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Prussic Acid Considerations for Sorghum & Sudangrass Crops

University of Minnesota Extension, Stearns County News
October 11, 2017         
Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Prussic Acid Considerations for Sorghum & Sudangrass Crops
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (10/04/17) —A couple of low temperature reports for October 10 include 25 in Little Falls and 27-28 in St. Cloud. Frost varies across the landscape, but this cold snap will probably give most of the corn and soybeans that weren’t ripe yet a reason to give up on the growing season.

For late planted crops and summer planted cover crops, there may be sorghum and sudangrass growers who have questions about prussic acid issues following a frost. Here are some key points:

Sorghum and sudangrass crops, under normal conditions should be 18 inches tall before grazing or green chopping to prevent prussic acid toxicity. Forage sorghums may not be safe for grazing or green chopping until after they are headed. All sorghums should be safe when harvested as dry hay. Wisconsin Extension Forage Specialist Dan Undersander writes that sorghum and sorghum-sudangrass “silage” is generally safe for feeding. It may contain harmful levels of prussic acid when harvested, but much of the prussic acid is expected to dissipate during fermentation. As a precaution, do not feed new silage from these crops for at least 3 weeks after harvest. Always transition to new feed gradually.

AFTER A FROST. After light frost, allow 7 to 10 days before grazing or green chopping. After a killing frost, wait until the plant has dried – approximately 7 days for grazing or green chopping. Do not graze “hungry” cattle on sorghum and sudangrass crops. Providing other feed before grazing reduces grazing consumption and risk. Watch for new shoots after any frost. Tender new shoots, like young plants, have more risk for prussic acid toxicity.

Prussic acid toxicity prevents animals from absorbing oxygen from the blood. Pink tissues will be brighter pink because of higher oxygen concentration in the blood. Cattle will behave like they are short of oxygen – heavy breathing, weak, tired, muscle trembling.

You might also talk with your veterinarian or nutrition advisor about feed issues.

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