Exercise Caution when Grazing Cattle in Cold Weather

By Jeanne Gibson, Purdue University

As frost begins to cover fields, cattle producers should protect grazing livestock against prussic acid poisoning and bloat, a Purdue University Extension beef specialist says.

Prussic acid, also commonly referred to as cyanide or hydrocyanic acid, is a potent, rapidly acting poison often used in rodent and vermin killers. It accumulates in a number of common plants, and once animals consume those plants, the toxin rapidly enters the blood stream, is transported throughout the body and inhibits use of oxygen by the cells in the animal's body.

"In essence, the animal suffocates," said Ron Lemenager.

Sorghums and related plant species can easily accumulate these toxic compounds following events such as frost that rupture plant cells.

Ruminants, or animals with stomachs divided into four chambers, are more susceptible to prussic acid poisoning than other animals because the ruminal microorganisms have enzymes that release the acid in the animal's digestive tract.

"Signs of prussic acid poisoning can occur anywhere from 15 minutes to a few hours after animals consume toxic forages," Lemenager said. "Animals are often found dead. Clinical signs, when noticed, occur in rapid succession and include excitement, rapid pulse and generalized muscle tremors, followed by rapid and labored breathing, staggering and collapse."

In addition to creating prussic acid in sorghum and related plants, frost also can create high levels of soluble protein in alfalfa, winter wheat and white clover. When an animal consumes these, the result is frothy or legume bloats, Lemenager said. Cattle suffering from bloat can die within an hour. Rapid swelling of the left side of the body and various signs of discomfort often can be symptoms.

"It is important to know when to graze cattle and when not to, " Lemenager said. "The best prevention of both prussic acid poisoning and bloat is to keep livestock from grazing on nights when frost is likely and to keep animals from grazing at least 5-7 days after a frost.”

Lemenager suggests that cattle be fed before they are let back out to graze so that they are full and less likely to over-eat infected plants. Producers also should wait until dew is off alfalfa plants before letting cattle out to graze and avoid grazing them on young plants, where the risk of bloat is often highest.