We have had a run of beautiful fall weather recently with temperatures above average but at some point that is going to turn and we are going to get our first frost of the season. When frosts and freezes begin, livestock owners need to be aware that some warm season forage plants such as sorghum, sorghum-sudan hybrids, sudangrass and even Johnsongrass can pose a threat of cyanide poisoning to grazing livestock. All of these species contain compounds known as cyanogenic glycosides. These compounds can be converted to cyanide, also known as prussic acid, a lethal poison.
Fortunately, the cyanogenic glycosides and the enzymes needed to convert them to cyanide are located separately within the plants cells and these plants species can be grazed under normal growing conditions with very little risk to livestock. However when plant cells are ruptured, as occurs in a frost or freeze situation, then the cyanogenic glycosides and the enzymes come together resulting in cyanide production, and now we have a high health risk to a grazing animal. The highest concentrations of cyanide or prussic acid are found in the leaves of immature plants while the lowest concentrations are in the stalks of mature plants.
According to a University of Kentucky Extension fact sheet entitled “Cyanide Poisoning in Ruminants”, signs of cyanide poisoning included “rapid labored breathing, irregular pulse, frothing at the mouth, dilated pupils, muscle tremors, and staggering.” Unfortunately affected animals die quickly, rarely surviving more than 1-2 hours after consuming a lethal amount and more typically dying within 5 to 15 minutes of developing clinical symptoms.
Prevention of cyanide poisoning is the management strategy that must be used. Prevention involves understanding the risk factors and exercising caution in permitting grazing or avoiding grazing altogether. Do not graze plants in the sorghum family on nights when frost is likely. High levels of cyanide are produced within hours after a frost. Do not graze after a killing frost until plants are dry, which usually takes 5 to 7 days. Since cyanide is a gas, it will have volatilized and dissipated from dead plant tissue in this time frame. After a non-killing frost (greater than 28 degrees F), do not allow animals to graze for two weeks because the plants usually contain high concentrations of toxic compounds. Another alternative to use these forages as livestock feed after frost is to harvest them as either a silage or dried hay crop. Hay should be properly dried before baling and fully cured before feeding. Silage made from these forages following a frost should be allowed to ferment for at least 8 weeks before feeding.
For more information about prussic acid or cyanide poisoning risk in forages and management strategies, contact the Wayne County Extension office at 330-264-8722.