Sorghum-sudangrass pasture poses prussic acid and nitrate poisoning risk

Kim Cassida, MSU Forage Specialist

posted 9/26/12

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 and nitrate poisoning. Forages in the sorghum family should never be fed to horses because of risk of a poorly understood but sometimes fatal urinary problem.

Prussic acid poisoning

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. When a killing frost is expected, animals should be removed from the pasture until the frost-killed herbage is completely dried (usually 5-6 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.

Nitrate poisoning

Sorghum-family forages accumulate nitrates when there is plenty of soil nitrogen, but not enough water or sunlight to drive plant cell growth. Both drought, which we saw plenty of this year, and cool cloudy weather, typical of fall in Michigan, can cause nitrate to accumulate. New growth is exceptionally high in nitrates for the first 3 to 4 days after a drought-breaking rain. Unlike prussic acid, more nitrate accumulates in stems than in leaves, and stem nitrate levels are greater in the lower portion of stems.
Unfortunately, forage preservation as hay or silage does not eliminate the risk of nitrate poisoning. Drying hay does not reduce nitrate levels of forage to any significant extent, so it is vital to test sorghum-family hay for nitrate before feeding if it was grown and harvested under conditions risky for nitrate accumulation, such as drought or cool cloudy fall weather. Ensiling will reduce nitrate levels in sorghum forages by 30 to 50%, but there may be enough nitrate left to cause poisoning if the level was high in the green forage. Suspect silage should be tested for nitrate before feeding. High nitrate forages can be safely fed if they are diluted with another forage so that the overall dietary NO3-N level is less than 1000 ppm. Feeding some grain as an energy source also helps animals detoxify nitrates.

Both prussic acid poisoning and nitrate poisoning are veterinary emergencies. Signs are similar and can occur as soon as 10 minutes after consuming a toxic dose. Both poisons interfere 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. These forages can be used safely, but require careful attention from producers to reduce the risks.