Cyanide
Leaves and stems of all Sorghum species can contain cyanoglycosides. Sudangrass generally has the least amount of cyanide production potential, while forage and grain sorghums, shattercane, and Johnson grass often produce dangerous concentrations. Sorghum-sudangrass hybrids are usually intermediate, but their cyanoglycoside content can vary widely. Use hybrids known to be lower in cyanogenic potential when grazing is planned.
Arrowgrass, velvetgrass, and white clover can also contain cyanoglycosides in their leaves. Chokecherry, pincherry, wild black cherry, apricot, peach, apple, and elderberry trees contain cyanoglycosides in leaves and seeds (pits). However, little or no cyanogenic potential is present in the fleshy part of the fruit. These trees can be hazardous around grazing areas because cattle often graze leaves for variety in their diet.
Plant Parts
Leaves usually produce 2 to 25 times more cyanide than do stems in forage grasses; seeds contain none. Young, upper leaves have more cyanogens than lower leaves. New shoots often contain very high concentrations of cyanogens. New shoots produced after frost can be especially hazardous.
Fertilization
High application rates of nitrogen fertilizer are often needed to obtain high yields of Sorghum species. However, cyanogenic potential also may increase with high nitrogen fertilization rates. This problem is most severe on soils deficient in phosphorus.
Maintain proper soil phosphorus levels according to soil test recommendations to reduce cyanide potential in forage. Use split applications of nitrogen when the total amount exceeds 60 pounds of N per acre to decrease the risk of cyanide toxicity.
Herbicides
Foliar herbicides such as 2,4-D can increase cyanogenic potential in forages for several weeks after application, so plan grazing operations accordingly.
Drought
Drought-stricken plants containing mostly leaves are slow to decrease their cyanogenic potential, so grazing stunted plants during drought is a common cause of cyanide poisoning of livestock.
Frost
Plants that are frozen may release hydrogen cyanide for several days thereafter. After wilting, hydrogen cyanide may be released and the plant’s cyanogenic potential will decrease. Dead plants usually have less cyanogenic potential. Do not graze or green chop forage for several days after a killing frost.
When only plant tops have been frosted, new shoots may regrow at the base of the plants. These can be very dangerous because of high cyanogenic potential and because cattle will selectively graze the new growth. Do not graze frosted summer annuals until regrowth of shoots is 15 to 18 inches tall, or until several days after the entire plant and shoots are killed by subsequent frost.