Dairy Pipeline: November 2002
Charles C. Stallings
Extension Dairy Scientist, Nutrition
When frost causes cells in certain species of plants to be destroyed there can be release of hydrogen cyanide, which can kill cattle. The greatest danger is when cattle are grazing species in the sorghum family such as Sudangrass, Johnsongrass, and sorghum hybrids. Also black cherry leaves can be a problem. It always is a good idea to allow sufficient growth before grazing these species. Generally this would be 24 inches for sudangrass and 30 inches for sorghum-Sudan hybrid crosses. Also do not graze pure stands of immature sorghum. If plants containing hydrogen cyanide are dehydrated and made into silage or hay the concentration will usually decrease to safe levels. Grazing fresh plants after frost is the greatest concern. Also new growth after frost can be very high in hydrogen cyanide and animals will select this growth. Therefore, animals should not graze sorghums for 5 to 7 days after frost or when new growth is present. Symptoms of poisoning include rapid breathing, convulsions, frothing at the mouth, and paralysis. Some of these are similar to nitrate poisoning. Because hydrogen cyanide is volatile and is lost rapidly it is difficult to measure. This is unlike nitrates, which are more stable, and laboratory measurements are possible. Care when grazing is the best prevention of prussic acid poisoning.