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Cover crops

Cover cropping has gotten contagious. At field days, on farm tours, and in computer forums, commodity-minded corn, wheat, and soybean farmers are clamoring to learn how to grow new and novel crops they likely last saw in their salad (radishes), soup (peas), or breakfast cereal (oats). There's a long list of benefits to such diversity, and it's proving to be enough to knock traditional crop rotations out of their rut. “Farmers are rapidly learning to appreciate the benefits that cover crops offer,” says Dwayne Beck, no-till guru and director of the Dakota Lakes Research Farm in Pierre, South Dakota.

“Cover cropping can solve many of the problems that long-term no-tillers have encountered. It helps dry the soil when it's too wet, keeps it moist when it's too dry, warms it when it's too cold, and keeps it cool when it's too hot. Cover crops look like they're the next step in no-till.”

Western Eyes On Moisture

By Larry Reichenberger

No-till farmers in South Dakota were among the first to give cover cropping its newfound popularity. Heavy wheat residue hampered their ability to plant corn in wheat/corn/soybean rotations.

Brassica crops (radishes, turnips, and canola) planted after wheat harvest provided a way to speed the decay of this residue — the low carbon-to-nitrogen ratio (C:N) of these crops increases microbial activity, which degrades the excess residue. Also, mixing in a winter small-grain crop (such as winter wheat, rye, or triticale) utilizes moisture in the late fall and early spring while also providing a living root system to improve trafficability.

Cover cropping has grown rapidly from that initial start, and it's now doing far more than drying out springtime soils. Innovative farmers are using cover crops to improve soil quality, reduce erosion, increase organic matter, fix atmospheric nitrogen, cycle nutrients, reduce compaction, suppress weeds, increase water infiltration, and even more.

Developing cover crop mixtures, or cocktails, has become a popular way to pursue several of these objectives at the same time and to fine-tune their impact. For example, Beck says South Dakota no-tillers quickly learned they could misuse brassicas as a cover crop because those plants destroyed too much wheat residue.

“It's critical for later in the season that we keep the soil covered, so we've learned to use grass crops, which have a high C:N ratio, in a mixture to balance the residue decomposition,” he says.

“There are different approaches depending on what you want to accomplish,” continues Beck. “Deep-rooting cover crops like radishes and sunflowers will cycle nutrients back to the surface and break compaction layers. Legumes will produce nitrogen, and grass crops with fibrous root systems will build soil structure. You've got to know what you want to do. Cover cropping isn't just a matter of planting 7 pounds of radishes.”

Wheat growers have a leg up on cover cropping, according to Bladen, Nebraska, farmers Keith and Brian Berns. “Going into wheat stubble after harvest provides a window of opportunity not only to get cover crops planted, but also for cover crops to develop enough growth to accomplish their objectives,” says Brian.

The brothers, who operate Green Cover Seed Company, have grown research and demonstration plots featuring more than 40 cover crop species and mixtures. “Most farmers we're dealing with plant a mixture, rather than a single species,” says Keith. “A popular one is a six-way mix that contains spring forage peas, common vetch, oilseed radish, dwarf essex rape, sorghum sudan, and pearl millet.”

Keith describes the role each of these components plays in the mix. “The grasses add biomass, which is ultimately turned into organic matter, and they balance the C:N ratio, which makes residue last longer. The radishes and rape will root deeply to break compaction and take up nutrients that have moved below the regular root zone. They also tie up carryover nitrogen, which forces the legumes to maximize their nitrogen fixation, and they lower the C:N ratio to allow some timing of decomposition and nutrient release.”

Such a six-way cover crop cocktail costs about $22 per acre. “If the goal is to provide livestock grazing, then we substitute some species that raise the seed cost to about $30 per acre,” says Keith.

Seed cost is a critical factor for cover crops, and some growers are looking at using more precise planters or air seeders, rather than drills, for seeding. “Some guys are planting peas or radishes, which are typically the most expensive, by singulating seeds in 30-inch rows with their planter. With interplant units, they can alternate peas and radishes in 15-inch rows. This allows them to cut seeding rates by one third to one half,” says Keith.

“Another unique idea,” says Keith, “is to use an air seeder along with an RTK guidance system to plant radishes in 30-inch rows while planting the rest of a cocktail mix between those rows. The radishes scavenge the nitrogen, break soil compaction, and decompose rapidly the following spring, along with any nearby wheat residue. This creates a biological strip-till system that leaves a nutrient-rich strip, clear of residue, where a row of corn can be planted.”

In drier areas of the Plains (where wheat-fallow-wheat is the traditional rotation), a shortage of soil moisture has emerged as a cover cropping concern for some farmers and researchers.

“Water is the most limiting resource for crop production in semiarid regions of western Nebraska, and what's used by a cover crop won't be available for the following wheat crop,” says Drew Lyon, dryland cropping systems specialist at the University of Nebraska.

“In several studies, we've found that winter wheat yields following a summer fallow replacement crop have been 25% to 60% less than winter wheat following a true summer fallow. This decline has been directly correlated to reduced soil moisture at planting time following the summer fallow replacement crop. If the cover crop is actually a forage crop, then its value can offset this loss in wheat yield. But with little or no direct economic return to a cover crop, dryland producers should be skeptical about the practice in this highly variable climate,” says Lyon.

Kansas State University agronomist John Holman felt the same way until he put cover cropping to the test for three years at his southwest Kansas location.

“We looked at various cover crops and cover crop mixtures, and we always found more soil moisture after the cover crops than after the fallow, provided our cover crops were terminated early. There was no difference in the subsequent wheat yields, but those soil-moisture findings were a surprise,” Holman says.

“We thought the cover crops would leave the soil drier at wheat-seeding time, but apparently the heavier residue cover increased water infiltration and reduced evaporation from the soil surface. In fact, some new research from North Dakota shows that a cover crop reduced peak surface soil temperatures by 20°F.,” he says.

Oilseed radishes (shown above) in a cover crop cocktail (shown far right) can break soil compaction, cycle nutrients from deep in the soil, and sequester nitrogen to encourage legumes to maximize nitrogen fixation. At their farm near Bladen, Nebraska, Keith and Brian Berns are experimenting with planting legume cover crops between corn rows (shown near right) to provide nitrogen.

Holman says these factors gave the cover-cropped plots an advantage in precipitation storage efficiency.

“In fallow systems, the precipitation storage efficiency really isn't very high. It typically runs around 20% when the soil is tilled and runs about 35% with a no-till fallow,” he says.

Holman notes that cover crops in his tests were terminated on June 1, so there was a period of several months for soil moisture to be replenished. He also found that haying the cover crops resulted in significantly lower soil-moisture levels.

The Berns brothers have also taken a look at the impact cover cropping has on soil moisture. In 2008 they were awarded a grant from SARE (Sustainable Agricultural Research and Education) to study the issue on their farm.

The study began with research into the water-holding capacity of the soils and the average precipitation received during the cover-cropping period (critical information for anybody considering cover cropping, according to Beck).

“From wheat harvest in July until corn planting the following May, we receive an average of 19.9 inches of precipitation,” says Brian. “Our silt loam soil holds 2.2 inches of water per foot, so our water-holding capacity in a 4-foot root zone is 8.8 inches. That leaves 10.8 inches of water that should be available to use for a cover crop without drying the soil.”

The study showed many cover crops could be produced within those limitations, but it surprisingly found that cover crop mixtures used the least water of all.

“We're not sure of all the factors involved,” says Keith. “This may occur because the plant roots in a mixture compete at different depths in the soil, while a single species cover crop roots at a similar depth. Whatever the reason, the cover crop cocktails left significantly more moisture in the soil profile.”

Covering Ground In The Corn Belt

By Rich Fee, Crops Editor

Despite their well-deserved reputation for conserving soil, limiting nitrogen losses, improving soil quality, and reducing weed pressure, cover crops are still not widely used in the heart of the Corn Belt.

Only 18% of the farmers in Iowa, Illinois, Indiana, and Minnesota who responded to a recent survey had ever used a cover crop; only 8% of them had planted a cover crop in the fall preceding the survey. Only 10.5% of the respondents in Iowa had used cover crops, while 28% of the respondents in Indiana had.

Jeremy Singer and colleagues at the USDA-ARS National Soil Tilth Laboratory (now called the National Laboratory for Agriculture and the Environment) surveyed 3,500 producers in those four states in 2006 to determine how widely cover crops were being used and why they weren't being used more widely. They received over 1,000 usable responses to the stratified random sample survey, funded by the Leopold Center.

Respondents who didn't use cover crops were asked to indicate why by choosing from a list of several possible reasons. The responses varied — sometimes widely — by state.

• 22%-34% said they didn't know enough about cover crops.

• 21%-30% said cover crops took too much time.

• 14%-24% said cover crops were too costly.

• 17%-35% said they already used no-till.

• 19%-23% said runoff wasn't a problem on their farm.

• <10% said cover crops reduced yields.

However, interest in cover crops has increased significantly since that survey was taken. “We are just getting bombarded with questions in Ohio,” says Alan Sundermeier, an Ohio State University Extension educator in Wood County. “It's amazing how much momentum we have now on cover crops. Oilseed radish cover crops are the hottest thing right now.”

Joel Gruver, an agronomist at Western Illinois University, says, “Higher grain prices and higher input costs seem to be pushing farmers to cover crops. Legumes can provide much of the nitrogen needed by corn, but Corn Belt farmers are more excited about non-legumes like radishes and annual ryegrass.”

At the same time, more information on managing cover crops is needed. “Widespread cover crop adoption is hampered in the Midwest by a lack of local selection and application information,” says Dean Baas with Michigan State University's Kellogg Biological Station Extension Land & Water unit.

Finally, government cost-share programs are making cover crops more attractive to many mainstream farmers. EQIP funding is available in many areas. In some places, state and local programs provide even more financial help. The Central Illinois Cover Crops Initiative is paying most of the seed and application costs for establishing cover crops in several counties this year through a special EQIP grant from NRCS.

Check These Cover Crop Info Sources

There are lots of fairly new publications and a few websites dedicated to cover crops. Here's a sampling.

• Managing Cover Crops Profitably. The third edition of this 244-page book was published in 2007 and is highly regarded. It can be downloaded for free at http://www.sare.org/publications/covercrops.htm. You can also purchase a print copy at that site for $19.

• You can access several publications at the Leopold Center's website for cover crop resources at http://www.leopold.iastate.edu/research/eco_files/cover_crops.htm. Start with the learning module about cover crops, then peruse the other listings.

• One of the priorities of the 4-year-old Midwest Cover Crops Council is to provide education and training to farmers and agencies to facilitate the adoption of cover crops. Their web address is http://www.mccc.msu.edu/links.html.

• A Penn State publication entitled Suppressing Weeds Using Cover Crops in Pennsylvania is available to download for free at http://pubs.cas.psu.edu/FreePubs/PDFs/uc210.pdf.

The general thinking is it will take both more knowledge and more dollars to get cover crops on more acres. Barry Fisher, the NRCS state agronomist for Indiana, told a meeting of the Midwest Cover Crops Council in March, “There are lots of dollars in crop share going out. But dollars alone won't get cover crops on the land. We need knowledge and a prescription for success.”

Collectively, those changes could lead to a significant increase in cover crop acres over the next few years.

Universities from California to Maryland, along with numerous other private and public organizations, are working with cover crops on local and regional bases. The sidebar on page 57 shows how to access information from these sources.

Gruver is evaluating how several different cover crops work in organic farming systems at Western Illinois University. As part of their coursework, students in one of Gruver's classes gather information about managing cover crops from organic and conventional farmers across the Midwest. That information is available at http://www.mccc.msu.edu/innovators.html.

“One of our most exciting experiments is a tillage system experiment featuring organic no-till soybeans drilled into rolled cereal rye,” says Gruver.

Another experiment compares conventional tillage, no-till, and bio strip-till, an approach that uses forage radishes to break up compaction. Last year, there were no significant yield differences for soybeans grown in the three systems.

Cover crop research and use are certainly not limited to organic farming. There are lots of cover crop systems that rely on herbicides to kill cover crops in the spring and help control weeds throughout the rest of the growing season.

It can be difficult to put a dollar value on soil quality, erosion control, and alleviating compaction, at least in the short run. It's somewhat easier to put a value on the nitrogen (N) scavenged or produced by a cover crop, even though it is difficult to know just how much nitrogen credit to give a particular cover crop in a particular year.

Dave Bishop, resource conservationist for the McLean County (Illinois) Soil and Water Conservation District, is helping spearhead the Central Illinois Cover Crop Initiative. He thinks the run-up in N prices three to four years ago has created much of the current interest in cover crops.

“That all happened in just months,” he says of the dramatic N price increase. “I believe farmers are thinking that if anhydrous can go from $400 (per ton) to $1,250, they need to have a Plan B.” Cover crops can be Plan B.

Legume cover crops such as red clover, cowpeas, and Austrian winter peas fix N from the air. Sundermeier says they can contribute from 50 to 150 pounds of N per acre.

Nonlegume cover crops scavenge leftover N from the soil. Some of that is then available to the next crop.

None of this is guaranteed, however. “Cover crops can replace nitrogen fertilizer, but not in every situation,” says Sundermeier. “After cereal rye, there may not be enough early nitrogen available for the new crop. And after a legume, the nitrogen will likely not be available until later in the growing season, depending upon when the crop decomposes. It all depends upon the carbon-to-N ratio.” 

6 Benefits of Cover Crops

1 Reduce soil erosion by wind and water.

2 Provide a home for beneficial insects.

3 Improve soil quality; add organic matter.

4 Suppress weeds.

5 Break up soil compaction.

6 Scavenge nitrogen and fix nitrogen from the air.

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