Increasing cropping intensity with cover crops
(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.
click image to zoomAverage sorghum yield response to preceding cover crop and nitrogen fertilizer over six years at Hesston. 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.
click image to zoomAverage sorghum yield response to preceding cover crop and nitrogen fertilizer over two years at 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, 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.