Frost and Prussic Acid in Some Warm Season Annuals

Rory Lewandowski, Extension Educator Wayne County

We have enjoyed a run of warm temperatures in recent weeks and the word “frost” nowhere to be found. A look at the calendar tells us that the odds of this continuing are growing slim. In preparation for the first frost of the season, I want to remind livestock owners that some warm season annual grass plants that include sorghum, sorghum-sudangrass hybrids, sudangrass and even that weed, Johnsongrass, all have the potential for prussic acid poisoning following a frost.

Plants in the sorghum family, including Johnsongrass, contain varying concentrations of cyanogenic glucosides. Cyanogenic glucosides are compounds composed of a carbohydrate or sugar molecule chemically bonded to a cyanide molecule. Frost ruptures cells and starts a process that frees the cyanide from its chemical bond leading to the formation of hydrocyanic acid, commonly called prussic acid. The formation of prussic acid occurs very quickly during a frost event.

Prussic acid is very toxic and is rapidly absorbed into the blood. It combines with hemoglobin in the blood to form cyanoglobin, which does not carry oxygen. Prussic acid poisoning symptoms include an increased rate of respiration, increased pulse rate, gasping, muscular twitching or nervousness, trembling, foaming at the mouth, spasms or convulsions. Death occurs from respiratory paralysis and can happen quickly, within a 15 to 20 minute time span.

Ruminant animals tend to be more susceptible to poisoning from cyanogenic glucosides than non-ruminant animals. When non-ruminant animals ingest forage with cyanogenic glucosides the forage goes directly into the stomach. The stomach is a strongly acidic environment that reacts with any free prussic acid to form low toxicity substances. In contrast, when the ruminant animal ingests forage with cyanogenic glucosides, the forage passes into the rumen. The rumen in a forage based diet is a mildly acidic to mildly alkaline environment that does not detoxify prussic acid. In addition, there are rumen micro-organisms and enzymes that contribute to the formation of prussic acid.

How much prussic acid or HCN is lethal? According to a publication from South Dakota State University on the topic of prussic acid poisoning of livestock, a dose of 1 gram of HCN is enough to kill a 1000 lb. cow. There are 454 grams in one pound to give you some perspective on this dose. However, ruminant animals can also detoxify HCN over time, so if small amounts are ingested livestock will not be harmed. The South Dakota State University publication says that a 1000 lb. cow can detoxify HCN at a rate of about 0.5 gram of HCN per hour. It is only when HCN is ingested and enters the bloodstream faster than it is being detoxified that livestock can be killed by prussic acid poisoning.

Livestock should not be allowed to graze any plants in the sorghum family immediately following a frost event. However, because prussic acid is actually a gas, it will dissipate from the plant as the plant dries out and also with the passage of time. For this reason, frosted plants can be chopped for silage or mowed for hay. Before feeding, the silage must have had time to fully ferment and the hay must be fully cured. Under these conditions, any prussic acid that was present at the time the forage was mowed or chopped will have dissipated.

The general recommendations regarding grazing plants in the sorghum family in the fall of the year are:

* Do not graze on nights when frost is likely. High levels of prussic acid are produced within hours after a frost.

* Do not graze after a killing frost until plants are dry, generally 5 to 7 days after the frost event.

* After a non-killing frost, do not allow animals to graze frosted plants that are shorter than 30 inches in height for 10 to 14 days. Plants above that height can be grazed after a 4-5 day period. New growth may appear at the base of the plant after a non-killing frost. This growth will contain high levels of prussic acid. Do not allow livestock to graze this growth. Wait for a killing frost and then give those plants another 2 weeks before livestock are allowed to graze.

For more information about prussic acid poisoning and prevention, contact your County Extension office.

Prussic Acid Poisoning Potential in Frosted Forages, OSU Extension C.O.R.N. Newsletter 2008-32
Prussic Acid Poisoning of Livestock, South Dakota State University, Extension Publication 4016
Prussic Acid, University of Arkansas, Extension Publication FSA3069