High nitrate levels pose problems

Aug 24, 2012
by DTN

Across the Corn Belt this summer, samples of droughtstressed corn and forage are streaming into agricultural labs for nitrate testing. While livestock deaths from toxic levels of nitrates have remained low this year, less immediate results of nitrate poisoning, such as the abortion of a fetus and lower rates of gain, may show up later this fall, experts told DTN.

Nitrates are normally converted into protein by a growing plant. But in severe droughts such as this one, a plant will halt the protein conversion as its growth stalls, yet continue to take up nitrogen from the soil. Nitrates can then accumulate to toxic levels in the stalks of plants such as corn, sudangrass, sorghum, and even weeds such as pigweed and waterhemp.

Not surprisingly, high nitrate levels have been a prominent concern for producers this summer.

“It’s what you would expect in a drought year,” Dave Mengel, a Kansas State University agronomist, told DTN. He oversees the university’s soil-testing lab, which has seen an unusually high number of nitrateheavy forage samples since late May. He estimates twothirds are above feed safety levels of 2,500 parts per million (ppm).

In the Hastings, NE, office of Servi-Tech Laboratories, supervisor Hans Burken said his staff is processing 100 high-nitrate forage samples a day, mostly from producers who have been forced to chop dead cornfields for silage. He said the majority will be usable for feed, but

only with very careful blending of additional forages to bring the nitrate concentration down to a safe level.

Most experts agree that proper ensiling can also reduce nitrate levels by up to 50 percent.

Extension offices have worked hard to keep producers aware of the danger of feeding corn and hay with high nitrate levels. The result has been an encouragingly low number of livestock deaths from nitrate poisoning this year. However, one insidious delayed effect of nitrate poisoning— the abortion of an animal’s fetus—will likely show up later this year during pregnancy checks, Robert Kallenbach, a forage specialist at the University of Missouri, told DTN. “We could see a lot of open cows this fall,” he said.

When a cow’s digestive system is presented with a high level of nitrates, her rumen may lack a sufficient number of microbes to convert the compound into protein. Instead, nitrates are absorbed directly into the bloodstream, where they block oxygen and starve cells, Kallenbach explained.

Over time, a cow can actually adapt to higher and higher levels of nitrates if she is consistently exposed to them, Sandy Johnson, an Extension livestock specialist at Kansas State’s Northwest Research and Extension Center. But while a mama cow may tolerate certain levels of nitrates in her feedstuff without dying, that same level could be too high for her fetus to survive. “She could look the same but lose the calf,” Kallenbach said. The abortion may not always be visible to the rancher, either, since in early pregnancies, the cow’s body will reabsorb the fetus.

Another result of cattle consuming high-nitrate feeds can be reduced average rate of gain, Johnson said. “It can impact an animal’s performance,” she said. In areas like western Kansas, producers are accustomed to cyclical droughts, she noted, and they are familiar with mixing feeds to achieve safe levels. “But what’s different this year is dealing with the magnitude of the corn crop’s failure and how many of our feedstuffs we expect to contain nitrates,” Johnson said.

As a result, ranchers will be forced to feed more nitrateheavy rations than usual.

Producers should also be aware of the potential of high nitrate levels in common weeds such as pigweed and waterhemp. Mengel said his lab recently tested some pigweed with nitrate levels ranging from 2,200 to 13,000 ppm, well above a safe feed level. In a year when forage is especially scarce, cattle may eat weeds they would normally ignore, Kallenbach said. “Some cattle actually kind of like pigweed.”

In areas with access to free manure all forage should be grazed carefully, Kallenbach added. Already, his lab has seen some samples of fescue and bermudagrass—not normally very susceptible to this problem—with high nitrate levels, most likely as the result of heavy fertilization. — Emily Garnett, DTN