Frost Danger

First Hard Frost Can Make Forages Poisonous

Victoria G Myers
By  Victoria G. Myers , Progressive Farmer Senior Editor
As cattle move between pastures this fall, keep an eye out for forages that develop prussic acid after a frost. (DTN/Progressive Farmer photo by Becky Mills)

It only has to happen once and the results of prussic acid poisoning can be devastating. Difficulty breathing, frothing at the mouth, dilated pupils, convulsions and death. It’s essentially a reaction to poisoning by cyanide. And it can take less than 30 minutes.

Managing around those forages most likely to cause these losses starts with awareness. Bruce Anderson, University of Nebraska Extension forage specialist, said sorghum-related plants are most commonly the ones that form prussic acid (cyanide) when freezing temperatures break plant cell membranes and allow formation of the toxin.

Texas A&M AgriLife Extension’s “Nitrates and Prussic Acid in Forages” includes an extensive list of possible sources of prussic acid, including some trees. They encompass, from high to low potential: johnsongrass, grain sorghum, sorghum almum, shattercane, sorghum-sudangrass hybrids, forage sorghum, sudangrass and sudangrass hybrids, arrowgrass, velvetgrass, white clover, birdsfoot trefoil, chokecherry, pincherry, wild black cherry, apricot, peach, apple, elderberry, and pearl and foxtail millet.

Prussic acid accumulation can occur when poor growing conditions prevent proper stem development; recent harvest or grazing causes slow growth of new tissue; nitrogen has been over-used; plants develop after prolonged drought; or plants are injured by frost, hail or herbicides. Standing forages can be green chopped and used as silages safely. They can also be used as hay, although sampling is recommended to ensure the prussic acid is no longer an issue prior to feeding.

Dennis Hancock, University of Georgia Extension forage specialist, told DTN the biggest danger from prussic acid lasts just 1 to 2 weeks after a hard frost. He said while most producers work around the potential problem by moving animals off of those areas where there is a lot of Johnsongrass or anything in the sorghum family, there is little sampling done for the toxin.

“This is such a short-lived thing that by the time most producers could take a sample and get back the results, the danger time has passed. Where producers know there are problems, they just move animals,” he said.

If animals are exposed to prussic acid accumulation, producers can dilute its effect by feeding other plant material and carbohydrates. Treatment for the condition, when caught in time, is often an intravenous mixture of sodium nitrite (a salt and anti-oxidant) and sodium thiosulfate (a medication). Producers suspecting prussic acid poisoning should call their herd veterinarian immediately.


Victoria Myers