After Light Frost, Keep Oklahoma Cattle Off Risky Forage for 14 Days

Keep cattle off frost-damaged sudangrass for at least 14 days, and off a killing-frozen field for at least seven days and until every plant is dry and brown. Those two minimums are the clearest field rules for protecting cattle after frost. Paul Beck’s Cow Calf Corner guidance, published November 11, 2026, sets out a practical grazing schedule.

At the Stillwater Mesonet, the median date for the first hard freeze, at 28°F, is November 8. That date had passed when Beck published, and no freeze was forecast through November 15. A low in the mid-30s can still damage pasture leaves, so a hard freeze is not the only trigger for prussic acid.

Many Oklahoma producers are growing forage sorghums, sudangrass and hybrid sorghum-sudangrass, while johnsongrass occupies many pastures this year. Some plan to stockpile these fields as fall forage for cows or weaned calves. Others have drilled small grains into standing cover crops, while volunteer corn, sorghum or sudangrass has emerged with wheat in crop residues.

What makes frost dangerous?

Drought-stressed or frost-hit sorghum, johnsongrass, sorghum-sudangrass hybrids and sudangrass accumulate compounds that release prussic acid when leaves are damaged by frost or chewed by cattle. Prussic acid is a form of cyanide that passes through the rumen wall into the bloodstream and prevents tissues from using oxygen carried by the blood.

The blood can remain bright cherry red even as the animal suffocates. Symptoms include anxiety, weakness, labored breathing and death. Beck warns: “Often the first sign a producer sees is a dead animal.”

Nitrate poisoning causes similar symptoms. In nitrate poisoning, the blood loses its ability to carry oxygen and may turn chocolate brown.

Prussic acid risk differs sharply by species. The susceptibility order gives producers a clear triage when several forage types share a field.

• Grain sorghum is the most susceptible.
• Johnsongrass ranks second.
• Sorghum-sudan hybrids rank below johnsongrass.
• Pure sudangrass has the lowest prussic acid susceptibility among these four plants.

Millets such as pearl millet and corn do not produce prussic acid, but both accumulate nitrate. That distinction matters because nitrate poisoning can resemble prussic acid poisoning in cattle.

Where does the plant concentrate the risk?

Prussic acid potential is higher in younger plants than older plants, higher in leaves than stems, and higher in upper leaves than lower leaves. Even a light frost can damage leaves while the rest of the plant remains green.

Scout pastures for frost-damaged leaves and check near the base of tall plants for young shoots. Regrowth can carry especially high prussic acid levels. Beck’s photographs show frost-damaged johnsongrass at top and sorghum-sudangrass at bottom, both capable of causing livestock deaths if consumed.

When can cattle return to the field?

Beck advises: “Do not allow animals to graze young plants or regrowth until plants reach 18 to 24 inches for sudangrass or 24 to 30 inches for sorghum-sudangrass.” The height limits keep cattle off the most hazardous young tissue, including plants recovering from frost.

• After a light frost that damages leaves but leaves plants alive, keep cattle off for at least 14 days, counting from the most recent frost. If new shoots appear, wait until the regrowth reaches grazing height, even if older stems are already 30 inches tall.
• After a killing freeze, keep cattle off for at least seven days and until the plants are completely dry and brown. Drying may take 10 days or longer, so the minimum waiting period does not end the risk on day seven.

Older stems do not override the condition of new growth. A field with 30-inch mature forage can still be unsafe if short shoots are available for grazing.

Can suspect forage become safe silage?

Harvesting a suspect stand for silage can reduce prussic acid levels. Allow the silage to ferment for at least eight weeks, then test it for prussic acid before feeding. The route creates a timed testing step rather than an immediate use for the standing crop.

Prussic acid can become an issue every year, making post-frost scouting the central safeguard. Oklahoma State University Cooperative Extension Fact Sheet PSS-9204, “Prussic Acid Poisoning”, provides the technical reference. Dr Barry Whitworth also explains how prussic acid poisoning occurs and how to keep a herd safe in a SunUpTV segment from July 22, 2024.

With no freeze forecast through November 15, the immediate task is to scout Oklahoma’s warm-season pastures. If another light frost arrives, start the 14-day hold from that date and inspect new shoots before turnout; a killing freeze requires at least seven days plus complete browning and drying.