Cover crops continue to grow in popularity, even in the face of depressed commodity prices. This is likely due to farmers' desire to protect their farm's soil and water, particularly in a time when there is increasing public scrutiny of the sustainability of agronomic practices.
As more farmers are trying cover crops, it is increasingly important that cover crop recommendations set them up for success. Overly optimistic recommendations can lead to a negative experience which may turn a farmer off to cover crops. A realistic recommendation can build small successes for the farmer and potentially lead to more acres covered and less soil lost to erosion.
Cover crops should be selected based on a farmer's goals and which cover can realistically meet those goals under Wisconsin growing conditions. Examples of farmer goals might be erosion control, growing nitrogen, producing an alternative forage and/or increasing organic matter. Typically, erosion control is one of the primary goals for using cover crops but this can be in combination with other goals.
Following are some examples of how cover crops can successfully fit into Wisconsin crop rotations.
There is plenty of growing season left after wheat harvest, allowing a wide range of cover crop options that can provide erosion control and other services such as growing nitrogen. Grasses, such as oats and barley, brassicas such as the oilseed radish (tillage radish), and several legumes are all options at this time of year.
Radishes deteriorate quickly in the spring so they should always be combined with a grass to provide enough spring residue to protect the soil. Barley and oats both make excellent partners for radishes with the added benefit of them all dying out naturally over winter. There have been claims that radishes provide "bio-tillage" to the soil. Research in Sheboygan County comparing radishes to a chisel plow pass suggests that the radish doesn't have the same impact as a plow but many farmers claim anecdotally that soil condition improves after radishes.
There are two annual legumes, crimson and berseem clovers, that can be planted after wheat and provide a nitrogen credit to a following corn crop. Extension research plots in Sheboygan County have shown berseem clover has a thicker stem that leaves more spring residue to better protect the soil. The plots have also been designed to analyze the nitrogen contribution that these clovers provide to corn but there haven't been enough study years for a recommendation. General Extension recommendation would be about a 40-60 pound credit from an annual clover.
Grass cover crops are generally the only options after corn silage since there is not enough time left in the growing season for oilseed radish or legumes. The date that the corn silage comes off should drive the selection of a grass cover crop. Barley and oats provide excellent fall cover and spring residue if they are planted early enough in the fall. In southern Wisconsin, they should generally be planted before Sept.15 to provide enough residue to protect the soil in the spring. Barley puts on more fall growth than oats so if there is a choice between the two, barley is preferable.
After Sept. 15, rye, wheat and triticale are the best choices since they continue to grow in the spring. For farmers who may take a spring forage cut, triticale and wheat both provide higher feed quality than rye. Triticale produces more tonnage than wheat so this is the better option of the two. Either way, farmers will need to plan on managing these cover crops the following spring with a herbicide ahead of planting their cash crop.
After corn and soybeans
Rye, triticale and wheat are the most reliable cover crop options after corn grain and soybeans. If soybeans come off early enough, the cover crop can be planted with a drill. For soybeans that come off later and for corn grain, farmers should consider using a highboy or airplane to plant the cover crops prior to harvest. In soybeans, cover crops can be planted by airplane when they are 50% senesced. In corn, the crop should be brown up to the ear before cover crops are planted. For optimal cover crop success, the corn or soybeans should be harvested less than two to three weeks after cover crop planting. Otherwise, the cover crop stand will be reduced.
Problems using rye
Agronomists in Wisconsin and surrounding states have observed some stunting in corn that followed a rye cover crop in certain years. Some USDA research in Iowa has found that there is likely a multitude of reasons for this. Physical planting problems into the thick rye residue and roots may be one of the major factors impacting the corn. The residue can prevent the seed slots from closing completely and cause sidewall compaction (particularly in wet soil), both leading to stunted corn. The research also suggests there is a slight increase in the prevalence of corn seedling diseases in corn that has been planted after rye, likely because rye can serve as a green bridge for some of the seedling diseases. And lastly, nitrogen deficiency may also cause the stunting if rye is tying up some of the soil nitrogen.
To avoid these stunting issues, farmers should be sure to plant their corn no sooner than 10-14 days after the rye has been terminated, apply some nitrogen at planting, and make sure not to plant in wet conditions. Having a corn planter with good down pressure on the press wheel can also help to close the seed slot completely and prevent some of these issues
Cover crop selection tool
For more information on selecting cover crops, farmers and agronomists can use the Midwest Cover Crop Council cover crop selector tool. This tool provides realistic recommendations tailored to a farmer's geographical region. This tool can be found at: http://www.mccc.msu.edu/selectorintro.html
Johnson is the Dane County Extension crops and soils educator.