By Kevin Shelley, UW Nutrient and Pest Management Program
Following harvest of winter wheat or other small grains in Wisconsin, if not planted to alfalfa, these fields are often left fallow. However, with more than 40 percent of the growing season remaining, planting a cover crop may be a good option. While the economics may not always be clear, many farmers are looking to cover crops to keep the soil covered, suppress some of the weeds that may otherwise grow, recycle and/or fix nutrients and improve soil health, and functioning with additional organic matter. Producing supplemental forages, managing field nutrient budgets and meeting conservation requirements are other objectives for which cover crops can provide value.
The choice of which cover crop(s) depends on a farmer’s objectives and needs and also the farm’s capabilities in terms of planting, management and termination. The cost and availability of good quality seed, versus anticipated benefits, are other factors to consider. Below are a few of the tried and true options for use in most parts of Wisconsin. All can be seeded with light tillage or no-till planting. However, good seed to soil contact at the appropriate depth for the species is essential for good germination and establishment.
Spring cereal grains, oats or barley can provide reliable mid-late summer cover and optional forage potential. They will grow rapidly in late summer and continue until a hard freeze. They will usually not over-winter in Wisconsin. These crops are often the best choice as a sequentially seeded soil cover or if fall-harvested forage is the main goal. They are more forgiving of temporary dry conditions than legume covers. Oats and barley have had equal yields in fall forage trials, 1-3 tons of dry matter (TDM) per-acre, with spring triticale slightly lower. 
Winter rye can be planted August-September for a late summer and over-winter cover. Stem elongation will not occur without vernalization (cold temperatures). Planted in August, rye will produce a thick cover but usually less than one TDM biomass before winter dormancy. It will grow rapidly in early spring. Rye as a cover crop should be terminated by late April before it grows too large and at least two weeks prior to planting if followed with corn.
Annual ryegrass (ARG) is actually a southern-US adapted winter annual. It is considered not cold tolerant but will sometimes over-winter in Wisconsin with mild conditions. It has rapid growth with good biomass production when summer seeded on most soil types. It has a shallow, fibrous root system desirable for erosion control. ARG can be a good compliment for brassicas and/or annual clover. However, although a somewhat popular and economical cover crop option, planting ARG is somewhat discouraged due to concerns with its potential to become a difficult to control weed. It can be a prolific seed producer, even in the seeding year, and several glyphosate resistant biotypes have been identified. If it over-winters, it can be difficult to control with herbicides.
Legumes such as berseem clover, crimson clover or field pea (annuals) as well as medium red clover (perennial) will accumulate biologically-fixed nitrogen (N) as they grow. The N is released back into the soil, becoming available for next year’s crop, after the legume plants die or are terminated. All are good choices for a wheat to corn grain crop rotation. Clovers may also be harvestable as forage by mid-late September.
The annual legumes will grow quickly when planted in mid-summer if moisture is sufficient. Berseem and crimson clovers may produce up to 2 TDM per-acre, but 1-1.5 TDM is more common. Research data on N credits is limited. A two-year UWEX field trial in Sheboygan County, Wisconsin has shown either a small credit and/or a 10 bu/acre corn yield increase each subsequent year. Berseem clover has a more upright growth habit and is better suited to mechanical forage harvest. Crimson has lower, more prostrate growth and is often used for winter grazing in southeastern US. If weed pressure is high, it may be advantageous to plant annual clovers in a mixture with oats.
Medium red clover (MRC) can be seeded after wheat harvest but is best when companion seeded early in the spring. A common method for MRC establishment is frost seeding, or broadcast seeding into fall-established wheat early the following spring. Early-planted MRC will normally yield more biomass and creditable N (60-80 lbs/acre) than sequentially seeded annual legumes. As a perennial, with vigorous growth potential the following spring, termination of MRC is best started, chemically or with tillage, in the fall.
Field pea is a large-seeded, cool season annual, best companion-seeded with a spring cereal grain to encourage climbing and minimize lodging. Pea-small grain mixtures can also be harvested as forage, with similar yield, but slightly higher forage quality and palatability than small grain forage alone. Field peas, however, provide only a minimal N credit to a subsequent crop and will increase the cost of the seed mix.
For more complete management and selection information on these and other mid-summer cover crop options, including brassicas (radish, turnips and rapeseed) and species mixtures, see the UW Extension Cover Crop Workgroup’s website, Cover Crops in Wisconsin at http://fyi.uwex.edu/covercrop/. From the home page, click on the Selecting Cover Crops for WI tab and then on Wheat.