in winter annual grass species such as goat grass, downy brome, and cultivated rye, and serious infestation of perennial noxious weeds such as quackgrass and Canada thistle. Other crops grown continuously have had similar types of problems.

No-till wheat yields following soybean (legume) were higher than following other previous grown crops in the rotation (Table 1). Studies from other states have shown corn yields better following soybean or alfalfa compared to corn on corn. Having a legume in the rotation provides a break in the biological cycle of various diseases, alters the herbicides used for better weed control and can result in less tendency to develop herbicide resistant weeds if and when herbicide "mode of action" is rotated between various crop species.

Table 1. Effect of previous crop on wheat yields under no-till, Fargo, ND.
	No-Till Wheat Yield
Previous Crop	Wheat Yield (bu/A)	Wheat on Previous Crop (%)
Wheat	33.3	100
Soybean	44.9	135
Sugarbeet	38.8	117
Sunflower	39.1	117
Corn	37.3	112
Flax	37.5	113
Barley	36.0	108
Source: Department of Crop & Weed Sciences at North Dakota State University. Eight (8) years.

Barley yields (Table 2) were increased when barley was seeded into soybean or fababean stubble. Barley yield increases obtained by seeding into fababean and legume stubble plus 54 pounds per acre of nitrogen were equivalent to yields obtained by the addition of 107 pounds per acre of nitrogen on barley stubble. Yields of barley on fababean stubble was equal to or superior to barley yields on soybean stubble. Yields and protein levels of barley were additionally increased on treatments where annual clover, fababean, and soybean were used as a green manure crop.

Table 2. The yield and protein content of barley as affected by different crop rotations. (Brandon, Manitoba).
Barley Grown On:	Yield	% Protein
	(bu/A)
I Summerfallow	62.3	11.5
2A Barley stubble	30.2	9.0
2B Barley stubble + 107 lbs/acre) of nitrogen	59.9	11.6
3A Fababean stubble	40.1	11.5
3B Fababean stubble (54 lbs/acre) of nitrogen	61.9	12.8
4A Soybean stubble	35.1	11.5
4B Soybean stubble (54 lbs/acre) of nitrogen	57.4	12.8
5 Clover green manure	84.0	14.7
6 Fababean green manure	79.6	14.4
7 Soybean green manure	77.2	14.2
Data represents six (6) station years. Barley seeded with 31 lbs./acre of P(2)O(5).

Diseases may remain in the soil from season to season living on plant residue or as resting spore bodies. Crop rotation can be helpful in controlling disease caused by fungi and bacteria. The biological cycle of the plant pathogens is broken and they cannot survive for extended periods in the absence of their host plants (Table 3).

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

Disease	Major Crops Attacked'	Best Control Methods
Common root rot	wheat barley, grasses	Rotation seed trt.
Ergot	rye,wheat,grasses	Rotation,tillage
Bacterial blights	wheat, barley grasses rye	Rotation, tillage
Blackleg	canola	Rotation, certified blackleg free seed
Scab	wheat, barley, corn, rye	Rotation
Tan spot	wheat, durum	Rotation, fungicide
Net blotch	barley	Rotation, fungicide
Septoria (different species)	wheat, barley	Rotation, fungicide
Septoria (different species)	wheat	Rotation,fungicide
Septoria (different species)	barley	Rotation, fungicide
Pasmo	flax	Rotation, variety
Wilt (flax)	flax	Rotation, variety
Rust (flax)	flax	Variety, rotation
Seedling blight	wheat, barley ,corn, oats, rye	Seed treatment
Smut (corn)	corn	Rotation
Bacterial wilt	alfalfa	Varety res.
Crown rot	alfalfa	Variety res.
Verticillium wilt	potato, sunflower, safflower	Rotation, variety
Sclerotina (white mold)	sunflower, dry bean, safflower, soybean, potato, canola	Rotation 4-5 years
Phoma	sunflower	Rotation
Wheat includes durum.Wheat includes durum.

Insects which have feeding habits restricted to certain crops can be controlled or at least reduced by rotations (Table 4). Susceptible crops should not follow each other in rotation. Wheat stem sawfly, for example, attacks wheat and spring rye, while wireworms attack wheat, spring rye, corn, potato, and sunflower, but oat, barley, sweet clover, and alfalfa are less susceptible.

Table 4. Insects controlled partially or entirely by rotation.

Insect	Susceptible Crop'
Wheat stem sawfly	Wheat, rye
Wheat stem maggot	Wheat, barley, rye
Hessian fly	Wheat
Wireworms	Since so many crops are susceptible to attack by wireworms, rotation will likely be of little benefit in control of this insect.
Alfalfa weevil	Alfalfa
Sweetclover weevil	Sweetclover
European corn borer	Corn borer migrates extensively. Corn is major crop damaged, also bean, potato, and millet.
Sunflower insects; moth, banded moth, stem weevil, midge, and beetle	Sunflower, wild and cultivated species.
Corn rootworm	Corn
Sugarbeet, maggot, and root aphid	Sugarbeet
Flea beetle	Canola, mustard

'Wheat includes durum.

Crop rotations can have a major influence on weed numbers and weed specie shifts. Changes in crop rotation sequences can also influence weeds present because of herbicide selection and use of certain residual herbicides. Rotation of herbicide "mode of action" also is important in control of and prevention of herbicide resistant weeds.

In no-till rotations, use of later planted crops allows application of herbicides for preplant burn-down of problem weeds. If early cool season weeds become a severe field problem, select a later planted crop, wait for early weed flushes and then control with herbicides prior to seeding.

Row crops are good for weed control if they are kept free of weeds with herbicides. The practice of delayed seeding is often used to permit weed emergence and tillage or chemical burndown for weed control before the crop is sown. Fall rye included in the rotation is an effective control practice for wild oat and other weeds due to the extreme competitive ability of winter rye. Perennial grasses and legumes grown for several years and pastured or cut for hay or silage are also good weed control crops. Crop rotation can be a critical component in reducing build-up in herbicide resistant weeds. Rotate crops, particularly those with different life cycles (winter annual such as winter wheat, summer annuals such as canola, corn, sunflower, field peas, and spring wheat, and perennials such as alfalfa). At the same time, remember not to use herbicides with the same mode of action on these different crops against the same weed unless other effective control practices are also included in the management system.

The length of growing season, precipitation, temperatures, problem soil areas, profitable market demands and federal farm programs all combine to limit the choice of crops to be included in rotations. Even with a limited choice of crops and cropping practices, crop rotation has value and should be carefully planned and followed for maximum farm profit. Rotations are especially helpful in maximizing water use controlling erosion, weeds, plant diseases and insects; and distributing labor and machinery requirements, but they require more forward planning and intensive management from the no-till producer.