Nitrogen-fixing cover crop options following small grains or peas

Jim Stute, Crops and Soils Agent, UW Extension – Rock County

Kevin Shelley, Outreach Educator, UW Nutrient and Pest Management Program

7/11/2006

Higher prices for nitrogen fertilizers increase the potential economic benefit of incorporating legume cover crops into cash crop rotations that include small grains, peas or other short-season crops in Wisconsin.  The legume cover may be able to add biologically fixed nitrogen (N) for the following crop while also smothering weeds and helping to build soil organic matter.

A number of field experiments comparing growth and N accumulation of several legume cover crops planted after harvest of peas or wheat have been conducted in Wisconsin since the early 1990’s. The following table presents N accumulation data for those legumes showing the best potential for mid-to-late summer seeding. This data shows herbage contribution only and does not account for N contained in roots which can be substantial, up to one-half the total. While N accumulation in the cover crop’s biomass provides an indication of creditable N for a following corn crop, trial results show that accumulated N and the optimal N rate for the following corn crop are not always directly correlated. However, response trials have shown corn yield is maximized in most cases with 60 to 80 lb./a of additional N (figure 1), verifying the UWEX credit of 50 to 120 lb. N/acre for most green manure (legume) cover crops.  Often there is no economic response to additional N. 

The decision to use a legume cover crop will be based, in part, on the expected value of the nitrogen provided relative to the cost of seed, seeding and subsequent control of the cover crop.  Soil quality and weed suppression benefits are relevant as well, but are likely less directly measurable.  The following table provides seeding information on recommended legumes.

Red clover will consistently produce significant creditable N only when it is companion seeded with, or over-seeded into, a small grain crop in the spring.  Hairy vetch can be a prolific N producer when planted later, but many farmers find difficulty dealing with it’s twining, vine-like herbage with later tillage or planting operations.  Further, un-controlled plants are prolific seed producers, which often become a weed management issue for subsequent crops. About 10 to 20 percent of vetch seed is “hard” which means it can lay in the soil for one or two seasons before germinating.  Hard seed content of annual medic can be even greater.  Berseem clover offers the advantages of quick establishment and good growth with moderate moisture and it dies over winter, making control the following spring a non-issue.  It is recommended to inoculate seed with the appropriate type of Rhizobia for best N fixation.

Two additional legumes, annual alfalfa and sweetclover are often tried as summer seeded covers, but not recommended for a variety of reasons including pest susceptibility and volunteer potential due to hard seed.  Other legume covers with reported potential for mid-late summer seeding include chickling vetch, crimson clover and field peas.  Additional experimental data is necessary for these in Wisconsin.  For additional information contact Jim Stute, 608-757-5696 or Kevin Shelley, 1-800-994-5853.