GARDEN CITY, Kan. – Clinical signs of prussic acid poisoning in cattle might include labored breathing and staggering. Prussic acid poisoning is a condition that occurs when cattle ingest forages with high levels of prussic acid, which inhibits the blood’s oxygen-carrying capacity and causes cattle to die very quickly. Producers should be on particular lookout for the condition this time of year.
“When we move into fall and flirt with that first frost, we have risk potential for prussic acid poisoning in livestock,” said Justin Waggoner, beef systems specialist at K-State Research and Extension’s southwest area office in Garden City.
High levels of prussic acid are common in several forages native to Kansas that include sorghum, Sudan grass and crosses of those types. The high levels are caused by anything that damages the cells in plant leaves, including that first frost.
“When plant cells are damaged due to frost, the plant cell wall ruptures and releases prussic acid, or hydrocyanic acid, into the surrounding leaf tissue,” Waggoner said.
Timing Fall Grazing
When the first frost hits a particular field, it burns the leaves and prussic acid content rises, Waggoner said. Over a period of time, five to seven days later, the prussic acid will volatilize. Once the plant becomes dormant, the risk of prussic acid is gone.
But, because prussic acid is volatile, it is hard to determine if it dissipates after one frost. If the plant doesn’t become dormant, there is still a risk for high prussic acid levels if another frost comes along.
Waggoner said producers often like to turn their cows out to graze sorghum stocks before a hard killing freeze, which could be risky for cattle.
“If you graze too early, the situation you run into is you get that early frost that doesn’t burn the entire field and make the plants uniformly go into dormancy,” Waggoner said. “Prussic acid levels may be high in one part of the field and relatively lower or non-existent in another part. We get another frost, and prussic acid just continues to spike and decline until all plants go into dormancy.”
Handling Forage Samples
Testing forages could help prevent prussic acid poisoning in cattle, but samples must be handled properly. Waggoner, along with K-State agronomist J.D. Holman, recently completed a study that examined the effectiveness of different sample handling methods for forage samples intended for prussic acid analysis.