As feed prices have risen, many livestock producers have turned to alternative means for providing feedstuffs for their livestock. One popular option has been to follow wheat harvest with a planting of sorghum. While a sorghum crop can be a valuable resource for feeding, it can also pose some hazards. When talking about toxins in a sorghum or sorghum cross plant, most commonly we are concerned with nitrate poisoning; especially during times of drought. However, there is another hazard that fewer people are aware of. It is prussic acid.
Prussic acid may also be referred to as hydrocyanic acid or HCN and is one of the cyanide toxins. It is produced by a glycoside called dhurrin. The dhurrin is in the epithelial cells of the plant. When these cells are damaged or stressed they can release the prussic acid. Some of the common causes of this are wilting, extreme heat or drought, livestock or mechanical trampling, and frost. Additionally, dhurrin can release prussic acid during times of rapid growth, causing a toxic build up in young, tender growth; which is also a very tasteful stage in the opinion of many livestock.
Prussic acid interferes with a red blood cells ability to transfer oxygen. So the oxygen becomes trapped in the blood and does not transfer to cells and tissue that need it. So in essence, the animal suffocates. Some signs of prussic acid poisoning are excessive salivation, rapid breathing, muscle spasms, staggering, and collapse. Death can occur in as little as ten minutes following the consumption of the toxin. Often times deceased victims will be found with forage still in their mouths.
Many people confuse prussic acid poisoning with nitrate poisoning. It is important that producers understand that the two conditions are caused by different means, have different modes of action in the animal’s illness and death, and most importantly have different active stabilities. In other words, as most of you are probably aware of, forage that has nitrate levels high enough to cause nitrate poisoning can maintain those toxic levels for extended periods of time (many months). Prussic acid, on the other hand, is a short lived condition and levels of the toxin should decrease in the plant a week or two after the damage has occurred.
There are a couple of situations where this rule fools some people. First is during young growth. If the plant is continuing rapid growth it is going to continue prussic acid production, so it may take more than a couple of weeks for it to be safe. Frost induced prussic acid can also fool producers. Sometimes only part of the crop will be frosted and a producer will assume that it is safe a week later. Then a second frost will come along and cause the remainder of the crop to become temporarily toxic.
Sorghum is not the only plant that can cause prussic acid poisoning problems. Other common suspects are johnsongrass, sudangrass, sorghum-sudan hybrids, choke cherry trees, and wild cherry.