Cover crops do not produce a marketable product, but they can potentially 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. Their effects on crop yields and soil water use also need to be considered. There are many factors to take into account when planting cover crops.
Nitrogen tie-up and residue longevity
Cover crops or mixtures with carbon to nitrogen ratios (C:N) greater than 40:1 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 20:1 generally cycle nitrogen more quickly. Nitrogen in these residues is relatively more available, and some 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.
Effect on yield of subsequent crops
Research with cover crops conducted at K-State demonstrated the influence of cover crops in different rotations.
Hesston. 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 when 60 pounds per acre or less of nitrogen fertilizer was applied to the sorghum. But the late-maturity soybean cover crop generally had no yield benefit when the nitrogen rate was 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.
Figure 1. Average sorghum yield response to preceding cover crop and nitrogen fertilizer over six years at Hesston.
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.
Manhattan. 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, cover crops with low C:N ratios (soybeans, winter pea, and winter canola) improved sorghum yields. If more than 80 pounds of nitrogen was applied, those cover crops had little effect on sorghum yields. The situation was much different where the cover crop was sorghum-sudangrass. Sorghum-sudangrass produces large amounts of residue with a high C:N ratio. As a result, this residue can immobilize residual and fertilizer nitrogen to a greater extent. Sorghum-sudangrass cover crops reduced grain sorghum yields unless 160 pounds of nitrogen fertilizer per acre was applied to the grain sorghum.
Figure 2. Average sorghum yield response to preceding cover crop and nitrogen fertilizer over two years at Manhattan.
In 2012, dry conditions pushed sorghum planting and emergence much later than desired at Manhattan, reducing yields to 65 bushels per acre and less. Sorghum did not respond to nitrogen rate, but did yield more after the double-crop soybeans (DCSB) and late-maturing soybean cover crop (LMSB) compared to no cover crop and compared to cover crops that included either sorghum-sudangrass or oats.
Table 1. Effect of cover crops on grain sorghum yield at Manhattan in 2012.
Preceding cover crop
Grain sorghum yield (bu/acre)
Garden City. 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. 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.
Over the period of years of this test – 2009 to 2012 – the effect of cover crops on wheat yields varied according to the yield environment of the wheat. When moisture was limiting and wheat yields following chemfallow were less than 35 bushels per acre, growing a cover or forage 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 in a continuous wheat cropping system 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 if they are harvested for hay or seed, but cover crops commonly reduced profitability compared to wheat-fallow. Grazing or cutting a cover crop for hay shifts it from being a true cover crop to being a forage crop.
Figure 3. Average wheat yield response to preceding crop or fallow at Garden City from 2009 to 2012. The crop rotation was wheat-fallow.
Cover crops mixtures
There has been interest in looking at mixtures of cover crops of more than two species. Mixtures containing many species have been called “cocktails,” and some have speculated having a mixture is more beneficial than single-species or two-species mixtures. It was speculated that these mixtures should contain around six species.
Therefore, the cover crop study at the Southwest Research Extension Center in Garden City was modified in 2012 to answer this question. Cover crops were grown in place of fallow in a wheat-grain sorghum-fallow no-till rotation. Cover crop treatments were safflower (grown for grain), spring pea (grown for grain), spring oat, spring triticale, spring pea, a mixture of spring pea and spring triticale, a mixture of spring pea and spring oat, and a cocktail (mixture of spring oat, triticale, pea, buckwheat, forage brassica and forage radish).
Wheat yields were measured in 2013. Wheat yields were highest following fallow in this low-yield environment (15 bu/acre). Growing a crop during the fallow period reduced yields. Similar to previous research, when moisture is limiting, growing a crop during the fallow period reduces yields. The cover crop cocktail was one of the lowest yielding treatments in 2013.
Figure 4. Effect of cover crops and cover crop mixtures on wheat yields at Garden City in 2013.
Cover crop characteristics
There are many reasons for planting cover crops in addition to the factors mentioned above. Other characteristics may be of overriding importance. A summary of cover crop characteristics is listed in the table below.
Kraig Roozeboom, Cropping Systems Agronomist
John Holman, Southwest Research-Extension Center, Cropping Systems Agronomist
Josh Jennings, CCA, Graduate Student, Agronomy