REDFIELD, Iowa (DTN) -- With the arrival of fall and colder weather, the first frost can create a potentially deadly risk for cattle grazing sorghum, sudangrass and related warm-season annual forages.

Cattle producers using cover crops or stockpiled forage should pay close attention to overnight temperatures and pasture conditions. Frost and other stresses can cause susceptible plants to release prussic acid, a form of cyanide that can kill cattle quickly.

Iowa State University Extension forage specialist Shelby Gruss said sorghum and sudangrass plants accumulate dhurrin when stressed. The compound can break down into prussic acid after plant tissue is damaged.

"If livestock consume forage with elevated prussic acid, it can lead to acute toxicity; muscle twitching, staggering, and even death within minutes," she said.

Prussic acid passes through the rumen wall into the bloodstream, where cyanide prevents the animal's tissues from using oxygen carried by the blood. Animals may show anxiety, weakness, staggering and labored breathing. In severe cases, death can occur rapidly, and the first sign a producer notices may be a dead animal.

The warm-season grasses most susceptible to prussic acid problems include grain sorghum, Johnsongrass, sorghum-sudangrass hybrids and sudangrass. Grain sorghum generally has the greatest risk.

Millets, including pearl millet, and corn do not produce prussic acid, although both can accumulate nitrates under stressful growing conditions.

FROST CHANGES GRAZING PLANS

Prussic acid risk is generally greater in young plants than mature plants and in leaves compared with stems. That makes frost-damaged leaves and young shoots particularly important when evaluating a pasture.

Even a light frost can injure leaves while leaving much of the plant green. Oklahoma State University Extension beef nutrition specialist Paul Beck recommends keeping cattle off susceptible forage for at least 14 days after a light frost that damages leaves but does not kill the plants. Producers should count from the most recent frost, meaning another frost restarts the waiting period.

A killing freeze changes the situation. After a hard freeze of around 28 degrees Fahrenheit, Iowa State recommends keeping livestock off susceptible forage for at least seven days. Beck likewise recommends waiting at least seven days after a killing freeze and until plants are completely dry and brown. Drying can take 10 days or longer, depending on conditions.

Simply watching the calendar isn't enough. Before returning cattle to a pasture, producers should walk the field and look closely for green regrowth. Young shoots can contain particularly high concentrations of compounds that produce prussic acid, even when the older plants appear safe.

Gruss recommends waiting until sudangrass regrowth is at least 18 inches tall or until a killing freeze. Beck recommends 18 to 24 inches for sudangrass and 24 to 30 inches for sorghum-sudangrass hybrids.

Producers should inspect the entire stand, including small plants and shoots growing beneath taller forage.

If there is uncertainty, forage can be tested for prussic acid before cattle are turned out. Iowa producers can submit forage samples to the Iowa State University Veterinary Diagnostic Laboratory or another laboratory offering prussic acid testing.

Harvesting questionable forage for silage is another option. Ensiling can reduce prussic acid levels, but Beck recommends allowing silage to ferment for at least eight weeks and testing it before feeding.

COOL-SEASON FORAGES POSE DIFFERENT RISK

Frost doesn't create a prussic acid problem in cool-season forages such as alfalfa or clover, but producers still need to use caution because frost can temporarily increase bloat risk.

"Even if cattle are adapted to grazing alfalfa, they should be removed from frosted fields," Gruss said. "Watch for wilting, typically five to seven days after the freeze, as this signals a reduced bloat risk."

Warm weather following a freeze can allow plants to resume growth and keep bloat concerns elevated. Risk falls substantially after a killing frost leaves more than half of the plants dormant.

Producers can further reduce bloat risk by providing access to a bloat-control product and making sure cattle are full before turning them onto alfalfa or clover.

Whether dealing with warm-season annuals or legumes, the safest approach after frost is to slow down. Check the plants, look for regrowth, and, when conditions are questionable, test the forage before allowing cattle to graze.

Jennifer Carrico can be reached at jennifer.carrico@dtn.com

Follow her on social platform X @JennCattleGal

(c) Copyright 2026 DTN, LLC. All rights reserved.