Keeping herds safe from the deadly plant poison

During stretches of drought or a frost the likelihood of prussic acid building up in Johnsongrass increases. If livestock graze Johnsongrass containing high concentrations of prussic acid the outcome is almost always fatal. 

When animals ingest the Johnsongrass containing prussic acid, the plant conversions taking place in the rumen form cyanide. “The effect of cyanide in the body is it doesn’t let the hemoglobin in the blood release oxygen,” John Jennings, Ph.D., forage extension specialist and professor, with the University of Arkansas Division of Agriculture explained. “It can pick up oxygen in the lungs, but it can’t release it to the tissues.”

The result is the animal internally suffocates. Typically, the time period between onset of symptoms and death is extremely short. “Most of the time, if it is prussic acid poisoning, the animals are found dead and not exhibiting symptoms because it happens so rapidly,” Jennings added. 

The time frame is so abbreviated, it is usually not feasible for a veterinarian to make it to the animal in time for treatment. If treatment is an option, the animal is given a combination of sodium nitrite and ¬sodium thiosulfate. This must be administered intravenously at a slow rate. 

An animal suffering from prussic acid poisoning shows signs of anxiety, weakness, muscle twitching, convulsions and labored breathing. One of the diagnostic symptoms of an animal that has died of prussic acid poisoning is, its blood is a bright, cherry red color because it is loaded with oxygen. 

The chances of prussic acid poisoning occurring increase when Johnsongrass wilts due to drought or frost. During times with little or no rain, Johnsongrass thrives while other forages thin out. “In years when we have drought conditions and all the other forage is grazed off tends to be when we see the most problems; because Johnsongrass can tolerate drought and it might still be standing and cattle tend to get into it,” Jennings stated.

In addition, problems with prussic acid poisoning become more prevalent during cold snaps producing frost. The freeze causes the Johnsongrass to wilt and the prussic acid to build up. 

“Some low-lying fields might get frost, and the hills might not, and maybe animals are in that entire pasture, so we get need to take precautions when it comes to that time of year,” Jennings added.

Preventing Prussic Acid Poisoning in Warm and Cold Weather Conditions

• Do not allow cattle to graze Johnsongrass following a hay cutting or recent grazing that leaves the Johnsongrass short (12-inch total plant height or less).

• If Johnsongrass is wilted from drought, wait four to seven days after a rain until the plant starts to grow again and look fresh, then turn cattle back into the pasture.

• If the Johnsongrass is 18 inches or taller it is safe to graze, as long as it is not wilted. 

• Johnsongrass that is wilted due to a killing frost should not be grazed until the forage appears dry and brown. 

• If the Johnsongrass is black from freeze damage it is dangerous and should not be grazed.

• Keep cattle off pastures with Johnsongrass if frost is likely.

Prussic acid dissipates as the forage dries out. When Johnsongrass is cut, dried and baled for hay the amount of prussic acid dispels over time. “It is just in that wilted condition, that it is toxic. Dry hay is safe from prussic acid,” Jennings said. 

Prussic acid can also be found in the leaves of wild cherry trees. Producers who have wild cherry trees on their property should keep in mind that if they are trimming the wild cherry trees or if a storm breaks off some tree limbs, the wilted wild cherry tree leaves will contain a dangerous amount of prussic acid. 


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