Prussic Acid Poisoning

Prussic acid is once again a potential issue for cattlemen in the county. We had a wet start to this year’s growing season, perhaps a little too wet for many crops. As expected, things have now turned around and we are now very dry. In hopes of taking advantage of some good moisture and to increase hay supplies there were lots of acres put into haygrazer and forage sorghum in the area this summer. Many have already had a good first cutting and have some regrowth that could be grazed. However, these plants along with johnsongrass are now getting stressed to the point that prussic acid will be a concern. You will need to be careful when grazing forage with the potential to build up high levels of prussic acid. If prussic acid is a concern, you should consider testing forages for high levels of prussic acid. While most labs can perform this test, the Texas Veterinary Diagnostic Laboratory does an excellent job, and we have the forms and collection instructions available at the office.

Prussic acid poisoning is a threat to grazing livestock that are grazing forages that may accumulate prussic acid. The threat of prussic acid accumulation is much greater when these forages are stressed from drought, frost, herbicide injury or other types of stress. Forages that have the potential to accumulate prussic acid at high levels include: Johnsongrass, grain sorghum, forage sorghum, and sorghum-sudangrass hybrids. Sudangrass, sudangrass hybrids, and pearl millet have a low to moderate chance of accumulating prussic acid.

Prussic acid in livestock occurs when they graze forage with high levels of prussic acid otherwise known as cyanogenic glucosides. These glucosides are actually sugar compounds that are broken down in the rumen; this frees the hydrocyanic acid (HCN) otherwise known as cyanide from the sugar. Cyanide poisoning in livestock causes respiratory stress and is very fast acting; death will occur quickly. Treatment is possible with sodium nitrate and sodium thiosulfate and treatment can be effective but must occur quickly after the initial symptoms to be successful.

Perhaps the best way to guard against prussic acid poisoning is to cut the forage for silage or hay. This will allow the HCN to volatilize. It is best to store the hay for some time to be sure volatilization has occurred. If you are forced to graze fields that have stands of grasses that are susceptible to storing moderate to high levels of prussic acid, I do recommend testing before turning cattle out. However, since prussic acid is volatile and the levels in the plants can change quickly, I also recommend that you never turn hungry cattle onto fields at risk, even those that have been tested. Make sure the cattle are full of hay or other forages before turning them out, this will prevent the cattle from overloading on the at-risk forage and lessen the risk of prussic acid poisoning. Turning out one or two initially, rather than the whole herd is also a good idea. As always watch your cattle closely for a day or two after turning them out on at risk fields.

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