By Rick Lundquist
A couple of weeks ago one of my business partners received a call from a client in Wisconsin asking whether he could turn some heifers out on some frost damaged sorghum. He advised him not to, because of the risk of toxicity from nitrates or from prussic acid. The farmer decided to chop it and bag it for silage. Some sorghum was inadvertently blown over the fence as he was chopping next to an adjacent field with grazing heifers. Shortly after this, two heifers were found dead next to the fence line.
Many of the same environmental factors are associated with both nitrate toxicity and prussic acid (cyanide) poisoning. Conditions that retard growth, such as drought, can cause nitrate to accumulate in the stalks and stems. Wisconsin and much of the Upper Midwest had a warm and dry September followed by an unusually cold and wet October, with several frosts. The frost, followed by re-growth of the sorghum can increase the risk of prussic acid accumulation in the leaves of the plant.
The forage was tested for nitrate and had a safe level (240 ppm), although this may not be conclusive because of the variability of nitrate levels within a single field. Nitrate levels above 1,000 ppm can be risky for pregnant animals, since abortions may result. Levels greater than 4,000 ppm are toxic and should not be fed. Nitrate is converted to nitrite in the rumen, which accumulates and reduces the oxygen carrying capacity of the blood by changing hemoglobin to methemoglobin. Methemoglobin cannot transport oxygen, so the animal suffocates. Treatment for nitrate poisoning is immediate intravenous administration of a 2% methylene blue solution by your veterinarian.
Lab results are still pending for cyanide, but cyanide poisoning seems likely given the conditions. Dhurrin, a cyanogenic glycoside, is highest in the leaves of new sorghum or the re-growth after frost. Grain sorghums are highest in cyanide potential, Sudangrass is lowest and the hybrids are intermediate. Perennial grass sorghums like Johnsongrass can also be highly toxic. The toxic dose of cyanide in cattle is 0.6 – 1.0 gram. Concentrations above 500 ppm are high risk, depending on the amount consumed. The signs of cyanide intoxication can occur within 10-15 minutes after ingestion of the plants. There is a rapid onset of distress, incoordination and labored breathing followed by death. There is no real treatment since the toxicity progresses so quickly.
Nitrate levels in silage will decrease as much as 40% after fermentation, but cyanide potential usually does not. Considering this, feeding the silage made from the sorghum on this Wisconsin farm would be very risky.
--Rick Lundquist is an independent nutrition and management consultant based in Duluth, Minn. You can contact him at firstname.lastname@example.org.
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