Watch out for nitrate poisoning in forages, forbs

News
Oct 23, 2009
by WLJ

—Reports received of common weeds poisoning livestock.

This year’s fall weather— rain and cloudy following a drought—and its effect on forages can be a recipe for nitrate poisoning of livestock, said a Texas AgriLife Extension Service expert.

And under these conditions, cattle don’t have to consume improved forages to be at risk, as many weeds also can build up high levels of nitrate, said Dr. Vanessa Corriher, AgriLife Extension forage specialist.

“In a recent incident, a Sabine County producer turned some cattle into a dry lot,” she said. “Though he supplied hay, the cattle apparently died of nitrate poisoning from eating pigweed in the lot.”

Corriher noted that livestock generally won’t consume weeds when they have quality hay available, but in this instance they did, and several cattle died as a result.

Forages and small grains that are susceptible to building up high levels of nitrate include sudangrass, sorghum-sudan hybrids, pearl millet, corn, wheat and oats, she said.

Weeds prone to build up high nitrate levels include Canada thistle, pigweed, smartweed, ragweed, lambsquarter, goldenrod, nightshades, bindweed, Russian thistle and stinging nettle.

Another risk factor is hay cut during or just after a drought period.

“This is especially risky if nitrogen was applied just prior to the hay harvest,” she said.

Though the high nitrate levels are associated with weather conditions, once the levels are built up in hay, the risk is not lessened over time, Corriher noted.

Nitrates are present in all forages, Corriher said. Strictly speaking, the nitrate poisoning should be called “nitrite” poisoning. With normal levels of nitrates, the range animal’s rumen converts the nitrate (NO3) into nitrite (NO2), which in turn is converted to ammonia, then into amino acids and then into proteins, she said.

But when nitrate levels are high in forages, the process becomes subverted, and high levels of nitrites are absorbed directly into the bloodstream through the rumen wall. The nitrite converts the hemoglobin in the blood into a form that cannot transport oxygen. The blood turns from a bright red to a chocolate color, and the animal essentially dies of asphyxiation, according to Corriher.

Corriher recommended producers regularly take forage samples from pastures and have them analyzed for nitrates, including samples of forages and weeds at various growth stages.

“Be sure to specify that you want nitrate analysis,” she said. “Standard nutritional analysis usually does not test for nitrates.”

Hay samples should be collected with a probe. Samples from several bales can be combined.

Unlike prussic acid poisoning, the risk of nitrate poisoning is not decreased over time, Corriher said. Hay harvested months ago could still contain the same high levels of nitrates it did when baled.

“Though the risk of nitrate poisoning is higher after a drought or an extended period of cool, wet weather, it’s something producers should be aware of year-round,” Corriher said.

AgriLife Extension’s Soil, Water and Forage Testing Laboratory can be contacted at 979/845-4816. Instructions and sample submittal forms can be found on the laboratory’s Web site at http://soiltesting.tamu.edu.

The Soil, Plant, Water Analysis laboratory at Stephen F. Austin State University in Nacogdoches, TX, also does forage analysis. Contact the lab at 936/468- 4500, or lyoung@sfasu.edu. — WLJ

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