Frosted sorghum pasture poses prussic acid poisoning risk

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Forage sorghum, sudangrass and sorghum-sudangrass hybrids (sudax) are exceptionally heat and drought tolerant annual crops that are used for cover, grazing, green chop, hay and silage. Because of the drought and hay shortage, many acres of these crops have been planted this year for emergency forage in Michigan. The recent rains boosted growth and many producers are grazing the forage. Care is needed when utilizing these forages for cattle, sheep and goats because of risks related to prussic acid poisoning. Forages in the sorghum family should never be fed to horses because of risk of a poorly understood but sometimes fatal urinary problem.

All sorghum family plants can cause prussic acid or cyanide poisoning in livestock. These plants contain a secondary compound called dhurrin, which is enzymatically converted to toxic prussic acid, also called hydrocyanic acid, in wilting forage. Dangerous wilting can be caused by drought, frost, cutting, trampling, or even just by chewing. Prussic acid poisoning can kill animals quickly. Animals develop some tolerance if they are continuously pastured on these forages, but producers should be alert to any conditions that cause sudden wilting of the forage. Leaves contain more toxin than stems. Toxicity potential is greatest in seedlings, lush, dark green new leaves, droughty forage and frosted forage. Because of concentration of toxin in new leaves, sorghum forages should never be grazed or fed as green chop until plants are at least 18 to 24 inches tall. This is approximately belly high on a mature cow.

Prussic acid poisoning is a veterinary emergency. Signs can occur as soon as 10 minutes after consuming a toxic dose. Prussic acid interferes with the ability of blood to carry oxygen, causing animals to die of suffocation. Affected animals show labored breathing, excitement, gasping, convulsions, paralysis, staggering and death. Usually animals are simply found dead because progression is rapid and easily missed when animals are not under continuous observation.

When a killing frost is expected, animals should be removed from the pasture until the frost-killed herbage is completely dried (usually five to six days). If new shoots form after frost, animals should not be allowed to graze until the new shoots are 18 to 24 inches tall, because they may preferentially graze the new growth which will be high in prussic acid.

When sorghum-family forages are cut for hay, prussic acid dissipates as the hay dries and hays are safe to feed once bales have reached the stable storage phase. This is also why frosted sorghum-sudangrass is safe to graze after it has field-dried to “standing hay.” “Green” sorghum hays that are still in the heating phase should not be fed. Prussic acid is destroyed by ensiling and is gone by the time fermentation is complete. A good rule of thumb is to wait three weeks after harvest before feeding hay or silage made from sorghum-family forages.

Sorghum-family forages can be used safely, but require careful attention from producers to reduce the risks.

For more information, contact Michigan State University Extension forage specialist Kim Cassida at 517-355-0271 ext. 1194.



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