(Editor’s Note: This article is an excerpt, slightly modified, from the new K-State publication Efficient Crop Water Use in Kansas, NF-3066, available at: http://www.ksre.ksu.edu/library/crpsl2/mf3066.pdf -- Steve Watson)
Cover crops do not produce a marketable product, but they benefit rotations by increasing organic matter, maintaining surface residue (which reduces evaporation), reducing nitrate leaching, reducing soil erosion, suppressing weeds, and adding diversity to crop sequences.
Cover crops or mixtures with carbon to nitrogen ratios (C:N) greater than 25:1 generally increase longevity of residue and may tie up available nitrogen, making it less available to the next crop. Cover crops or mixtures with C:N ratios less than 25:1 generally cycle nitrogen more quickly. Nitrogen in these residues is relatively more available, and a sizable fraction may be released in time to be used by a following summer annual crop or may speed the breakdown of accumulated low-nitrogen residues from previous crops such as wheat, corn, or sorghum.
Research with cover crops conducted at K-State demonstrated the influence of cover crops in different rotations. The figure below shows the influence of late-maturity soybeans and sunn hemp in a wheat-sorghum rotation at Hesston. A late-maturing soybean cover crop increased grain sorghum yields with 60 pounds per acre or less of nitrogen fertilizer, but generally had no yield benefit compared to no cover crop when nitrogen rate increased to 90 pounds per acre. Sunn hemp resulted in greater sorghum yields at all nitrogen rates, although the yield benefit was less with more fertilizer nitrogen.
When averaged over nitrogen application rates, the long-term grain sorghum yield benefits from late-maturing soybean and sunn hemp cover crops amounted to 8.8 and 14.9 bushels per acre, respectively.
Sorghum response to cover crops in a wheat-sorghum-soybean rotation at Manhattan was similar. With less than 80 pounds per acre of fertilizer nitrogen, sorghum planted after double-crop soybeans or cover crops with C:N ratios less than 25:1 (late-maturity soybeans, winter pea, winter canola) yielded more than sorghum after no cover crop. Application of 160 pounds of nitrogen fertilizer per acre was required for sorghum planted after the sorghum-sudangrass cover crop to produce yields comparable to sorghum after other cover crops or after no cover crop. Sorghum-sudangrass produced large amounts of residue with a high C:N ratio that likely immobilized much of the residual and fertilizer nitrogen.
A summary of cover crop characteristics is listed in the table below. Grazing or cutting a cover crop for hay shifts it from being a true cover crop to being a forage crop. Timing of termination of cover crops is important and depends on what crop is being planted next, especially in more water-limited environments.
Annual forages or cover crops were grown in place of fallow in a wheat-fallow no-till cropping system between 2009 and 2012 at Garden City (see figure below). Wheat yields were similar whether the previous crop was harvested for forage or left standing as a cover crop. Wheat yield following the previous crop or fallow was dependent on precipitation during fallow and the growing season.
When moisture was limiting and wheat yields following chemfallow were less than 35 bushels per acre, growing a crop during the fallow period reduced wheat yield. When wheat yields following fallow were greater than 70 bushels per acre, only winter triticale, grain peas, and continuous wheat grown in place of fallow reduced yield.
Averaged across all years, wheat yield following continuous wheat was 41 percent less, and following grain peas was 21 percent less compared to wheat-fallow. Wheat-fallow averaged 56 bushels per acre. Cover crops never increased wheat yields. Annual forages and grain peas can increase profitability, but cover crops commonly reduced profitability compared to wheat-fallow.