It’s that time of year when producers should watch cattle for signs of prussic acid poisoning, including labored breathing and staggering, warns Justin Waggoner, a Kansas State University beef systems specialist.

When cattle ingest forages with high levels of prussic acid, it reduces their blood’s oxygen-carrying capacity and can cause death.

“When we move into fall and flirt with that first frost, we have risk potential for prussic acid poisoning in livestock,” he says.

Sorghum, sudangrass and crosses of those types of forages can produce high levels of the poison.

“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 says.

First frost burns leaves, increasing prussic acid content. Five to seven days later, the acid will volatilize. Once the plant becomes dormant, the risk of prussic acid is gone.

Because the acid is volatile, it is hard to determine if it dissipates after one frost. If plants don’t become dormant, there is still a risk for high prussic acid levels with another frost, he says.

Producers who usually graze cows on sorghum stocks before a hard-killing freeze may want to rethink that option.

Early frosted fields may have plants haven’t uniformly moved into dormancy, Waggoner says. “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.”

He suggests testing forages, but samples must be handled properly. Waggoner and J.D. Holman, K-State agronomist, recently completed a study examining the effectiveness of different sample-handling methods for forage samples intended for prussic acid analysis.

One sample was sent directly to the lab on the day of collection. Other samples were placed in a refrigerator, freezer, a sealed plastic bag and an unsealed plastic bag. Bag samples were left in a pickup for seven days.

Compared to the fresh sample taken right to the lab, refrigerated and freezing showed no difference in prussic acid content.

Waggoner says the open bag sample placed in the pickup had 400 parts per million (ppm) less prussic acid compared to the fresh sample, which showed that it was not an efficient sampling method.

“Interestingly enough, in the plastic bag we sealed, prussic acid content was basically maintained,” he says. “It was slightly lower, so we did lose a little bit of the prussic acid. But, at the same time, it was still relatively high compared to the sample delivered directly to the lab.”

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Producers may also want to run prussic acid tests of packed forages, such as silage.

“Silage is packed relatively tightly, and it doesn’t go through the same wilting process that hay does, when hay is allowed to dry in a field. So the prussic acid content could still be relatively high. You might want to run a sample if you put up a forage for silage that was potentially high in prussic acid.”

After testing fresh forages or silage, producers can safely feed cattle crop with less that 500 ppm of prussic acid. Anything between 500 ppm and 1,000 ppm is potentially toxic but could be fed as long as it’s not the only feed source. Anything that tests above 1,000 ppm for prussic acid should not be feed to cattle.

For more of Waggoner’s findings, check the K-State Research and Extension bookstore publication, Roundup 2013. Another resource: Forage Facts MF3040.

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