Finding the ‘just right’ volume of cover crop growth

Written by Jonathan Eisenthal

How can farmers get the maximum benefit from cover crops in a corn-soybean operation? One scientist tackled the topic at the Fourth Annual Minnesota Nitrogen Management Conference, held recently in St. Cloud.

Matt Ruark

Matt Ruark, an associate professor in soil fertility and nutrient management at the University of Wisconsin, has been looking at a number of cover crop species, including winter rye, spring barley and various clovers. Cover crops can be used to control erosion and nutrient loss, and can also contribute nutrients to help the cash crop succeed. It turns out that the volume of biomass generated by the cover crop is crucial to meeting those goals.

“You’ve got a small window. You’ve got to have enough biomass that you have good erosion control, (and) to take up nitrogen…but you don’t want too much,” Ruark told an audience of farmers, crop consultants and fellow scientists. The sweet spot falls in the one-third to half ton of biomass, according to Ruark. Short of that mark and farmers will miss out on the environmental benefits of cover crops; too much biomass will cause a detrimental agronomic effect from cover crops.

When his experimental plots developed three-quarters of a ton of spring barley, or one-and-a-quarter ton of winter rye, those strips showed a significant yield drag compared to strips where no cover crop was used. Ruark noted, however, that these high cover crop volumes occurred in years with excellent growing conditions and the cash crop yields were still bountiful—there was nothing visually wrong with the corn plants and they still yielded 230 to 250 bushels per acre. The yield drag was only apparent through the comparison strips where no cover crop had been planted.

Ruark, who also serves as the faculty advisor for the UW-Discovery Farms program, presented other research in which he examined which cover crop species can deliver additional plant-available nitrogen. A key metric is the ‘carbon-to-nitrogen’ (C-N) ratio of the plant. Those with a high C-N ratio spend most of their nitrogen in the process of decomposing their carbon, while those with a low C-N ratio deliver additional nitrogen for the use of farm crops. This is a cost benefit—allowing farmers to put on less commercial fertilizer. There is also an environmental benefit in having nitrogen in a form that’s less apt to leach from the field. In this research, clovers came out as the clear winners.

You can read more about this research at AgWater Exchange, the Discovery Farms blog. You can also learn more about Ruark and his research team at his web page.

The “Nitrogen: Minnesota’s Grand Challenge & Compelling Opportunity Conference” where Ruark spoke took place Feb. 6 in St. Cloud, and was organized by the University of Minnesota Extension Service with the support of the Minnesota Corn Growers Association.

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