A recent survey revealed that thousands of farmers are planting cover crops and reporting benefits from the practice.
While only a few respondents to the fifth annual cover crop survey were from Mississippi, the study revealed more landowners appreciate the practice of growing crops to protect and enrich the soil. Most respondents were from the Midwest in the survey conducted by the Sustainable Agriculture Research and Education program and the Conservation Technology Information Center.
Cover crops are sown between growing seasons and usually not harvested for profit. In some areas of Mississippi, cost-share programs are available for cover crops under the U.S. Department of Agriculture Natural Resources Conservation Service’s practice code 340.
Large agricultural producers and small gardeners alike know that soil health is just as important to success as water and sunshine.
Soil health is a key benefit from planting a cover crop, such as this cereal rye on a northeast Mississippi farm.
Of 2,012 survey respondents, 88 percent reported using cover crops, with years of experience ranging from just one year to more than 10. When the first survey was conducted in 2012, farmers reported planting an average of 217 acres of cover crops. That number steadily rose to an average of 400 acres in 2016, and it is projected to reach 451 acres in 2017.
Paralleling a continued emphasis on sustainable soil management, 86 percent of respondents noted soil health as a key benefit of planting cover crops. Fifty-four percent reported that soil health benefits began in the first year of use.
Despite the benefits and growing popularity of integrating cover crops into production systems, maximizing the benefits of the practice takes time, careful planning and research. If not implemented properly, cover crops can most certainly create pitfalls. Here are three tips for getting the most out of a cover crop.
First, identify your goals for the upcoming season and select the appropriate cover crop or mix of cover crops. Some reliable choices for the Southeast include cereal rye, oats, wheat, radish, winter pea and vetch.
Second, proper timing is one of the most important aspects of planting success. By this time of year, it may be best to just start researching for 2018. Optimal planting time for many cover crops is relative to the potential for frost. In Mississippi, the average first frost dates range from Oct. 26 to Nov. 5. Sow seeds about 30 days before the first anticipated fall frost date in your part of the state.
Third, choose a cover crop that is easy to kill when you need to prepare for the upcoming cash crop planting season. The timing, effort and method of termination are also crop-dependent.
Many local, independent businesses carry seed for cover crops, often available by the pound — a plus for small gardens. Co-ops and garden centers can order seed, as well.
Crop rotations with small grains in the sequence allow for an adequate seasonal window to establish variety of cover crop blends following grain harvest. However, producers with strict corn-soybean rotations are limited in their options for cover crop species, since there is not enough growing degree days left for cover crops to grow after primary grain crop has been harvested. One cover crop that has caught attention and has consistently worked in South Dakota environments where pre-dominant rotation is corn-soybean is winter rye.
About Winter Rye
Winter rye is known for its winter hardiness allowing late fall planting and puts on a rapid growth the following spring. Furthermore, adding a cool season small grain component into a corn-soybean rotation would not only add diversity the cropping system but also help break pest pressures in the field. Winter rye is also known for its inherent ability to suppress weeds because of its allelopathic characteristics, i.e. its ability to produce biochemical compounds that inhibits germination, growth, and reproduction of other plants. On a long term basis incorporating cover crops would also help improve soil health and provide supplemental forage.
Fitting Into Rotation
Considering growing habits of all three crops is essential when determining the order of winter rye within the cropping sequence. Planting rye after corn, and ahead soybeans, seems to be a better fit than to grow rye before corn. This way corn residue provides protection to rye seedlings. In addition, soybeans can tolerate later planting in the spring better than corn which allows rye to accumulate more spring growth. Rye biomass in the spring can be terminated as cover or utilized as forage depending on the farm need. Research conducted in various locations of Southeast SD for the last few years has shown no negative impact on soybean yields when grown on rye cover crop residue. On the other hand corn yield tends to suffer following a rye cover crop which could be due to allelopathic effects of growing rye cover crop or the micro climate created by the rye residue on the soil surface at the time of corn seeding. It is suggested to terminate rye 2-3 weeks prior to corn planting to avoid any negative impact on corn plant health and grain yield.
Seeding rate is about 40 lbs/ac as a cover crop, however, it can be increased to 75 lbs/ac if weed suppression is the primary objective.
Aerial seeding can be done during mid to late corn seed-filling stage (early Sept). Research results show that aerial seeded (or broadcast method) rye produces about 80% of the spring biomass of drill-seeded following grain harvest.
Producers of small grains such as wheat, oat, barley, etc. are suggested not to use winter rye as a cover crop because it may act as significant contaminant or weed in small grain crops.
As winter rye accumulate rapid growth in the spring, it is a good practice to look out for short or medium term spring weather so that rye can be terminated early when conditions are drier than usual.
Plan cover crop mix for weed suppression as well as soil health.
Keep costs down with lower cost cereal rye and ryegrass.
Integrate herbicide program planning with cover crop planning for maximum return.
Ralph Upton has cut his herbicide costs, controlled herbicide resistant marestail and waterhemp, improved soil health and reducd erosion on his south central Illinois fields. And while the weather impacted his herbicide program, weed suppression by cover crops kept working.
“Where I had cover crops, I seldom saw weeds,” says Upton. “Waterhemp is hitting us real hard. We’ve had issues in the past, but nothing compared to this year.”
Wet, cold weather in April delayed planting until May 10, with additional rain delays keeping him out of the field until June 1. As a result, he planted only 200 acres of corn compared to his normal 800 to 900 acres. That same weather also led to poor herbicide burndown. After years of relying on glyphosate he tried alternatives, only to discover apparent PPO as well as glyphosate resistance in his waterhemp.
Fields with cover crops aren’t immune to waterhemp. “I did get a few coming through and where I did, I sprayed the field, but I should have spot sprayed,” he says. “We had corn fields with a hairy vetch cover crop that we didn’t put any herbicide on. Where I had to replant I did use herbicide, but even there it was on only eight to ten acres out of 80.”
Stifled winter annuals, marestail
Mark Anson also reports significant weed suppression with delayed weed seed germination using cover crops. Weed control combined with other benefits justifies Anson Farms continuing to invest in cover crops while looking for ways to reduce other costs. The 20,000-acre operation in southwest Indiana and southeast Illinois has about 65 percent of its acres in cover crops.
“We find that cover crops control winter annuals really well and my brother, who does a lot of the spraying, has pointed out fields that used to have heavy marestail pressure have very few out there,” says Anson. “We seeded cover crops in a river bottom field, leaving a 100 ft. perimeter bare. The cover cropped area was clean, but the 100 ft. perimeter was heavy with weeds.”
“We are clearly seeing weed suppression with cover crops,” says Larry Steckel, row crop weed specialist, University of Tennessee. “If you have a good cover crop stand, you don’t have to worry about marestail and winter annuals. Sometimes you avoid that extra burndown needed for marestail and it’s a big help with summer annuals like Palmer pigweed. A decent cover crop stand can delay pigweed 20 to 30 days which buys time and puts less stress on resistance management.”
Steckel notes seeing the most consistent weed control with wheat/vetch, cereal rye/vetch or cereal rye/crimson clover combinations. He does admit that they can require a two-pass program to terminate, such as dicamba, a few weeks before planting and gramoxone behind the planter.
He also has found that cover crops can complicate pre-emerge herbicide programs. “Atrazine in corn or metribuzin in soybeans work better than others,” says Steckel. “Prowl or Dual seems to hang up on top (the soil) while encapsulated acetoclor works well.”
Green planting helps
Mike Plumer has three years of research (eight replications each year) to back up claims of marestail suppression with cover crops. “We saw 95 to 98 percent control,” reports Plumer, consultant, Conservation Agriculture and a former University of Illinois extension specialist. “The best control was with 60 to 80 pounds of cereal rye seeded the second or third week of October – You can go lighter and still get weed control if planted early enough to tiller. If you’re shooting for full season weed control of a broad spectrum of weeds, let cereal rye grow until planting. If planting cereal rye into corn stubble, 25 to 30 pounds of nitrogen in fall can increase stand, improve growth going into winter and increase growth in the spring improving weed control.”
Upton tries to get vetch planted on soybean stubble by mid-October even if that means planting a shorter season soybean variety as he did this year. He aims to plant 35 to 40 pounds of cereal rye and 10 to 12 pounds of ryegrass on corn stubble by Thanksgiving.
He feels he is getting physical suppression from the rye combination, as well as some allelopathic affect. As he has focused more and more on weed suppression, he waits longer to terminate which adds more complications. “To get more biomass, we need to let it grow longer,” he says. “Increasingly, I will be terminating right at planting with glyphosate on the rye and 2, 4-D on the hairy vetch.”
Convince landlords of value
This year Upton had too much biomass. Delayed planting allowed up to 25 percent more biomass than usual. “The planter didn’t get set up right to handle it and that affected the stand which in turn allowed some water hemp to slip through,” he says.
Even with the complications, Upton, like Anson, has no plans to back off on his cover crop program. He is even considering expanding it to rental ground. “It has been hard to justify the cost on land you may not farm the following year,” he says. “However, I’m seeing enough weed control and other benefits that I may be able to convince landowners to work with me on it.”
In Minnesota, small grain harvest is complete and sugar beet, sweet corn, corn silage, and pea harvest is well underway. With these early harvested crops, producers have an opportunity to consider planting a cover crop this fall, a practice that has many benefits. This may be a particularly good year to try cover crops, since soil moisture in many parts of the region is ideal for germination.
Cover crop success stories have been shared in almost every local and national Ag magazine over the past five years. Cover crops have many benefits, including improving water infiltration, reducing soil erosion, scavenging excess nitrogen and phosphorus, providing nutrients, using excess moisture, extending the grazing season, improving soil health, and providing a food source for pollinators.
How and where should a producer get started with cover crops? The best approach is to keep it simple. In fields where wheat was just harvested, one option is to allow the wheat to reseed itself without tilling the land. As long as the soil is covered, this would be considered a cover crop. However, this may cause issues by creating a wet mat of plant residue on the soil surface in the spring. A better option may be to seed cereal rye into the volunteer wheat, so the rye can utilize the extra moisture in the spring. When starting out, using one or two cover crops is an excellent way to get acquainted with their benefits. With lower commodity prices, expensive seed mixes are not necessary. The most popular choices are forage radish and cereal rye. Before choosing a cover crop, consider the following questions:
1. What is my crop rotation and harvest timing? If sugar beets have just been harvested, brassicas will not be needed in the cover crop mix (i.e. turnip, canola, or radish), as there isn’t enough time for adequate growth to justify the cost. Instead, plant a small grain or grass. If the field is going to wheat next spring, then do not use cereal rye as a cover crop. Instead, choose oats, so that any volunteers that come up in the wheat crop can be killed.
If cover crops are planted after August 15, use cool season crops. These may include forage pea, barley, wheat, and triticale. In our northern climate, there may not be enough time to truly benefit from a legume’s nitrogen credit when they are planted in late-summer.
Herbicide carryover and rotation restrictions are another consideration when planting cover crops. Look at herbicide product labels or call your agronomist for exact restrictions. University of Wisconsin has a helpful, in-depth fact sheet that can be used as a guide: Herbicide rotation restrictions in forage and cover cropping systems.
2. Do I want a cover crop that will overwinter? A cover crop that overwinters will provide the best soil protection through the winter and into spring. However, it generally will need to be terminated before the cash crop is planted in the spring. A combination of cereal rye, which will overwinter, and radish (which will winter kill) is an example of a mix where the rye provides added soil protection, while the radish residue decomposes quickly. For a quick growing grain that does not overwinter, seeding barley in the fall is a good option. While typically winterkilled, it should be noted that turnips overwintered in parts of Minnesota and North Dakota in the 2015-2016 winter. However, if radish in the mix and there aren’t grazing cattle, then another cover crop that serves the same purpose as radish isn’t needed.
3. How should cover crops be seeded? There are several choices for seeding cover crops. They include a no-till drill, slurry seeded (keep manure agitated for a more even spreading of seed), broadcast and lightly incorporated for seed to soil contact, or by plane. If the soil is moist, all of these seeding methods are viable options. If the soil is dry, leaving the seeds on the soil surface will reduce germination and coverage during the fall. Oats are better equipped to be seeded a little deeper if soil moisture is a concern. Peas also need to be seeded deeper than most other cover crops. In general, when broadcasting or flying on cover crops, smaller seeds are a better choice: radish, turnip, flax, dwarf essex rapeseed, cereal rye or barley. Larger seeded crops, like peas, sunflower, etc., should not be broadcast or flown on.
Producers are encouraged to consult with seed dealers and other sources for selecting the right mix and rates for specific field and crop rotation situations. Consider visiting U of M and NDSU Extension websites, Midwest Cover Crop Council, and NRCS publications for more information.
Cover crops can be an excellent addition to crop rotations as they improve water infiltration and reduce erosion. Trying one to two species of cover crops and planting them in the early harvested fields will set-up the fields for long-term success.
As planting season is quickly approaching, it is time to terminate some of our overwintered cover crops. Annual ryegrass is one of the more challenging cover crops to terminate with herbicides due to its aggressive growth and extensive root system. Glyphosate has thus far been the preferred herbicide for ryegrass termination, however the following points will increase termination success.
Use a high rate of glyphosate. A minimum of 36 fluid ounces per acre of Roundup PowerMax, or 1.25 pounds acid equivalent (ae) per acre glyphosate, is recommended, with up to 72 fluid ounces per acre (2.5 pounds ae per acre glyphosate) in situations where the cover crop is extremely dense, greater than 8 inches tall or starting to flower. When using higher rates of glyphosate, growers should consider how this fits into their maximum total quarts allowed for preplanting through pre-emergence and the annual total according to the Roundup PowerMax label.
Annual ryegrass interseeded into seed corn in St. Joseph County, April 2016.
Make sure the weather is appropriate for glyphosate uptake. Glyphosate is a systemic herbicide, so the plant needs to be actively growing for it to be effective. Minimum air temperatures need to be greater than 45 degrees Fahrenheit for three or more days after application, with warmer temperatures resulting in greater uptake and faster activity.
Ammonium sulfate is needed. As with all glyphosate applications, it is important to add ammonium sulfate at 17 pounds per 100 gallons of spray solution. The ammonium sulfate counteracts the effects of hard water on glyphosate and helps with glyphosate uptake into the annual ryegrass for more effective control.
In December 2015, Purdue University Extension released an article, “Successful Annual Ryegrass Termination with Herbicides,” that provides an excellent summary of their research findings, including tank mixtures that can help increase the effectiveness of annual ryegrass termination. Their current recommendation is as follows:
36 fluid ounces per acre Roundup PowerMax + 1 fluid ounce per acre Sharpen + 17 pounds per 100 gallons ammonium sulfate + 1% v/v methylated seed oil
As with any herbicide application, attention needs to be paid to any rotation restrictions for the subsequent cash crops, which can be found on the herbicide label or Michigan State University Extension’s “Weed Control Guide for Field Crops” (E0434). For example, higher rates of Sharpen (i.e., 1.5-2 ounces) would require a minimum of 14-30 days between application and soybean planting.
Spring has sprung in the majority of Minnesota. It’s now time to manage cover crops that were planted last summer or fall. Spring management of cover crops is as varied as the different farming operations across Minnesota. The plan of action any given farmer decides to do primarily depends on two things: 1) His or her reasons for using cover crops in the first place, and 2) the specific cover crops used.
Termination of cover crops
Winter-killed cover crops such as spring small grains (oats, spring wheat), some legumes (Crimson clover, field pea), and some non-leguminous broadleaves (buckwheat, oilseed radish) will have died over the winter. However, be aware that some winters may result in situations that allow cover crops that normally winter-kill to survive into spring. Some cold tolerant cover crops, turnips for example, may overwinter if there is plenty of snow cover. You may see a few scattered plants or possibly a patch in a protected area. Another possibility is spring germination of a seed that didn’t germinate in the fall. If either one of these situations occurs, treat it like a winter-hardy cover.
Winter-hardy cover crops like winter cereal rye, winter wheat, and hairy vetch go dormant over the winter and begin growing again in the spring. Poor fall growth or harsh, open winters may result in some plants not surviving the winter, so you may see spotty or limited growth. However, if you have any winter-hardy cover crops, you need to have a plan in place and be prepared with chemical or mechanical termination.
Chemical termination by a nonselective herbicide is often the choice for conventional farmers that need to manage winter-hardy cover crops. A general guideline for successful chemical termination is to spray when the cover crop is about 4-8 inches tall. Younger, smaller plants are easier to kill and tend to have a lower carbon to nitrogen ratio for faster decomposition. Cool spring temperatures can lead to cover crops that aren’t actively growing; if the covers aren’t actively growing then they will have trouble taking-up the herbicide and control can be compromised. If the spring weather is not cooperating or if you made the decision to allow the cover crop to grow taller, a second herbicide application may be needed to fully control the cover crop. Always consult product labels before using an herbicide and check for any restrictions to ensure use of the product will not impact your cropping rotation or forage plans.
Mechanical methods of cover crop termination primarily include use of tillage, a roller crimper, or a mower. These methods can be used alone or in combination with each other or an herbicide.
Different spring tillage equipment options can affect the termination success. A field cultivator operated at a 3-4 inch depth will cut most cover crop roots and bury the plant resulting in better termination. This is not necessarily true for coulter carts or vertical tillage equipment, since they move soil up and down which has little effect on cover crop, or weed, termination. One pass with a piece of tillage equipment may not be enough soil and root disturbance to kill the cover crop so be prepared to use two passes if needed.
A roller crimper or a mower can also successfully terminate winter-hardy cover crops. However, these methods are successful when the cover crops have reached maturity (flowering or heading stage of growth). This often puts the timing of these methods of termination into late-May. If you choose to use a roller crimper, be sure to plant in the same direction, and use a properly calibrated planter that has sufficient attachments or down-pressure to move aside or cut through the residue.
Cover crop residue management
The amount of residue left in the spring will depend on which cover crops were used and when they were planted. Brassicas and legumes begin decomposing and breakdown faster than grasses because of a lower carbon to nitrogen ratio in the plant material. The age of your cover crop stand will also affect decomposition. The earlier you planted the cover crop, the more mature the cover crop is at termination. This results in a higher carbon to nitrogen ratio and slower decomposition. The key for spring management of cover crop residue is planter set-up. Be sure to calibrate your planter so that it can sufficiently handle the cover crop residue situation. This may mean that you will need to re-calibrate the planter, even within a field if there are areas with measurable changes in residue type or amount.
Additional things to keep in mind
Be on the lookout for potential spring pest issues in cover crops fields, particularly with winter-hardy cover crops. Make a point to scout your cover crop fields and/or be prepared to terminate as soon as the weather allows.
It is normally recommended to wait 7-14 days after cover crop termination before planting the year’s cash crop. This is mainly to allow some time for the cover crop residue to begin breaking down and for the surface soil moisture to recharge. That being said, there are farmers that successfully plant into standing cover crops or plant within days or hours of spraying.
One of the best sources of cover crop information is experience. If you have access to neighbors, crop consultants, agency personnel, or Extension Educators with experience managing cover crops, be sure to contact them. The majority of people with cover crop experience are more than willing to share what they have learned.
One of the most significant contributions that legume cover crops make to the soil is the nitrogen (N) they contain. Legume cover crops fix atmospheric N in their plant tissues in a symbiotic or mutually beneficial relationship with rhizobium bacteria. In association with legume roots, the bacteria convert atmospheric N into a form that plants can use. As cover crop biomass decomposes, these nutrients are released for use by cash crops. Farmers should make an effort to understand this complex process because it will help them to select the proper legumes for their cropping plan, calculate when to incorporate cover crops and plant cash crops that follow, and plan fertilizer rates and schedules for those cash crops. Above all, they need to inoculate legume seed before planting with the appropriate Rhizobium species.
The N associated with cover crop biomass undergoes many processes before it is ready to be taken up for use by cash crops. The process begins with biomass N, which is the nitrogen contained in mature cover crops. From 75 to 90 percent of the nitrogen content in legume cover crops is contained in the above ground portions of the plant, with the remaining N in its roots and nodules (Shipley et al., 1992).
When legume or grass cover crops are killed and incorporated into the soil, living microorganisms in the soil go to work to decompose plant residues. The biomass nitrogen is mineralized and converted first to ammonium (NH4) and then to nitrate compounds (NO3) that plant roots can take up and use. The rate of this mineralization process depends largely on the chemical composition of the plant residues that are involved (Clement et al., 1995), and on climatic conditions.
Determining the ratio of carbon to nitrogen (C:N) in the cover crop biomass is the most common way to estimate how quickly biomass N will be mineralized and released for use by cash crops. As a general rule, cover crop residues with C:N ratios lower than 25:1 will release N quickly. In the southeastern U. S., legume cover crops, such as hairy vetch and crimson clover, killed immediately before corn planting generally have C:N ratios of 10:1 to 20:1 (Ranells and Wagger, 1997). Residues with C:N ratios greater than 25:1, such as cereal rye and wheat, decompose more slowly and their N is more slowly released.
A study conducted in 1989 reported that 75 to 80 percent of the biomass N produced by hairy vetch and crimson clover residues was released eight weeks after the cover crops were incorporated into the soil (Wagger, 1989a). This amounted to 71 to 85 pounds of N per acre. However, not all of the released N was taken up by the subsequent corn crop. The corn utilized approximately 50 percent of the N released by both residues. (This value may be con-sidered the N uptake efficiency of corn from legume residues. This value is similar to the N uptake efficiency of corn from inorganic fertilizer sources, such as ammonium nitrate.) The N not taken up by the following crops may still contribute to soil health. Living microbes in the soil may use the nitrogen to support population growth and microbial activity in the soil.
The impact of cover crops on pathogens—agents in the soil, such as bacteria or viruses, that cause disease—can be good, bad, or nonexistent. This impact varies broadly depending on individual circumstances and situations. A cover crop can act as a host for soilborne pathogens, or it can serve as an effective form of biological control for other plant pathogens. Incorporating cover crop residues can, in some cases, provide an organic food base that encourages pathogen growth (Phillips et al., 1971). On the other hand, some cover crops, such as brassicas (cabbage and mustard), can actually decrease soil pathogen populations (Lewis and Papa-vizas, 1971; Subbarao and Hubbard, 1996).
The impact of a cover crop on a pathogen involves many variables. Principally, it depends upon the pathogen’s nature and life cycle requirements. For example, if the pathogen survives best on surface residue and the cover crop residue is left on the soil surface as mulch, then the pathogen may survive until the next crop is planted and the level of disease may increase (Fawcett, 1987). Many plant diseases are associated with surface residue, for example, fungal and bacterial leaf blights (Boosalis and Cook, 1973).
At the same time, the increases in soil organic matter provided by cover crops can enhance biological control of soilborne plant pathogens. This comes about both through direct antagonism and by competition for available energy, water, and nutrients (Sumner et al., 1986). Organisms that cause disease can also be affected by changes in temperature, moisture, soil compaction, and bulk density, as well as nutrient dynamics. Whether or not the cover crop is in the same family of plants (taxonomically related) to the subsequent cash crop can also influence whether or not disease cycles are interrupted or prolonged.
Nematodes are enough of a concern in the sandy soils of the southeastern U. S. to give them individual attention when consider-ing disease management. The root-knot nematode (Meloidogyne spp.) is particularly troublesome in the Southeast. Agricultural scientists have more questions than answers concerning how to reduce pop-ulations of nematodes with cover crops. They are struggling to find a selection of crop rotations with cover crops that can address a wide variety of nematodes that have a very diverse host range (Reddy et al., 1986). They are also unclear, at this point, as to how some cover crops reduce the population levels of certain nematode species.
For instance, some green manure or cover crops placed in a rotation can reduce damage by one nematode species but not others. In a study in Florida, the warm-season legumes, which included pigeonpea, crotalaria, hairy indigo, velvetbean, and joint vetch, reduced root-knot nematode damage in a subsequent snapbean crop when the crop was compared to one produced in fallow. These same cover crops, however, were no more effective than fallow in reducing damage from sting (Belonolaimus longicaudatus) and lesion (Pratylenchus brachyurus) nematodes.
In some cases, a cover crop can reduce populations of one parasitic nematode but serve as a host that increases populations of other nematodes. While two researchers (McSorley and Gallaher) reported in 1991 that sorghum-sudangrass cover crops reduced levels of root- knot nematodes, Rhoades and Forbes (1986) found that a sorghum-sudangrass cover crop increased populations of B. longicaudatus and M. incognita nematodes. Farmers attempting to use crop rotations for controlling one nematode species must be aware that these rotations could benefit other damaging nematodes present in the field (McSorley and Dickson, 1995). Potential rotation crops should be evaluated for their effects on as many different damaging nematodes as possible.
Cover crops can be both a blessing and a drawback because they attract both beneficial and harmful insects to farm fields (Altieri and Letourneau, 1982; Andow, 1988). When a cover crop matures or dies, both the beneficial and pest insects may move to cash crops. The resulting effect on insect pest populations on the farm (an effect that also depends on several environmental factors) can present frustrating dilemmas for a farmer. For example, in a study in 1991, researchers found that a rye cover crop helped to reduce fruitworm populations in the tomato crop that followed it. But the rye cover also led to increased stinkbug damage (Roberts and Cartwright, 1991).
To create the best situation, a farmer grows a cover crop to attract beneficial insects before the damaging insects arrive. The beneficial insects are attracted by the moisture, shelter, pollen, honeydew, nectar, and potential insect prey associated with the cover crop. These beneficial insects subsist in the cover crop and then move into the vegetable crop to attack arriving pest insects. Several studies show that this approach is often successful. Researchers in Georgia reported high densities of big-eyed bugs, lady beetles, and other beneficial insects in vetches and clovers that moved into ensuing tomato crops (Bugg et al., 1990). In a more recent study, a researcher reported that assassin bugs destroyed Colorado potato beetles feeding on eggplant that had been planted into strip-tilled crimson clover.
Cover crops and surface crop residues can be used to control or inhibit weeds in subsequent cash crops in three basic ways:
• By smothering and shading them so they don’t receive adequate air and light.
• By outcompeting them for nutrients.
• By producing an effect known as allelopathy, the toxic effect on weed seed germination and seedling growth that occurs as residues of some cover crops decompose.
The primary way to suppress weed seed germination and growth is to have a vigorous cover crop stand. Such a stand will simply out-compete weed seeds for light and nutrients (Teasdale and Daughtry, 1993). When the cover crop is killed, its thick residues remain on the surface and hinder weed growth by physically modifying the amount of natural light, soil temperature, and soil moisture that is necessary for weed seed germination.
It’s important to note that suppressing weeds by smothering them becomes less effective as cover crop residues decompose. How fast residues decompose depends on several variables. For instance, warm temperatures, rainfall, and field tillage can speed up the decomposition rate. Another important factor is the C:N ratio, the carbon-to-nitrogen ratio of different kinds of crop residues. Residues with a high C:N ratio, such as mature small grain cover crops like rye (which has a C:N ratio of around 50), have a much slower decom-position rate than legumes like hairy vetch (which has a C:N ratio of around 12). Mix-tures of legumes and small grains have an intermediate rate of decomposition (a C:N ratio of around 25).
Cover crop residues also interfere with weed emergence through the allelopathic effect (Creamer at al., 1996a). Scientists are still researching the many (and sometimes mysterious) allelopathic effects that one plant has on another through its allelo-chemicals, the chemicals a plant releases into the environment that can be toxic to other plants. Some scientists believe that the specific allelopathic effects of certain plants are enhanced by chemicals produced by actinomycetes, algae, fungi, or other microbes associated with particular plant root systems in the upper soil layers (Putnam, 1988). Where and how these allelochemicals originate is often hard to discern. Each chemical’s biological activity may be reduced or enhanced by other factors, such as microbe action in the soil and oxidation. Other factors, such as environmental conditions, insects, or disease pressure, can speed up the detrimental effects of allelochemicals on weeds.
In one study, researchers found that cereal rye residues on the soil surface suppressed most common annual broadleaf and grassy weeds for four to eight weeks (Smeda and Weller, 1996). Thus, using a rye cover crop could eliminate the need for a soil-applied herbicide at transplanting without depressing yield. The authors indicated, however, that post-emergence weed control of escaped weeds might be necessary in some years.
Researchers have reported that the cover crops listed in Table 1 have shown allelopathic effects on certain weeds. We should note that the allelopathic effects of crimson clover and hairy vetch are more apparent if the cover crop is incorporated rather than left on the surface in no-till management (Teasdale and Daughtry, 1993).
Type: summer or cool-season annual broadleaf grain
Roles: quick soil cover, weed suppressor, nectar for pollinators and beneficial insects, topsoil loosener, rejuvenator for low-fertility soils
Buckwheat is the speedy short-season cover crop. It establishes, blooms and reaches maturity in just 70 to 90 days and its residue breaks down quickly. Buckwheat suppresses weeds and attracts beneficial insects and pollinators with its abundant blossoms. It is easy to kill, and reportedly extracts soil phosphorus from soil better than most grain-type cover crops.
Buckwheat thrives in cool, moist conditions but it is not frost tolerant. Even in the South, it is not grown as a winter annual. Buckwheat is not particularly drought tolerant, and readily wilts under hot, dry conditions. Its short growing season may allow it to avoid droughts, however.
Quick cover. Few cover crops establish as rapidly and as easily as buckwheat. Its rounded pyramid- shaped seeds germinate in just three to five days. Leaves up to 3 inches wide can develop within two weeks to create a relatively dense, soil shading canopy. Buckwheat typically produces only 2 to 3 tons of dry matter per acre, but it does so quickly—in just six to eight weeks. Buckwheat residue also decomposes quickly, releasing nutrients to the next crop.
Weed suppressor. Buckwheat’s strong weed suppressing ability makes it ideal for smothering warm-season annual weeds. It’s also planted after intensive, weed-weakening tillage to crowd out perennials. A mix of tillage and successive dense seedings of buckwheat can effectively suppress Canada thistle, sowthistle, creeping jenny, leafy spurge, Russian knapweed and perennial peppergrass. While living buckwheat may have an allelopathic weed-suppressing effect, its primary impact on weeds is through shading and competition.
Phosphorus scavenger. Buckwheat takes up phosphorus and some minor nutrients (possibly including calcium) that are otherwise unavailable to crops, then releasing these nutrients to later crops as the residue breaks down. The roots of the plants produce mild acids that release nutrients from the soil. These acids also activate slow-releasing organic fertilizers, such as rock phosphate. Buckwheat’s dense, fibrous roots cluster in the top 10 inches of soil, providing an extensive root surface area for nutrient uptake.
Thrives in poor soils. Buckwheat performs better than cereal grains on low-fertility soils and soils with high levels of decaying organic matter. That’s why it was often the first crop planted on cleared land during the settlement of woodland areas and is still a good first crop for rejuvenating over-farmed soils. However, buckwheat does not do well in compacted, droughty or excessively wet soils.
Quick regrowth. Buckwheat will regrow after mowing if cut before it reaches 25 percent bloom. It also can be lightly tilled after the midpoint of its long flowering period to reseed a second crop. Some growers bring new land into production by raising three successive buckwheat crops this way.
Soil conditioner. Buckwheat’s abundant, fine roots leave topsoil loose and friable after only minimal tillage, making it a great mid-summer soil conditioner preceding fall crops in temperate areas.
Nectar source. Buckwheat’s shallow white blossoms attract beneficial insects that attack or parasitize aphids, mites and other pests. These beneficials include hover flies (Syrphidae), predatory wasps, minute pirate bugs, insidious flower bugs, tachinid flies and lady beetles. Flowering may start within three weeks of planting and continue for up to 10 weeks.
Nurse crop. Due to its quick, aggressive start, buckwheat is rarely used as a nurse crop, although it can be used anytime you want quick cover. It is sometimes used to protect late-fall plantings of slow-starting, winter-hardy legumes wherever freezing temperatures are sure to kill the buckwheat.
Buckwheat prefers light to medium, well-drained soils—sandy loams, loams, and silt loams. It performs poorly on heavy, wet soils or soils with high levels of limestone. Buckwheat grows best in cool, moist conditions, but is not frost-tolerant. It is also not drought tolerant. Extreme afternoon heat will cause wilting, but plants bounce back overnight.
Plant buckwheat after all danger of frost. In untilled, minimally tilled or clean-tilled soils, drill 50 to 60 lb./A at 1/2 to 11/2 inches deep in 6 to 8 inch rows. Use heavier rates for quicker canopy development. For a fast smother crop, broadcast up to 96 lb./A (2 bu./A) onto a firm seedbed and incorporate with a harrow, tine weeder, disk or field cultivator. Overall vigor is usually better in drilled seedings. As a nurse-crop for slow growing, winter annual legumes planted in late summer or fall, seed at one-quarter to one-third of the normal rate.
Buckwheat compensates for lower seeding rates by developing more branches per plant and more seeds per blossom. However, skimping too much on seed makes stands more vulnerable to early weed competition until the canopy fills in. Using cleaned, bin-run or even birdseed-grade seed can lower establishment costs, but increases the risk of weeds. As denser stands mature, stalks become spindly and are more likely to lodge from wind or heavy rain.
Buckwheat is used most commonly as a mid-summer cover crop to suppress weeds and replace bare fallow. In the Northeast and Midwest, it is often planted after harvest of early vegetable crops, then followed by a fall vegetable, winter grain, or cool-season cover crop. Planted later, winterkilled residue provides decent soil cover and is easy to no-till into. In many areas, it can be planted following harvest of winter wheat or canola.
In parts of California, buckwheat grows and flowers between the killing of winter annual legume cover crops in spring and their re-establishment in fall. Some California vineyard managers seed 3-foot strips of buckwheat in row middles, alternating it and another summer cover crop, such as sorghum-sudangrass.
Buckwheat is sensitive to herbicide residues from previous crops, especially in no-till seedbeds. Residue from trifluralin and from triazine and sulfonylurea herbicides have damaged or killed buckwheat seedlings. When in doubt, sow and water a small test plot of the fast germinating seed to detect stunting or mortality.
Few pests or diseases bother buckwheat. Its most serious weed competitors are often small grains from preceding crops, which only add to the cover crop biomass. Other grass weeds can be a problem, especially in thin stands. Weeds also can increase after seed set and leaf drop. Diseases include a leaf spot caused by the fungus Ramularia and Rhizoctonia root rot.
Plant buckwheat as an emergency cover crop to protect soil and suppress weeds when your main crop fails or cannot be planted in time due to unfavorable conditions.
To assure its role as habitat for beneficial insects, allow buckwheat to flower for at least 20 days—the time needed for minute pirate bugs to produce another generation.
Buckwheat can be double cropped for grain after harvesting early crops if planted by mid-July in northern states or by early August in the South. It requires a two-month period of relatively cool, moist conditions to prevent blasting of the blossoms. There is modest demand for organic and specially raised food-grade buckwheat in domestic and overseas markets. Exporters usually specify variety, so investigate before planting buckwheat for grain.
Buckwheat can become a weed. Kill within 7 to 10 days after flowering begins, before the first seeds begin to harden and turn brown. Earliest maturing seed can shatter before plants finish blooming. Some seed may overwinter in milder regions.
Buckwheat can harbor insect pests including Lygus bugs, tarnished plant bugs and Pratylynchus penetrans root lesion nematodes.
Buckwheat has only about half the root mass as a percent of total biomass as small grains. Its succulent stems break down quickly, leaving soils loose and vulnerable to erosion, particularly after tillage. Plant a soil-holding crop as soon as possible.
Buckwheat is nearly three times as effective as barley in extracting phosphorus, and more than 10 times more effective than rye—the poorest P scavenger of the cereal grains.
As a cash crop, buckwheat uses only half as much soil moisture as soybeans.