Common Name
Johnson grass
Botanic Name
Sorghum halepense (L.) Pers.
Plant Family
Poaceae (Gramineae)
A common weedy grass of roadsides, waste areas and alluvial bottom land.
Animals Affected
Cattle, sheep, goats, and horses
Toxic Principle
Cyanogenic glycoside dhurrin which is hydrolysed by rumen microorganisms to free hydrogen cyanide (HCN). All parts of the plant are poisonous especially if wilted or in regrowth after the grass is cut. The Cyanide blocks the action of the cellular enzyme cytochrome oxidase thus preventing hemoglobin from releasing oxygen to the tissues. Death results rapidly from anoxia. Johnson grass may also accumulate toxic levels of nitrates especially under drought conditions or heavy fertilization. Horses are not affected by the acute effects of cyanide or nitrate in sorghums. When sorghums are grazed over a period of time they may cause neurological disease resulting in degeneration of the nerves of the hindquarters. Affected animals (horses, cattle, sheep) show hind leg weakness, ataxia, and urinary incontinence.
Johnson grass is a coarse, drought tolerant, perennial grass, with long hairless, broad leaves, growing to 6 feet when in flower. It spreads by rhizomatous scaly roots, and by seed. Seeds are yellow-purple and are produced in large branched clusters. Sudan grass (Sorghum bicolor) and its hybrids are annuals.
Excessive salivation.
Without stressing the animal, sodium thiosulfate and sodium nitrite solution should be given intravenously. A mixture of 1ml 20% sodium nitrite and 3ml of 20% thiosulfate should be prepared and given at the rate of 4 ml of the mixture per 100lbs body weight. Sodium thiosulfate should be given orally via stomach using 30gm dissolved in a gallon of water to reduce further absorbtion of cyanide from the rumen.
Cardiovascular system
Mucous membranes appear cherry-red. Venous blood is cherry-red in color. Stressing the animal rapidly leads to collapse and death.
Respiratory System
Initially animals show difficulty in breathing. Open mouth breathing is common as the animal becomes oxygen deprived.
Nervous System
Nervousness and weakness precede death.
Renal System
Urinary incontinence
Reproductive System
Abortions may occur if the cow does not herself die. Foals, calves, and lambs may be produced with deformed legs (arthrogryposis) if the dams consume sorghum hay or grass during pregnancy.
Rumen contents or plant material can be tested for cyanide using the sodium picrate test. Commerical test kits for cyanide are available.
Special Notes
Hay made from some cyanide containing sorghum grasses is known to cause a chronic and irreversible disease in horses, cattle and sheep characterized by urinary incontinence and hind leg weakness and ataxia. Some varieties of sorghum contain lathyrogens -substances converted from cyanide or nitriles that cause demyelination/degeneration of the peripheral nerves of the hind legs and urinary bladder. Once affected, animals do not recover and are usually euthanized because of permeant damage to the nerves controlling the bladder and hind legs. Unless it is certain that the sorghum hay is from species of sorghum that have been selected to be free of cyanide, it is risky to feed such hay to horses or cattle for extended periods. Johnson and Sudan grass can also be a cause of nitrate poisoning. NITRATES: See Kochia weed for comments on managing nitrates in forages for cattle.
Sudan grass hybrid.