Is there a fallow period in your crop rotation after wheat harvest? If so, you have an excellent opportunity to seed various legume cover crop species to supply nitrogen (N) to your following crop. Like biological fertilizer factories, legume cover crops fix nitrogen from the atmosphere and incorporate it into their plant tissue. Upon decomposition, nitrogen from the cover crop is released into the soil where it can be used by the next crop. Here are some results from an experiment in 2011-12 where we planted different types of legume cover crops after wheat harvest at the Penn State Agronomy Research Farm at Rock Springs.
Because there is a month or more of warm summer weather after wheat harvest, summer annual legume cover crop species may have the potential to fix worthwhile quantities of nitrogen before winterkilling. Of the three summer annual cover crop species we planted on August 5, 2011, fava bean accumulated an aboveground biomass nitrogen content of 140 lbs N/ac, soybean accumulated 70 lbs N/ac and sunn hemp only accumulated 35 lbs N/ac prior to winterkill.
Sunn hemp is a tropical legume species and the planting date after wheat harvest in central Pennsylvania did not leave enough growing degree days left in the season to make the cover crop worthwhile. Sunn hemp growth would be greater when planted in southeastern Pennsylvania after wheat harvest in mid-July, or better yet planted in early June as a full season summer cover crop.
The soybean cover crop accumulated a modest but acceptable quantity of N. The best type of soybean to use as a cover crop is typically a forage-type soybean, but seed can be difficult to come by. Instead, we used a maturity group 3.6 grain-type soybean variety seeded at 31 lbs/ac. At this seeding rate the stand was a little bit sparse, so increasing the seeding rate to 50 lbs/ac likely would have boosted the per acre biomass N accumulation.
Fava bean, drilled with a seeding rate of 100 lbs/ac, emerged with the highest N accumulation of the three winterkilled species we tested in 2011. Fava bean thrives in the cooler weather of late summer and early fall and continued to grow through October, several weeks longer than soybean and sunn hemp did.
We also planted a winter-hardy legume cover crop, Medium red clover. By late April 2012, the red clover had accumulated 140 lbs N/ac in its aboveground biomass, a similar amount as the fava bean had accumulated the previous fall.
Following the cover crops, we planted a corn crop and applied rates of nitrogen fertilizer between 0 and 160 lbs N/ac to measure the yield response of the corn to nitrogen fertilizer. We found that red clover supplied the most nitrogen to the corn crop, with a fertilizer equivalency of 160 lbs N/ac. Soybean and fava bean supplied similar quantities of N with a fertilizer equivalency of 100 lbs N/ac. Sunn hemp’s nitrogen contribution to the following corn crop was negligible.
Despite the similar biomass N accumulation of the fava bean and red clover cover crops, red clover was more effective at supplying N to the following corn crop. This is likely because the N release from red clover terminated in the spring was in better synchrony with corn N uptake. Nitrogen released in early spring from the winter-killed fava bean cover crop may have leached into the subsoil before the corn crop could take it up. Our work to understand the dynamics of N supply from cover crops, particularly how cover crop mixtures might improve efficiency, is continuing and we will keep you up to date on our findings.