2005 Year in Review

From the Fields

Source: Jim Morrison, Extension Educator, Crop Systems, Rockford Extension Center, University of Illinois, Phone 815-397-7714, FAX 815-397-8620, Email morrison@uiuc.edu

With the anticipation or the occurrence of the first frost in the fall, questions are asked about the affect on harvesting and feeding of the sorghum family of annual grasses and 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. Sudangrass and sorghum-sudangrass hybrids require 28 degrees for a killing frost, but even a light frost requires special management. When a large amount of the substance is consumed in a short period of time by ruminants, the dose can be lethal. The prussic acid potential is higher in the early stages of growth, but decreases until fall or frost.

The above-mentioned grasses in the sorghum family 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 the general height of the plant. University of Illinois suggests that sudangrass and sudangrass hybrids be at least 18 inches tall and sorghum-sudangrass hybrids be at least 24 inches tall before harvesting. The difference is due to the fact that sudangrass has a lower prussic acid potential than sorghums.

If at the proper height, frosted sudans and sorghums can be fed once the plant tissue has dried. This 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 at least 8 to 10 days before harvesting to allow for adequate drying.

Frost causes the sorghum family to grow as annuals and thus produce new tillers or suckers from the base of the plants. These tillers are very high in prussic acid. A rule of thumb says that a 6-inch plant has the same prussic acid as a 6-foot plant. Horses should never be allowed to graze sorghums or sudangrass at anytime as these forages may result in paralysis and urinary disorders.

Two forage crops that do not produce prussic acid are pearlmillet and alfalfa. Pearlmillet, like the sorghum family, is a summer-annual grass but has a higher leaf to stem ratio than other forages in the sorghum family, but at harvest leave a minimum of a 6-inch stubble.

There is no known toxic compound in alfalfa that results from a frost. One 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 a 6- to 8-inch stubble so the stems can catch snow and remain above any ice layer.

by  Editor, theCity1.com
October 17, 2005