Crop Rotation with Cover Crops
CROP ROTATION WITH COVER CROPS
Readers’ note: > indicates progression to another crop; / indicates a mixture of crops growing at the same time.
One of the biggest challenges of cover cropping is to fit cover crops into your current rotations, or to develop new rotations that take full advantage of their benefits. This section will explore some of the systems used successfully by farmers in different regions of the U.S. One might be easily adapted to fit your existing crops, equipment and management. Other examples may point out ways that you can modify your rotation to make the addition of cover crops more profitable and practical.
Whether you add cover crops to your existing rotations or totally revamp your farming system, you should devote as much planning and attention to your cover crops as you do to your cash crops. Failure to do so can lead to failure of the cover crop and cause problems in other parts of your system. Also remember that there is likely no single cover crop that is right for your farm. Before you start:
Review Benefits of Cover Crops and Selecting the Best Cover Crops for Your Farm.
Decide which benefits are most important to you.
Read the examples below, then consider how these cover crop rotations might be adapted to your particular conditions.
Talk to your neighbors and the other “experts” in your area, including the contact people listed in Regional Experts.
Start small on an easily accessible plot that you will see often.
Be an opportunist—and an optimist. If your cropping plans for a field are disrupted by weather or other conditions outside of your control, this may be the ideal window for establishing a cover crop.
Consider using an early-maturing cash crop to allow for timely planting of the cover crop.
Cover crops can be used for feed. Consider harvesting or grazing for forage or alternative livestock such as sheep and goats.
The ideas in this book will help you see cover crop opportunities, no matter what your system. For more in-depth scientific analysis of cover crops in diverse cropping systems, see several comprehensive reviews listed in Appendix F (77, 106, 362, 390).
COVER CROPS FOR CORN BELT GRAIN AND OILSEED PRODUCTION
In addition to providing winter cover and building soil structure, nitrogen (N) management will probably be a major factor in your cover crop decisions for the corn>soybean rotation. A fall- planted grass or small grain will scavenge leftover N from the previous corn or soybean crop. Legumes are much less efficient at scavenging N, but will add N to the system for the following crop. Legume/grass mixtures are quite good at both.
Keep in mind that corn is a heavy N feeder, soybeans benefit little, if at all, from cover crop N and that you have a shorter time for spring cover crop growth before corn than before soybeans.
Precaution. If you use herbicides, be sure to check labels for plant-back or rotation intervals to ensure that your cover crop isn’t adversely affected.
Cover crop features: rye provides winter cover, scavenges N after corn, becomes a long-lasting (6 to 12 week) residue to hold moisture and suppress weeds for your soybeans; hairy vetch provides spring ground cover, abundant N and a moderate-term (4 to 8 week) mulch for a no-till corn crop; field peas are similar to vetch, but residue breaks down faster; red clover is also similar, but produces slightly less N and has less vigorous spring regrowth; berseem clover grows quickly to provide several cuttings for high-N green manure.
Here are some options to consider adapting to your system:
In Zone 7 and warmer, you can grow a cover crop every year between your corn and full-season beans. Also, you can use wheat or another small grain to replace the cover crop before beans, in a three-crop, two-year rotation (corn>wheat>doublecrop beans). In all cases, another legume or a grass/legume mixture can be used instead of a single species cover crop. Where it is adapted, you can use crimson clover or a crimson/grass mixture instead of vetch.
In cooler areas, plant rye as soon as possible after corn harvest. If you need more time in the fall, try overseeding in rowed beans at drydown “yellow leaf” stage in early fall, or in early summer at the last cultivation of corn. Seeding options include aerial application where the service is economical, using a specialty high-body tractor with narrow tires, or attaching a broadcast seeder, air seeder or seed boxes to a cultivator.
Precaution. Broadcast seeding of cover crops into standing crops is less dependable than other seeding methods. Success will depend on many factors, including adequate rainfall amount and distribution after seeding.
Kill the rye once it is about knee-high, or let it go a bit longer, killing it a couple of weeks before planting beans. Killing the rye with herbicides and no-tilling beans in narrow rows allows more time for cover crop growth, since you don’t have to work the ground. If soil moisture is low, consider killing the rye earlier. Follow the beans with hairy vetch or a vetch/small grain mixture.
Legumes must be seeded at least 6 weeks before hard frost to ensure winter survival. Seed by drilling after soybean harvest, or by overseeding before leaf drop. Allow the vetch (or mixture) to grow as long as possible in spring for maximum N fixation.
Harvesting sweet corn, seed corn or silage corn opens a window for timely cover crop planting. Harvest or graze the small grain or legume / small grain mixture in spring if needed for feed. See individual cover crop chapters for management details.
In Pennsylvania, Ed Quigley seeds rye or spring oats after corn silage harvest. The oats can be cut for silage in fall if planted by early September. Rye can be made into rylage or sprayed before no-till corn the following spring.
Worried about planting your corn a bit late because you’re waiting for your cover crop to mature? Research in Maryland, Illinois and elsewhere suggests that no-tilling corn towards the end of the usual window when using a legume cover crop has its rewards. The delay can result in greater yields than earlier planting, due to greater moisture conservation and more N produced by the cover crop, or due to the timing of summer drought (82, 84, 300). In Pennsylvania, however, delayed planting sometimes reduced corn yields following rye (118).
Check your state variety trial data for a shorter season corn hybrid that yields nearly as well as slightly longer season corn. The cover crop benefit should overcome many yield differences.
Worried about soil moisture? There’s no question that growing cover crops may consume soil moisture needed by the next crop. In humid regions, this is a problem only in an unusually dry spring. Time permitting, allow 2 to 3 weeks after killing the cover crop to alleviate this problem. While spring rainfall may compensate for the moisture demand of most cover crops by normal planting dates, cover crops can quickly dry out a field. Later in the season, killed cover crop residues in conservation tillage systems can conserve moisture and increase yields.
In dryland areas of the Southern Great Plains, lack of water limits cover crop use. (See Dryland Cereal Cropping Systems).
In any system where you are using accumulated soil moisture to grow your cash crop, you need to be extra careful. However, as noted in this section and elsewhere in the book, farmers and researchers are finding that water-thrifty cover crops may be able to replace even a fallow year without adversely affecting the cash crop.
Corn>Rye>Soybeans>Small Grain>Hairy Vetch. This rotation is similar to the corn>rye>soybeans rotation described above, except you add a year of small grains following the beans. In crop rotation research from different areas, many benefits accrue as the rotation becomes longer. This is because weed, disease and insect pest problems generally decrease with an increase in years between repeat plantings of the same crop.
Residue from small grains provides good organic matter for soil building, and in the case of winter grains, the plants help to prevent erosion over winter after soybeans loosen up the soil.
The length of the growing season will determine how you fit in cover crops after full-season soybeans in the rotation. Consider using a short- season bean if needed in order to achieve timely planting after soybean harvest. Calculate whether cover crop benefits will compensate for a possible yield loss on the shorter season beans. If there is not enough time to seed a legume after harvest, use a small grain rather than no cover crop at all.
The small grain scavenges leftover N following beans. Legume cover crops reduce fertilizer N needed by corn, a heavy N feeder. If you cannot seed the legume at least six weeks before a hard frost, consider overseeding before leaf drop or at last cultivation.
Precaution. Because hairy vetch is hard seeded, it will volunteer in subsequent small grain crops.
An alternate rotation for the lower mid-South is corn>crimson clover (allowed to go to seed) > soybeans > crimson clover (reseeded) > corn. Allow the crimson clover to go to seed before planting beans. The clover germinates in late summer under the beans. Kill the cover crop before corn the next spring. If possible, choose a different cover crop following the corn this time to avoid potential pest and disease problems with the crimson clover.
Precaution. In selecting a cover crop to interseed, do not jeopardize your cash crop if soil moisture is usually limiting during the rest of the corn season! Banding cover crop seed in row middles by using insecticide boxes or other devices can reduce cover crop competition with the cash crop.
3 Year: Corn>Soybean>Wheat/Red Clover. This well-tested Wisconsin sequence provides N for corn as well as weed suppression and natural control of disease and insect pests. It was more profitable in recent years as the cost of synthetic N increased. Corn benefits from legume-fixed N, and from the improved cation exchange capacity in the soil that comes with increasing organic matter levels.
Growers in the upper Midwest can add a small grain to their corn>bean rotation. The small grain, seeded after soybeans, can be used as a cover crop, or it can be grown to maturity for grain. When growing wheat or oats for grain, frost-seed red clover or sweet-clover in March, harvest the grain, then let the clover grow until it goes dormant in late fall. Follow with corn the next spring. Some secondary tillage can be done in the fall, if conditions allow. One option is to attach sweeps to your chisel plow and run them about 2 inches deep, cutting the clover crowns.
Alternatively, grow the small grain to maturity, harvest, then immediately plant a legume cover crop such as hairy vetch or berseem clover in July or August. Soil moisture is critical for quick germination and good growth before frost. For much of the northern U.S., there is not time to plant a legume after soybean harvest, unless it can be seeded aerially or at the last cultivation. If growing spring grains, seed red clover or sweet-clover directly with the small grain.
An Iowa study compared no-till and conventional tillage corn>soybean>wheat/clover rotation with annual applications of composted swine manure. Berseem clover or red clover was frostseeded into wheat in March. Corn and soybean yields were lower in no-till plots the first year, while wheat yield was not affected by tillage. With yearly application of composted swine manure, however, yield of both corn and soybean were the same for both systems beginning in year two of the 4-year study (385).
Adding a small grain to the corn>soybean rotation helps control white mold on soybeans, since two years out of beans are needed to reduce pathogen populations. Using a grain/legume mix will scavenge available N from the bean crop, hold soil over winter and begin fixing N for the corn. Clovers or vetch can be harvested for seed, and red or yellow clover can be left for the second year as a green manure crop.
Using a spring seeding of oats and berseem clover has proved effective on Iowa farms that also have livestock. The mix tends to favor oat grain production in dry years and berseem production in wetter years. Either way the mixture provides biomass to increase organic matter and build soil. You can clip the berseem several times before flowering for green manure.
Precaution. Planting hairy vetch with small grains may make it difficult to harvest a clean grain crop. Instead, seed vetch after small grain harvest. Be sure to watch for volunteer vetch in subsequent small grain crops. It is easily controlled with herbicides but will result in significant penalties if found in wheat grain at the elevator.
Full-Year Covers Tackle Tough Weeds
The couple experimented with many different cover crops on their north-central Pennsylvania farm while adapting a system to battle quackgrass. Originally modeled on practices developed on a commercial herb farm in the Pacific Northwest, the Nordells continue to make modifications to fit their ever-changing conditions.
In the fallow year between cash crops, the Nordells grow winter cover crops to smother weeds and improve soil. Combined with summer tillage, the cover crops keep annual weeds from setting seed. Cognizant of the benefits of reduced tillage, they continue to modify their tillage practices—reducing tillage intensity whenever possible.
Regular use of cover crops in the year before vegetables also improves soil quality and moisture retention while reducing erosion. “Vegetable crops return very little to the soil as far as a root system,” says Eric, a frequent speaker on the conference circuit. “You cut a head of lettuce and have nothing left behind. Growing vegetables, we’re always trying to rebuild the soil.”
Continual modification to their system is the name of the game. When they set up their original 4-year rotation in the early 80’s, tarnished plant bugs were not an issue on their farm, but in the 90’s they became a major problem in lettuce. The problem—and the solution!—was in their management of yellow sweetclover.
In their original rotation, sweetclover was overseeded into early cash crops such as lettuce. After overwintering, the sweetclover was mowed several times the following year before plowing it under and planting late vegetable crops. When the tarnished plant bugs began moving in—possibly attracted by the flowering sweetclover—the Nordells realized that mowing the sweetclover caused the plant bugs to move to the adjacent lettuce fields. It was time to change their system.
Fully committed to the use of cover crops, they first tried to delay mowing of the sweetclover until after lettuce harvest. Eventually, they decided to revamp their clover management completely. They now plant sweetclover in June of the second or fallow year of the rotation. This still gives the sweetclover plenty of time to produce a soil-building root system before late vegetables. It flowers later, so they are no longer mowing it and forcing tarnished plant bugs into lettuce fields.
Yellow blossom sweetclover—one of the best cover crop choices for warm-season nitrogen production—puts down a deep taproot before winter if seeded in June or July, observes Eric. “That root system loosens the soil, fixes nitrogen, and may even bring up minerals from the subsoil with its long tap root.”
Originally part of their weed management program, Eric points out that the clover alone would not suppress weeds. It works on their farm because of their successful management efforts over a decade to suppress overall weed pressure using intensive tillage, crop rotation and varied cover crops. The same concept applies to the tarnished plant bug. Never satisfied with a single strategy rather than a whole- system approach, the Nordells also began interseeding a single row of buckwheat into successive planting of short term cash crops like lettuce, spinach and peas.
The idea was to create a full-season insectiary in the market garden, moderating the boom and bust cycle of good and bad insects. They also hoped that the buckwheat would provide an alternate host for the plant bugs. The strategy seems to be working. Data collected as part of a research project with the Northeast Organic Network (NEON) found very few tarnished plant bugs in their lettuce but lots in the buckwheat insectiary.
The two pronged cover crop approach using buckwheat and a different management regime for sweetclover seems to be doing the trick. The next step, currently being evaluated, is to mix Italian ryegrass with the sweetclover to increase root mass and sod development between June planting and frost.
Rye and vetch are a popular combination to manage nitrogen. The rye takes up excess N from the soil, preventing leaching. The vetch fixes additional nitrogen which it releases after it’s killed the following spring. With the August seeding, the Nordells’ rye/vetch mixture produces“ a tremendous root system” and much of its biomass in fall.
The Nordells plow the rye/vetch mix after it greens up in late March to early April, working shallowly so as not to turn up as many weed seeds. They understand that such early kill sacrifices some biomass and N for earlier planting of their cash crop—tomatoes, peppers, summer broccoli or leeks—around the end of May.
Thanks to their weed-suppressing cover crops, the Nordells typically spend less than 10 hours a season hand-weeding their three acres of cash crops, and never need to hire outside weeding help. “Don’t overlook the cover crops’ role in improving soil tilth and making cultivation easier,” adds Eric. Before cover cropping, he noticed that their silty soils deteriorated whenever they grew two cash crops in a row. “When the soil structure declines, it doesn’t hold moisture and we get a buildup of annual weeds,” he notes.
The Nordells can afford to keep half their land in cover crops because their tax bills and land value are not as high as market gardeners in a more urban setting. “We take some land out of production, but in our situation, we have the land,” Eric says. “If we had to hire people for weed control, it would be more costly.”
To order a video describing this system ($10 postpaid) or a booklet of articles from the Small Farmers’ Journal ($12 postpaid), write to Eric and Anne Nordell, Beech Grove Farm, 3410 Route 184, Trout Run, Pa 17771.
COVER CROPS FOR VEGETABLE PRODUCTION
Vegetable systems have many windows for cover crops. Periods of one to two months between harvest of early planted spring crops and planting of fall crops can be filled using fast-growing warm-season cover crops such as buckwheat, cowpeas, sorghum-sudangrass hybrid, or another crop adapted to your conditions. As with other cropping systems, plant a winter annual cover crop on fields that otherwise would lie fallow.
Where moisture is sufficient, many vegetable crops can be overseeded with a cover crop, which will then be established and growing after vegetable harvest. Select cover crops that tolerate shade and harvest traffic, especially where there will be multiple pickings.
Cover crop features: Oats add lots of biomass, are a good nurse crop for spring-seeded legumes, and winterkill, doing away with the need for spring killing and tilling. Sorghum-sudangrass hybrid produces deep roots and tall, leafy stalks that die with the first frost. Yellow sweetclover is a deep rooting legume that provides cuttings of green manure in its second year. White clover is a persistent perennial and good N source. Brassicas and mustards can play a role in pest suppression and nutrient management. Mixtures of hairy vetch and cereal rye are increasing used in vegetable systems to scavenge nutrients and add N to the system.
In Zone 5 and cooler, plant rye, oats or a summer annual (in August) after snap bean or sweet corn harvest for organic matter production and erosion control, especially on sandy soils. Spray or incorporate the following spring, or leave unkilled strips for continued control of wind erosion.
If you have the option of a full year of cover crops in the East or Midwest, plant hairy vetch in the spring, allow to grow all year, and it will die back in the fall. Come back with no-till sweet or field corn or another N-demanding crop the following spring. Or, hairy vetch planted after about August 1 will overwinter in most zones with adequate snow coverage. Allow it to grow until early flower the following spring to achieve full N value. Kill for use as an organic mulch for no-till transplants or incorporate and plant a summer crop.
You can sow annual ryegrass right after harvesting an early-spring vegetable crop, allow it to grow for a month or two, then kill, incorporate and plant a fall vegetable.
Some farmers maximize the complementary weed-suppressing effects of various cover crop species by orchestrating peak growth periods, rooting depth and shape, topgrowth differences and species mixes. See Full-Year Covers Tackle Tough Weeds.
3 Year: Winter Wheat/Legume Interseed> Legume>Potatoes. This eastern Idaho rotation conditions soil, helps fight soil disease and provides N. Sufficient N for standard potatoes depends on rainfall being average or lower to prevent leaching that would put the soil N below the shallow-rooted cash crop.
2 Year Options: For vegetable systems in the Pacific Northwest and elsewhere, plant a winter wheat cover crop followed by sweet corn or onions. Another 2-yr. option is green peas > summer sorghum-sudangrass cover crop > potatoes (in year 2). Or, seed mustard green manure after winter or spring wheat. Come back with potatoes the following year. For maximum biofumigation effect, incorporate the mustard in the fall (see Brassicas and Mustards).
1 Year: Lettuce>Buckwheat>Buckwheat> Broccoli>White Clover/Annual Ryegrass. The Northeast’s early spring vegetable crops often leave little residue after their early summer harvest. Sequential buckwheat plantings suppress weeds, loosen topsoil and attract beneficial insects. Buckwheat is easy to kill by mowing in preparation for fall transplants. With light tillage to incorporate the relatively small amount of fast-degrading buckwheat residue, you can then sow a winter grass/legume cover mix to hold soil throughout the fall and over winter. Planted at least 40 days before frost, the white clover should overwinter and provide green manure or a living mulch the next year.
California Vegetable Crop Systems Innovative work in California includes rotating cover crops as well as cash crops, adding diversity to the system. This was done in response to an increase in Alternaria blight in LANA vetch if planted year after year.
4 Year: LANA Vetch>Corn>Oats/Vetch> Dry Beans>Common Vetch>Tomatoes>S-S Hybrid/ Cowpea >Safflower. The N needs of the cash crops of sweet corn, dry beans, safflower and canning tomatoes determine, in part, which covers to grow. Corn, with the highest N demand, is preceded by LANA vetch, which produces more N than other covers. Before tomatoes, common vetch works best. A mixture of purple vetch and oats is grown before dry beans, and a mix of sorghum-sudangrass and cowpeas precedes safflower.
In order to get maximum biomass and N production by April 1, LANA vetch is best planted early enough (6 to 8 weeks before frost) to have good growth before “winter.” Disked in early April, LANA provides all but about 40 lb. N/A to the sweet corn crop. Common vetch, seeded after the corn, can fix most of the N required by the subsequent tomato crop, with about 30 to 40 lb. N/A added as starter.
A mixture of sorghum-sudangrass and cowpeas is planted following tomato harvest. The mixture responds to residual N levels with N-scavenging by the grass component to prevent winter leaching. The cowpeas fix enough N for early growth of the subsequent safflower cash crop, which has relatively low initial N demands. The cover crop breaks down fast enough to supply safflower’s later-season N demand.
Precaution. If you are not using any herbicides, vetch could become a problem in the California system. Earlier kill sacrifices N, but does not allow for the production of hard seed that stays viable for several seasons.
Start Where You Are
We’ll use a basic Corn Belt situation as a model. From a corn>soybean rotation, you can expand to:
Corn>Cover>Soybean>Cover. Most popular choices are rye or rye/vetch mixture following corn; vetch or rye/vetch mixture following beans. Broadcast or drill covers immediately after harvest. Hairy vetch needs at least 15 days before frost in 60° F soil. Rye will germinate as long as soil is just above freezing. Drill for quicker germination. Consider overseeding at leaf-yellowing if your post-harvest planting window is too short.
If you want to make certain the legume is well established for maximum spring N and biomass production, consider adding a small grain to your rotation.
Corn>Soybean>Small Grain/Cover. Small grains could be oats, wheat or barley. Cover could be vetch, field peas or red clover. If you want the legume to winterkill to eliminate spring cover crop killing, try a non-hardy cultivar of berseem clover or annual alfalfa.
If you have livestock, a forage/hay market option or want more soil benefits, choose a longer-lived legume cover.
Corn>Soybean>Small Grain/Legume> Legume Hay, Pasture or Green Manure. Yellow sweetclover or red clover are popular forage choices. An oats/berseem interseeding provides a forage option the first year. Harvesting the cover crop or terminating it early in its second season opens up new options for cash crops or a second cover crop.
Late-season tomatoes, peppers, vine crops or sweetcorn all thrive in the warm, enriched soil following a green manure. Two heat-loving covers that could be planted after killing a cool-season legume green manure are buckwheat (used to smother weeds, attract beneficial insects or for grain harvest) and sorghum-sudangrass hybrid (for quick plow-down biomass or to fracture compacted subsoil).
These crops would work most places in the Corn Belt. To get started in your area, check Top Regional Cover Crop Species to fill various roles, or Cultural Traits and Planting to find which cover crops fit best in your system.
|ANNUAL and PERENNIAL MEDIC cultivars can fix N on low moisture and can reduce erosion in dryland areas compared with bare fallow between crop seasons.|
COVER CROPS FOR COTTON PRODUCTION
In what would otherwise be continuous cotton production, any winter annual cover crop added to the system can add rotation benefits, help maintain soil productivity, and provide the many other benefits of cover crops highlighted throughout this book.
Hairy vetch, crimson clover, or mixtures with rye or another small grain can reduce erosion, add N and organic matter to the system. Drill after shredding stalks in the fall and kill by spraying or mowing prior to no-till seeding of cotton in May. Or, aerially seed just before application of defoliant. The dropping leaves mulch the cover crop seed, aiding germination. Rye works better than wheat. Yields are usually equal to, or greater than yields in conventional tillage systems with winter fallow.
Balansa clover, a promising cover crop for the South, reseeds well in no-till cotton systems (see Up-and-Coming Cover Crops).
1 Year: Rye/Legume>Cotton. Plant the rye/legume mix in early October, or early enough to allow the legume to establish well before cooler winter temperatures. Kill by late April, and if soil moisture permits, no-till plant cotton within three to five days using tined-wheel row cleaner attachments to clear residue. Band-spray normal preemergent herbicides over the cleaned and planted row area. Cotton will need additional weed control toward layby using flaming, cultivation or directed herbicides. Crimson clover, hairy vetch, Cahaba vetch and Austrian winter peas are effective legumes in this system.
Multiyear: Reseeding Legume>No-Till Cotton> Legume>No-Till Cotton. Subterranean clover, Southern spotted burclover, balansa clover and some crimson clover cultivars set seed quickly enough in some areas to become perpetually reseeding when cotton planting dates are late enough in spring. Germination of hard seed in late summer provides soil erosion protection over winter, N for the following crop and an organic mulch at planting.
Strip planting into reseeding legumes works for many crops in the South, including cotton, corn, sweet potatoes, peanuts, peppers, cucumbers, cabbage and snap beans. Tillage or herbicides are used to create strips 12 to 30 inches wide. Wider killed strips reduce moisture competition by the cover crop before it dies back naturally, but also reduce the amount of seed set, biomass and N produced. Wider strips also decrease the mulching effect from the cover crop residue. The remaining strips of living cover crop act as in-field insectary areas to increase overall insect populations, resulting in more beneficial insects to control pest insects.
Watch for moisture depletion if spring is unusually dry.
Be sure to plant cotton by soil temperature (65° F is required), because cover crops may keep soil cool in the spring. Don’t plant too early!
A delay of two to three weeks between cover crop kill and cotton planting reduces these problems, and reduces the chance of stand losses due to insects (cutworm), diseases or allelopathic chemicals.
Additional mid-summer weed protection is needed during the hot-season “down time” for the reseeding legumes.
Reseeding depends on adequate hard seed production by the clovers. Dry summer weather favors hard seed production while wet summers reduce the percentage of hard seed.
DRYLAND CEREAL-LEGUME CROPPING SYSTEMS
Soil moisture availability and use by cover crops are the dominant concerns in dryland production systems. Yet more and more innovators are finding that carefully managed and selected cover crops in their rotations result in increased soil moisture availability to their cash crops. They are finding ways to incorporate cover crops into flexible rotations that can be modified to capitalize on soil moisture when available while preventing adverse effects on cash crops. This delicate balance between water use by the cover crop and water conservation—particularly in conservation tillage systems—will dictate, in part, how cover crops work in your rotation. See also Managing Cover Crops in Conservation Tillage Systems.
Perennial legumes provide numerous benefits to grain cropping systems in the Northern Plains, including increased grain yield, nutrient scavenging, carbon sequestration, breaking weed and insects cycles and for use as feed (129).
Cover crop features: perennial medics persist due to hard seed (of concern in some systems), providing green manure and erosion control; field peas and lentils (grain legumes) are shallow-rooted yet produce crops and additional N in years of good rainfall.
An excellent resource describing these rotations in detail is Cereal-Legume Cropping Systems: Nine Farm Case Studies in the Dryland Northern Plains, Canadian Prairies and Intermountain Northwest (258).
7 to 13 Years: Flax>Winter Wheat>Spring Barley>Buckwheat>Spring Wheat>Winter Wheat>Alfalfa (up to 6 years) >Fallow
System sequences are:
Flax or other spring crops (buckwheat, wheat, barley) are followed by fall-seeded wheat (sometimes rye), harvested in July, leaving stubble over the winter;
Spring-seeded barley or oats, harvested in August, leaving stubble over the winter;
Buckwheat, seeded in June and harvested in October, helps to control weeds;
A spring small grain, which outcompetes any volunteer buckwheat (alternately, fall-seeded wheat, or fall-seeded sweetclover for seed or hay).
The rotation closes with up to 6 years of alfalfa, plowdown of sweetclover seeded with the previous year’s wheat or an annual legume green manure such as Austrian winter peas or berseem clover.
There are many points during this rotation where a different cash crop or cover crop can be substituted, particularly in response to market conditions. Furthermore, with cattle on the ranch, many of the crops can be grazed or cut for hay.
Moving into areas with more than 12 inches of rain a year opens additional windows for incorporating cover crops into dryland systems.
9 Year: Winter Wheat>Spring Wheat> Spring Grain/Legume Interseed>Legume Green Manure/Fallow>Winter Wheat> Spring Wheat>Grain/Legume Interseed> Legume> Legume. In this rotation, one year of winter wheat and two years of spring-seeded crops follow a two or three-year legume break. Each legume sequence ends with an early summer incorporation of the legume to save moisture followed by minimal surface tillage to control weeds. Deep-rooted winter wheat follows sweet-clover, which can leave surface soil layers fairly dry. Spring -seeded grains prevent weeds that show up with successive winter grain cycles and have shallower roots that allow soil moisture to build up deeper in the profile.
In the second spring-grain year, using a low-N demanding crop such as kamut wheat reduces the risk of N-deficiency. Sweetclover seeded with the kamut provides regrowth the next spring that helps to take up enough soil water to prevent saline seep. Black medic, INDIANHEAD lentils and field peas are water-efficient substitutes for the deep-rooted—and water hungry—alfalfa and sweetclover. These peas and lentils are spring- sown, providing back-up N production if the forage legumes fail to establish.
While moisture levels fluctuate critically from year to year in dryland systems, N levels tend to be more stable than in the hot, humid South, and adding crop residue builds up soil organic matter more easily. Careful management of low-water use cover crops can minimize soil water loss while adding organic matter and N. Consequently, dryland rotations can have a significant impact on soils and the field environment when used over a number of years.
These improved soils have higher organic matter, a crumbly structure, and good water retention and infiltration. They also resist compaction and effectively cycle nutrients from residue to subsequent crops.
Remember, the benefits of cover crops accrue over several years. You will see improvements in crop yield, pest management and soil tilth if you commit to cover crop use whenever and wherever possible in your rotations.