Johnsongrass can be both beneficial and deadly

As producers in the Ozarks saw last year, Johnsongrass can be a rather prolific forage in times of drought or summer heat. While livestock seem to enjoy it, grazing Johnsongrass requires careful management or it can turn problematic.

“Johnsongrass is listed as a noxious weed by the Missouri Department of Agriculture,” Jill Scheidt, agronomy field specialist with the University of Missouri Extension explained. “Johnsongrass can accumulate toxic levels of prussic acid or nitrates.”

Prussic acid is a large issue surrounding Johnsongrass because it builds up in the plant’s young/new growth, and after it has been exposed to stress, such as frost, herbicide exposure and drought – which is rather ironic, since it often becomes a prolific forage during a drought.

“Last year, during our dry spell, Johnsongrass was about all some folks had for their cattle to eat,” Eldon Cole, livestock field specialist with MU Extension, said. Early spring and late fall (when the plant has new growth or has been exposed to frost), and during drought are all times to be aware of the prussic acid risk.

Scheidt recommends producers follow the “Rule of 2” when prussic acid is a concern: wait two weeks or for 2 feet of growth following a frost or drought before grazing.

Following this rule gives the prussic acid levels time to decline. Scheidt and Cole both noted that prussic acid does not stay in the forage after it has been cut for hay.

Nitrates are another issue with Johnsongrass, and these do not dissipate when the plant is harvested for hay.

“The nitrate question of risk shows up when drought stress occurs. We have a field test that is used and if it shows the possibility of high levels, we always suggest a quantitative test run by a lab,” Cole said. “A level of up to 4,400 parts per million (0.44 percent nitrate) doesn’t usually bother animals. Levels in the 4,400-4,800 ppm (0.44 to 0.88 percent nitrate) is probably OK for non-pregnant cattle, but for pregnant ones, limit the nitrate containing forage to 50 percent of their diet. As nitrates increase the risk rises and when the level is 1.5 percent or greater, it’s really iffy whether it should be fed at all. For sure, if it’s above 1.75 percent don’t feed it to any class of livestock.”

Signs of nitrate poisoning show up fast.

“Symptoms may occur if the animals have only been on it 30 minutes or less,” Cole said. “And will include rapid respiration rates, salivation, then they will stagger, collapse and die.”

If an animal survives two hours after showing nitrate poisoning symptoms, they typically make a full recovery.

“Ruminants are more susceptible than other animals such as hogs,” Cole said. “Cattle seem more susceptible than sheep.”

If there are nitrates in hay or in the Johnsongrass in the pasture, giving the livestock additional feed or forages can reduce the risk of nitrates.

“Corn, or its by-product feeds, are good supplement feeds,” Cole advised.

Producers with Johnsongrass are encouraged to test it to minimize risk.

“The best thing to do is test feeds that are risky before turning animals on it. We usually advise owners to not turn the whole herd in on Johnsongrass or forages that pose high risks. Turn a few “monitor” animals in and watch them closely to see if any suspicious signs surface in the first 45 to 60 minutes. Know how to get hold of a veterinarian as they can help save some animals from both prussic acid and nitrates,” Cole said.


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