Dry conditions can do more than just put yield limits on corn. These situations can also be cause for concern for growers harvesting their crop for silage.
Maurice Eastridge, Ohio State University Extension ruminant nutrition specialist, says that drought-stressed corn can accumulate nitrates that are toxic to livestock feeding on fresh crop or one harvested for silage. Nitrates are major constituents of fertilizers and, when processed properly by the plant, are converted to harmless proteins. Corn and some forages, like sorghum and sudangrass, are prone to nitrate toxicity.
"The problem arises when the plant takes up nitrates from the soil, but doesn't convert that into proteins," explains Eastridge. "Plants that are most at risk for accumulating nitrates are drought-stressed plants because they have not had enough moisture to convert nitrates. Nitrate toxicity is also a problem right after a rain event when the drought-stressed plant takes up nitrates too quickly."
Peter Thomison, an OSU Extension agronomist, says that delaying harvest is the best recommendation to minimize nitrate toxicity, especially if drought-stressed corn is suddenly subjected to rainfall.
"You don't want to harvest right after a rainfall," says Thomison. "If growers can wait two or three weeks after rainfall before harvesting to allow that nitrogen to be metabolized into amino acids and proteins, then that's the safest for the animal."
Once nitrates enter the body, they are converted to nitrites. Those nitrites accumulate in the blood and compete with iron on hemoglobin, cutting off oxygen. Symptoms of nitrate toxicity include labored breathing, pale mucous membranes and chocolate-brown-colored blood. The condition can be fatal if left untreated.
"In general, forages high in nitrates should not be grazed or fed as green chop or hay. Animals who feed on a fresh crop are most at risk for nitrate toxicity," said Eastridge. "And the variation of the nitrate concentration in the plant itself is important to note. The lower the area on the plant closest to the soil, the higher the nitrate concentration."
Research shows that the lower one-third of a corn stalk can have a nitrate concentration of as much as 5,000 parts per million. By comparison, the leaves accumulate less than 20 parts per million in nitrates.
Eastridge notes that corn, or other forages, stored after harvest cause less nitrate toxicity problems because the fermentation process reduces those toxic levels rapidly — anywhere from 35 percent to 80 percent over a 30-day period.
"Usually by the time the plant material has undergone fermentation and the material is fed to the animal, the nitrate level in the product has dropped low enough to where it's not much of a concern," said Eastridge.
Thomison recommends that silage be tested for nitrate levels just to be on the safe side.
"Take tissue samples in those fields that one may not be certain about, or of those drought-stressed plants that may have experienced recent rainfall," he suggests.
For more information on nitrate toxicity and how to manage, check out OSU Extension's Ohioline fact sheet "Nitrites in Dairy Rations" at: http://ohioline.osu.edu/asfact/0003.html.
The Ohio State University