Prussic Acid Threat Rises in Drought-Stressed Sorghum Forages

Drought-stressed sorghum forages can concentrate prussic acid at levels that kill livestock within hours, and South Dakota State University Extension is telling producers to manage the risk before turning cattle out, not after.

Kim Ricardo, SDSU Extension forage field specialist, explained the mechanism in an interview out of Brookings, S.D. "Really what happens is that drought stresses those plants, of course, and then that damage to those plant tissues and cells creates a greater concentration of this acid," Ricardo said.

Prussic acid triggers rapid cyanide poisoning in livestock, which can be fatal if left untreated or in severe cases. The compound disrupts oxygen utilization at the cellular level, Ricardo said. "Some symptoms include, of course, you know, labored breathing. Those animals are probably going to be staggering around, really just having some difficulties breathing. They are just not going to act quite right," she said.

Her advice: get ahead of it. "It's kind of something you want to be on top of and try to manage, you know, as more of a preventing this from happening rather than trying to fix it once it does happen," Ricardo said.

Which forages carry the risk?

Prussic acid appears most commonly in four sorghum-family forages:

• forage sorghum
• sorghum sudangrass hybrids
• sudangrass
• Johnson grass

Fields that have been hayed or drought-stressed pastures where forage growth has stunted carry the greatest risk. Grazing poses the biggest concern of all, Ricardo said, because the acid concentrates in the young, leafy green parts of the plant — precisely the tissue cattle select first on new regrowth after stress or frost.

How does prussic acid differ from nitrate toxicity?

Producers frequently confuse the two conditions. "I think a lot of times producers get the two confused or think that they are the same thing, but in reality they're not," Ricardo said.

Nitrates occur in most plant species, so a far wider range of forages can accumulate nitrate toxicity. "So there's a wider variety of forage species susceptible to nitrate toxicity. So that is kind of one thing to recognize with prussic acid is that not all of your forage species are going to be at risk for it," Ricardo said. "So, that kind of helps manage it in some ways."

The narrower host range for prussic acid means growers who know their stand composition can rule the hazard in or out before turnout.

What should producers do before grazing?

Ricardo laid out concrete thresholds. Wait at least 10 days to two weeks after a killing frost before grazing sorghum species, so the plant dries out and the acid dissipates. Plants that have reached 18 to 24 inches in height carry somewhat lower risk, so grazing is typically advised only after forages hit that height.

Never turn out hungry cattle. Full animals consume less of the hazardous forage at once.

Producers should also give livestock access to safe feed and forage before exposure, which reduces the chance animals gorge on dangerous quantities.

Can growers test for it?

Yes. Ricardo said producers can take forage samples to a laboratory if they suspect prussic acid, and most labs request overnight shipping so the sample arrives fresh.

One takeaway: after drought or frost, test, wait the full window, and fill cattle on safe feed before turnout.