Vet urges livestock producers to check nitrate, prussic acid levels in forages
Topsy-turvy weather this summer and fall posed plenty of challenges for livestock producers--one of which is the possibility of increased nitrate and prussic acid levels in forages, according to Kansas State University veterinarian Larry Hollis.
Producers in some parts of the state saw new plant growth after much-needed rains this fall, following months of severe drought. Testing of some forage supplies, however, indicated nitrate levels above safe levels in some cases, said Hollis, who is a beef cattle specialist with K-State Research and Extension.
Dave Kehler director of the K-State Research and Extension office in El Dorado, Kan., is encouraging producers to test before turning cattle out on new-growth forages.
"We have sent samples of corn, forage sorghum, milo and millet," Kehler said. "Only one of those tests came back with a safe nitrate level. One was medium (7,070 parts per million dry matter basis). The others have been high (10,000 to 15,000 ppm) and some others over 18,000."
Kehler said that all prussic acid tests that he's aware of in his area have been low, but they were all taken before a recent fall freeze.
"A lot of this unusual nitrate situation is a result of producers fertilizing their crops with nitrogen and the nitrogen not being utilized from the soil because of the low level of grain or forage production resulting from the drought," Hollis said. "The available nitrogen in the soil was only partially utilized by the original plant growth. Some of the original growth was already high in nitrate because of poor movement of nitrogen into the upper parts of the plants (leaves and corn ears or sorghum heads). The regrowth of plants that were still alive, or new growth of volunteer plants from scattered corn or milo seed, pulled a lot of available soil nitrogen into these live plants, resulting in the high nitrate levels currently being detected."
In addition to the nitrate levels, Hollis is encouraging producers to check for prussic acid levels in any forages that were green and growing at the time of the recent freezing nights.
"If leaves on the plants had already turned brown and dried up prior to any recent rain, then prussic acid is not a concern," he said. "If re-growth of green leaves, development of tillers, or volunteer growth of plants occurred, then prussic acid could be a problem."
Hollis also fielded at least one report of high nitrate levels in both turnips and oats.
Given the extreme weather conditions, we are seeing toxic levels in plants that we don't normally suspect," Kehler said. "For this reason, we are encouraging producers to have tests done."
"The bottom line is, test for nitrates before turning cattle out on any new crop growth," Hollis said.
More information can be found in the K-State publications "Nitrate Toxicity" online at http://www.ksre.ksu.edu/library/crpsl2/mf3029.pdf and "Prussic Acid Poisoning" http://www.ksre.ksu.edu/library/crpsl2/mf3040.pdf.
How do high nitrate levels affect livestock?
Nitrate in itself is not toxic to animals, but at elevated levels, it causes a disease called nitrate poisoning. Nitrates normally found in forages are converted by the digestion process to nitrite, and in turn the nitrite is converted to ammonia. The ammonia is then converted to protein by bacteria in the rumen. If cattle rapidly ingest large quantities of plants that contain high levels of nitrate, nitrite will accumulate in the rumen. Nitrite is 10 times as toxic to cattle as nitrate.
Nitrite is absorbed into red blood cells and combines with hemoglobin (oxygen carrying molecule) to form methemoglobin. Methemoglobin cannot transport oxygen as efficiently as hemoglobin, so the animal's heart rate and respiration increases, the blood and tissues of the animal take on a blue to chocolate brown tinge, muscle tremors can develop, staggering occurs, and the animal eventually suffocates.