A killing frost can have deadly consequences to livestock grazing Sudan grasses, sorghums, and the sorghum-Sudan grass hybrids.
Why it happens
Warm-season annual plants including Sudan grasses, sorghums, and hybrids contain dhurrin, says Travis Meteer, University of Illinois Extension commercial ag educator.
“When the plant tissue is frozen, enzymes in those plants convert dhurrin into hydrocyanic acid or hydrogen cyanide (commonly known as prussic acid),” Meteer.
When it happens
A killing frost of 28 degrees can activate the production of prussic acid, but even light frosts may require careful monitoring. Other times of concern include:
Times of new growth. Forages should be at least 18 to 24 inches tall before harvesting, Meteer says.
Times of regrowth, especially following drought, grazing, or environmental stresses.
Plants grown in soils with high levels of nitrogen coupled with low levels of phosphorus and potassium.
Hungry livestock that over-consume plants are at greater risk. Cattle and sheep are at the highest risk because microbes in the rumen quickly break down the compounds into cyanide.
What it does to livestock
Livestock who ingest prussic acid suffocate. Death can occur within minutes of exposure, South Dakota State University Extension reports. The prussic acid inhibits oxygen utilization by the cells.
When it’s safe
After a frost, wait until the plant tissue has dried, Meteer says.
Light frost impacting the tops of plants may only need 4 to 5 days of drying.
Killing frost required a complete dry down, usually from 1 to 2 weeks.
Consider supplementing with other feedstuffs to dilute the diet.
Explore dhurrin-free varieties or consider Pearl millet for your summer annual grass.
Consider grazing cornstalks or stockpiled cool season grasses instead during fall and winter.
Management Practices from Oklahoma State University Extension
The following management techniques will help prevent or minimize losses related to prussic acid.
Do not turn hungry cattle into the pasture. Feed some hay before turning in cattle late in the afternoon.
Prevent selective grazing of lush young regrowth by using rotation grazing.
Allow plants to reach at least 18 to 24 inches in height before grazing is allowed. This management practice permits some dilution of the hydrocyanic acid.
Do not graze after frost until all plants are field cured. It is necessary to make sure you have had a killing frost because sorghum plants will initiate tillering [side shoots or secondary stems that sprout from the base of grass plants] with a light frost. The tillers will be exceptionally high in hydrocyanic acid. It is recommended that at least seven days (one week) be allowed after a killing frost before grazing.
If environmental conditions in your area are such that hydrocyanic acid poisoning is a consistent problem, check with your seed dealer about varieties or hybrids that would have a low hydrocyanic acid accumulation potential.
Take a soil sample from the area to be planted. Do not apply excessive nitrogen fertilizer. Keep soil phosphorus, and pH at an appropriate level for producing sorghum. Soil sampling is essential in managing for hydrocyanic acid.
Dig deeper
Dangers of frosted forages, Travis Meteer, University of Illinois Extension
Prussic Acid Poisoning, Warren Rusche, South Dakota State University Extension
Prussic Acid Poisoning, Oklahoma State University Extension