Along with acorn poisoning, as discussed a recent article, another potential dangerous crop to ruminant animals is Johnsongrass. Johnsongrass is a warm season perennial grass that spreads by underground stems called rhizomes and seeds.
Seed production is very prolific and can lay dormant for 15-20 years before germinating. Many questions asked about Johnsongrass are related to the control of and the possibilities of it being poisonous to farm animals.
Johnsongrass can be toxic to livestock, but only under certain conditions. The same is true for sudangrass, milo and sorghum-sudangrass. Wild cherry trees can also produce toxic levels, and poisoning occurs most often when animals consume wilted leaves after trees have been damaged by storms or pruning.
Johnsongrass is toxic when under stress, including for about 72 hours after a “killing” frost. After a “burn back” frost, it can be toxic for at least 10 days and possibly longer.
When the plant is under stress, it produces a chemical called prussic or hydrocyanic acid (HCN), or more commonly called cyanide. Smaller, younger growth plants, especially those less than a foot tall, produce much more of the toxin than more mature, older plants,.
Ruminant animals (cattle, sheep and goats) appear to be the most susceptible to prussic acid poisoning. Reports of poisoning in swine and horses are rare. Calves still nursing are much more susceptible to the toxin than older, more mature cattle, but older cattle can be poisoned if they eat enough of the plants. Drinking soon after eating the plants raises the likelihood of poisoning.
Symptoms of prussic acid poisoning include anxiety, progressive weakness and labored breathing, gasping, increased pulse rate, muscular twitching and convulsions, and death may follow rapidly when lethal amounts of HCN are consumed.
The dead animals may be found without visible symptoms of poisoning. Animals affected by prussic acid poisoning may be treated with a sodium nitritesodium thiosulfate combination, injected intravenously and very slowly. Dosage and method of administration are critical. Consult a veterinarian to correctly diagnose prussic acid poisoning and to determine the proper treatment.
Because it’s hard to eradicate Johnsongrass in pastures, it’s best to pull cattle out of fields with Johnsongrass for at least three days after a killing frost and at least 10 days after a burn bac frost.