Cover crop residue can complicate nitrogen management in corn

COLUMBIA, Mo. – Farmers who rely on surface-applied nitrogen may have difficulty if they apply to fields with high surface residue, says University of Missouri Extension nutrient management specialist John Lory.

This is especially true for UAN solution, Lory says. Residue can temporarily tie up nitrogen fertilizer because it is taken up by microorganisms decomposing the residue.

“This is particularly a problem with liquid nitrogen applied on top of the residue,” says Lory.

More farmers are planting cover crops, so they are planting more corn into residue. Cereal rye is a popular cover crop because it grows well, is inexpensive and reduces soil erosion. “Farmers can plan ahead for this problem by injecting nitrogen into soil, either liquid or anhydrous ammonia,” Lory says. “This avoids contact and tie by the residue.”

The next best option is dry fertilizer where the prills may fall through the residue to the soil surface, he says.

MU Extension nutrient management specialist Peter Scharf’s research shows that rye can reduce N availability to corn even when the N is injected. This is likely due to rye uptake of soil N that otherwise would have been available to the corn. It is most pronounced if the N rate is below what it needs to be. This is one reason why early termination of rye before planting corn is a good idea.

Cover crops can lead to challenges with nitrogen management in corn. A farmer may have a system that relies on surface application of nitrogen. Challenges with timely termination of the cover crop because of weather or other factors may lead to a situation with heavy cover crop residue where the planned surface application may not work as well as in the past.

“The good news is that corn is very resilient when it comes to nitrogen management,” Scharf says. “There is time to apply nitrogen to stressed corn. University of Missouri research shows that early N deficiencies can be overcome by applying more N. You are likely to get full yield if you apply nitrogen by the time the corn is chest-high, and rescue applications are economically beneficial in terms of yield response until tasseling.”

Excess rain stopped some growers from getting into fields to burn down cereal rye cover crops this year. Cereal rye headed out and biomass increased.

Scharf’s rule of thumb: Nitrogen loss occurs when 12 or more inches of rain fall on poorly drained soils in May and June. Well-drained soils face loss at 16 inches for the period April through June. Much of the state is on target to reach that mark, with some of southern Missouri already past it, he says. More rainfall may cause nitrogen to leach below the root zone in well-drained soils. May’s cool weather has kept denitrification down.

Scharf reports excess rainfall and serious N deficiencies in parts of Missouri in 2008, 2009, 2010, 2011, 2013 and 2015. Deficiencies caused yield loss. 2016 is shaping up to join the list.