Cover Cropping For N Return: Penn State Research Results

July 23, 2013 10:13 AM

Researchers at Penn State's Agronomy Research Farm at Rock Springs, PA have released results from a 2011-12 experiment on the N returns of four different legume cover crops after wheat harvest. Late planted cover crops are best when allowed to grow to their full nitrogen fixing potential before knockdown. In fact, this research shows that winterkill allows for the best nitrogen capture as the plant generally has a longer life.

The Penn State research included fava beans, grain-type soybeans and sunn hemp. The best type of soybean to use is a forage-type soybean, but seed can be hard to find. The results of the study showed that 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 legume species and the planting date after wheat harvest in central PA 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 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 research 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 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.
  • The researchers 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.


Charlie White, Penn State Extension worker writes, "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."

White continues, "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."

Timing plays its role here, and selecting the right cover crop can make a tremendous difference in available nitrogen in the spring. If corn prices are going to continue on at a low, growers would do well to take advantage of all nitrogen capturing opportunities. Look for a crop that will winterkill or terminate close to spring and provide supplemental N when emerging plants can use it to get a leg up on vegetative growth.

Based on a report by:
Charlie White, Extension Associate, Sustainable Agriculture, Penn State 

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