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Frost and forages.pdf
Frost and forages
by Jim Morrison, Extension Educator, Crop Systems and Justin Sexten Extension Specialist, Animal Systems / Beef

Producers anticipating the first frost in the fall often ask questions regarding harvesting and feeding the sorghum family of annual grasses as well as alfalfa. After a frost, certain precautions and management strategies may need to be taken with these crops.

Sudangrass, sorghum-sudangrass hybrids, and sorghums contain a compound called dhurrin. When the plant tissue is frozen, enzymes in the plant convert dhurrin into hydrocyanic acid or hydrogen cyanide, also referred to as prussic acid. Prussic acid poisoning can be rapid and lethal. Ruminants can show symptoms such as excessive salivation, rapid breathing and muscle spasms within 15 minutes to an hour after consuming toxic forages. The prussic acid potential is higher during early plant growth, but decreases until fall or frost. Sudangrass and sorghum-sudangrass hybrids require 28 degrees for a killing frost, but even a light frost requires special management.

Sudangrass, sorghum-sudangrass hybrids, and sorghums can be safely harvested or grazed after a frost if: (1) the crop was safe to harvest before the frost and (2) the plant tissue is allowed to dry.

Safe harvest or grazing is based upon general plant height. University of Illinois suggests sudangrass and sudangrass hybrids be at least 18 inches tall and sorghum-sudangrass hybrids be at least 24 inches tall before harvesting. Sudangrass has a lower prussic acid potential than sorghums allowing for earlier harvest.

If at the proper height, frosted sudans and sorghums can be fed after the plant tissue has dried. Drying allows the prussic acid to dissipate from the plant material. With a light frost, only the tops of the plants may be damaged and one should delay harvesting or grazing 4 to 5 days. With a killing frost (28 degrees or colder), one should wait for adequate dry down, generally 1 to 2 weeks.

Frost causes the sorghum family to grow as annuals and thus produce new tillers or suckers from plant bases. These tillers are very high in prussic acid. A rule of thumb indicates that a 6-inch plant has the same prussic acid content as a 6-foot plant.

The safest way to utilize frosted material of questionable prussic acid content is to ensile it. Prussic acid content is reduced substantially by the fermentation process (typically requiring at least 4 weeks). A second method is to make the crop into hay if dry-down conditions permit. Forages can be limit-fed as green chop after plants dry down. Grazing is the least desirable method of using questionable sudans and sorghums.

Horses should never be allowed to graze sorghums or sudangrass at anytime as these forages may cause cystitis which can result in paralysis and urinary disorders.

Pearl millet and alfalfa do not produce prussic acid. Pearl millet is a summer-annual grass like the sorghum family but has a higher leaf to stem ratio and requires a minimum of a 6-inch stubble at harvest.

There is no known toxic compound in alfalfa resulting from a frost. Producers should wait a day or two until the frosted parts of the plant dry before grazing alfalfa to reduce the increased bloat potential and the laxative effect. A killing frost for alfalfa is generally considered to be 24 degrees F for a few hours. If desired, a harvest can be made from an established stand after a killing frost or after mid-October, but leave 6- to 8-inch stubble so the stems can catch snow and remain above any ice layer to prevent winter kill.

Jim Morrison, (815) 397-7714, morrison@uiuc.edu
Justin Sexten, 618-242-9310, sexten@uiuc.edu
Date: 10/10/2005

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