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Johnsongrass (Sorghum halepense) is a perennial, drought-resistant grass that causes toxicity in cattle and horses. In pastures that are not mowed or maintained, Johnsongrass is an opportunistic weed that takes over weaker grasses during both drought and non-drought periods. If horses are supplied with healthy forage or nutritious feed, particularly during drought periods, grazing horses will not typically consume Johnsongrass. Unfortunately, for those horses that do graze on Johnsongrass, even in small amounts (over long periods of time), they may develop a devastating syndrome that is caused by nerve damage to the spinal cord.
The best ways to reduce chances of Johnsongrass poisoning is both to maintain healthy forage for grazing horses, and to minimize any contact between the horse and the grass. This can be accomplished by ridding pastures and fields of Johnsongrass growth, although this is a strenuous task. Extremely sturdy and resilient, Johnsongrass is almost impossible to eradicate from fields, though it can be managed in pastures with a combination of monthly hay cutting and frequent mowing.
Johnsongrass is a member of the Sorghum family of grasses, and is found throughout pastures and hayfields around the world. It is a noxious weed capable of causing great harm to livestock, particularly to horses and sometimes to cattle. For those experienced with sorghums, Johnsongrass is sometimes used as forage for cattle. Under certain, warm-weather conditions, cattle find Johnsongrass in pastures and graze it without health consequences. However, the danger of Johnsongrass is its ability to transition from an acceptable source of forage, to one potentially lethal, almost overnight. As soon as Johnsongrass becomes stressed, especially due to drought or frost, it begins to accumulate hydrogen cyanide (prussic acid) in its leaves, as well as nitrates. As the stressful conditions subside, cattle can continue to consume Johnsongrass, and typically strip an entire pasture within two years. This is not the case in horses. Due to its potentially high levels of cyanogenic glycosides and nitrates, Johnsongrass can be lethal to horses if ingested.
Johnsongrass spreads widely due to the thousands of seeds that are produced during summer periods. The seeds travel through wind and water to mix with food sources such as hay and grain. The grass contains toxins called b-cyanoalanine and cyanogenetic, and accumulates nitrates and hydrogen cyanide.
Poisoning due to Johnsongrass ingestion is associated with serious health conditions in horses, including neuropathy, lower spinal damage and photosensitization.
Johnsongrass is a noxious member of the sorghum family of grasses. Horses often have a toxic reaction to Johnsongrass, especially after ingesting the plant over time. Cattle display a greater tolerance to Johnsongrass, but only when the weed is unstressed by drought or frost. All plants in the sorghum family contain some number of cyanogenic glucosides, compounds that connect to cyanide. Johnsongrass has high levels of glucosides, which place horses at risk of devastating conditions, including cystitis-ataxia syndrome.
Johnsongrass toxicity may lead to neurologic problems and lower spinal cord damage in horses, particularly after periods of prolonged grazing or if a horse ingests it as a contaminant of hay. Horses that continually graze sorghums may develop cystitis-ataxia syndrome. Cystitis is inflammation of the bladder, and ataxia refers to a lack of muscle coordination. Affected horses will develop signs of incoordination, dribble urine, and then develop a flaccid paralysis of the tail and hind legs. Mares will open and close the vulva as if in heat, and abortions and other reproductive irregularities may occur. Like females, males will display incontinence in addition to ataxia and incoordination.
Diagnosis is based on owner reporting of environmental conditions (presence of Johnsongrass) and signs and symptoms of sickness. A physical exam will be conducted. If the horse is poisoned, clinical signs will be obvious. In cases of incontinence, urine testing and a bladder palpation may be performed if the diagnosis is questionable.
There is no treatment for Johnsongrass poisoning. Supportive measures may show success if the amount ingested was not excessive. The offending grass must be removed from the diet, and resulting damage to the kidneys or other organs will be treated specifically. If the horse has been neurologically impacted or nerve damage appears to be severe and permanent, the horse will likely not improve and may be euthanized.
Prevention is key when it comes to lessening the chance of environmental poisoning. Grasses and plants should be identified and pasture areas should be frequently surveyed for new growth. There are specialists who assist horse owners with the management of toxic grass like Johnsongrass. Mowing of Johnsongrass is essential, and hay and hay bales should be scattered about to mitigate the grass. Hay and feed may be tested for the presence of toxic seeds and grass.
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