Nitrate levels won't decrease in corn that has stopped growing because of severe drought stress, a South Dakota State University specialist said.
Drought can lead to high nitrate levels because it interrupts the process by which plants convert the nitrate they've taken from the soil into ammonia, which in turn is incorporated into plant protein. As a result nitrate can accumulate in the plant.
Fifty-three percent of the corn samples tested at SDSU as of July 30 showed potentially toxic levels of nitrates. Hall cautioned that producers shouldn't expect any change in their nitrate levels if they're in an area where drought has shut off any potential new growth in corn.
"Essentially there are two types of corn out there. There is some corn that is stressed to the point that a rain would not allow it to recover," Hall said. "And you're going to have some where an additional rain would allow some growth to occur. If growth occurs, it has the potential to lower the nitrate content somewhat. But you've got to be prudent and still do a nitrate test on it if you're going to feed it for forage."
Producers also should wait five to seven days after a rain before harvesting drought-stressed corn so that the plant has an opportunity to metabolize nitrate, Hall said.
In areas where corn is so severely stressed that it will not recover and grow more, ensiling is the best option for producers. Letting such corn stand in the field may change moisture content slightly over time, Hall said, but will have no effect on nitrate levels.
"If it's stressed to the point that when it gets a rain on it and nothing happens, the nitrate content in that is still there," Hall said. "In that corn there is only one thing that will drop the nitrate content in those stalks. That is the fermentation process through the ensiling process."
Proper fermentation during ensiling can lower the nitrate level in forages by anywhere from 20 to 60 percent. More information about proper ensiling is available in SDSU Extension Extra 4017, "Harvesting and Feeding Drought-Stressed Corn." The publication is available online at: http://sdces.sdstate.edu/drought/.