Johnsongrass and Other Sorghums Can Cause Toxicity When Grazed

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Often when we look out at a pasture and see some green "grass" we think our horses are fine and have plenty of nutritious grazing ahead of them. When we are feeding a grass or mixed grass hay, we often don't consider what "grass" is in that hay. However, not all plants are created equal, and some can cause problems with our horses, such as the plants in the sorghum family. Johnsongrass is a common undesirable pasture weed in some parts of the country. In this article from the University of Kentucky College of Agriculture, Food and Environment you will learn more about the problems that can occur if horses eat these grasses for a period of time, sometimes as little as a few weeks.

When drought affects your pastures or the fields from which your hay is coming , one of the things some horse owners worry about is Johnsongrass poisoning. Johnsongrass (Sorghum halepense) is a drought-tolerant noxious weed that can infiltrate pastures and hay fields. In pastures that are not mowed and maintained, drought conditions can cause a dying off of many grasses while Johnsongrass survives and flourishes. Horses grazing these fields can potentially ingest large amounts of Johnsongrass if supplemental hay is not provided.

All sorghums, including Johnsongrass, can be associated with these major disease syndromes:

  1. neuropathy (nerve damage);
  2. teratogenesis (damaging effects to the fetus);
  3. photosensitization;
  4. nitrate intoxication; and
  5. acute cyanide poisoning.

For cattle, nitrate and cyanide poisoning are the major risks associated with Johnsongrass. However, for horses, neuropathy and teratogenesis are the most important risks, and rarely if ever do photosensitization, nitrate, or acute cyanide poisoning occur with Johnsongrass ingestion in horses. 

In horses, symptoms of poisoning can occur after a few weeks to months of continuously grazing Johnsongrass or other sorghums, at any growth stage of the plant. Hay containing sorghums has also been incriminated. Horses gradually develop ataxia, incoordination, difficulty backing and dribbling urine, progressing to flaccid paralysis of the tail and hind legs. Mares repeatedly open and close the vulva as if in heat and have continuous urine dribbling and scalding of the hind legs. Abortions and fetal malformations such as arthrogryposis (fused joints) can occur during any stage of pregnancy. Males exhibit an extended and relaxed penis and urinary incontinence in addition to ataxia and incoordination.

The mechanism by which sorghums cause these problems is not well understood, but involves damage to the spinal cord and problems with innervations to the bladder and hind end. Inflammation of the bladder, and sometimes the kidneys, can occur. The condition is sporadic, and not all horses eating sorghums are affected. The amount of sorghum that needs to be ingested for clinical signs to occur has not been established, but poisoning generally requires continuous exposure to large amounts of sorghum for several weeks or longer.

There is no specific treatment for the condition, but if sorghum is removed from the diet and treatment for bladder and kidney problems is initiated soon after the start of clinical signs, some horses can improve. However, the nerve damage is permanent, and once ataxia and incoordination occur, the prognosis is poor.

Prevention is important and includes minimizing exposure to Johnsongrass and other sorghums by controlling these plants in hay fields and pastures, and by not feeding hay containing sorghums. Johnsongrass can be controlled in pastures by mowing and close grazing; control in hay fields is more problematic. Consult a weed extension specialist for more information on controlling Johnsongrass.

Cynthia Gaskill, DVM PhD, clinical veterinary toxicologist at the University of Kentucky Veterinary Diagnostic Laboratory, provided this information.