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What is Redroot Pigweed Poisoning?

Redroot pigweed (species Amaranthus retroflexus L.) is an annual plant that is native to North America, and grows freely across cultivated fields and pastures, and along roadsides in the United States and Canada. Redroot pigweed has an unremarkable, weed-like appearance that blends dangerously among other pasture plants. Growing to heights of three to four feet, the unexceptional pigweed is rough to the touch, and sprouts small, prickly flowers, and long, oval-shaped leaves that are dull-green in color. Pigweed’s nondescript presentation makes identification and removal a challenge for horse owners unaware of toxic weeds. Though it is most poisonous in the case of grazing cattle, pigweed is also toxic to goats, sheep and horses, or any animal sensitive to nitrates. In all of these animals, symptoms of poisoning may occur soon after ingestion, or accumulate over a period of weeks.

Classified as a nitrate accumulator, the pigweed plant contains varying, unpredictable concentrations of two chemicals: nitrates and oxalates. Nitrates most often develop in both its leaves and stems. When consumed in a large enough quantity, these chemicals may lead to toxicosis in your horse. While poisoning may occur soon after ingestion, it may also develop over a period of weeks as the nitrates continue to accumulate. Horses are most likely to experience toxicosis (the condition caused by the poison) when the nitrate in the plant turns into nitrate in the gastrointestinal tract. More worrisome is the absorption of nitrate into the cardiovascular system of the horse. When absorbed into the blood, nitrate may convert hemoglobin (the essential molecule responsible for spreading oxygen throughout the tissues) into methemoglobin, a “corrupt” form of hemoglobin that is incapable of spreading the oxygen necessary for healthy functioning of the muscles, brain and heart. The horse may also develop hypothyroidism, a condition also known as goiter.

Oxalates, the acids that keep the kidneys and urinary tract healthy, also amass in unhealthy quantities within this potentially dangerous plant. In this case, renal (kidney) failure is possible. Although cattle are most susceptible to nitrate poisoning, large amounts of consumption or high concentrations of the pigweed may threaten the life of the horse. Observable symptoms of nitrate poisoning in horses include labored breathing (due to the actions of the oxalates), tremor, collapse or even sudden death. 

Fortunately, many instances of nitrate poisoning in horses do not have fatal outcomes. This is attributable to the same factor that typically keeps horses from gobbling large amounts of poisonous plants: taste. Noxious plants are about as palatable to horses as they are to humans. Under normal conditions, livestock do not happily choose to have a toxic lunch. Primarily during periods of drought or when pastures are overgrazed, horses may turn to a plant like pigweed due to hunger. If healthy and tasty forage is available, horses choose that form of nutrition. For these reasons alone, it’s essential to provide your horse with safe hay or other sources of nutrition.

Redroot pigweed is an invasive, drought-resistant weed that is moderately poisonous to many types of livestock, particularly cattle, sheep and horses.

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Symptoms of Redroot Pigweed Poisoning in Horses

Due to nitrate poisoning in redroot pigweed, horses may suffer from life-threatening damage to their essential organs. At greatest risk are the kidneys, the heart, brain and urinary tract. If you observe any of the following symptoms in your horse, immediately contact the veterinarian. 

  • Difficulty breathing
  • Bluish-colored mucous membranes
  • Weakness
  • Goiter
  • Tremors
  • Sudden death

Causes of Redroot Pigweed Poisoning in Horses

Redroot pigweed poisoning in horses stems from substantial consumption of the plant or else small to moderate amounts at an uncommonly high concentration. A nitrate accumulator, pigweed contains two potentially noxious chemicals: nitrates and oxalates. 

Horses encounter pigweed not only in pastures, but also when unwittingly present in other healthy forage. It’s important to observe seasonal or weather-based changes in the plants in your pasture. Environmental factors that may increase the concentration of nitrates include draughts, excessively cloudy weather, or just before flowering.

Diagnosis of Redroot Pigweed Poisoning in Horses

If poisoning is suspected, a veterinarian should be called immediately. The best way to help your horse is to bring along a sample of any suspicious plant. 

Diagnosis will rely on owner report, plants samples and any perceived changes in behavior and symptoms. The veterinarian will consider conditions other than toxicity, and administer testing to the horse, including blood samples. In the unfortunate case of death, diagnosis will depend on autopsy.

Treatment of Redroot Pigweed Poisoning in Horses

In case of toxicity, your vet will develop an immediate treatment plan. If in time, the vet will try to empty the stomach of the horse to flush out remaining poison. In some cases, activated charcoal is effective because it works to absorb any ingested poison. Laxatives may be administered to speed up elimination. Symptoms such as diarrhea will be treated separately from the poisoning. Troubled respiration typically responds to an inhaler or nebulizer.

Recovery of Redroot Pigweed Poisoning in Horses

Fortunately, redroot pigweed is a plant that can be controlled by frequent, yearly mowing, and hand-pulling of large plants. Chemical control is also used on resistant pigweed; discuss these treatments with the veterinarian in order to get a recommendation for environmental management or referral to a specialist. Pastures should be walked daily in search of new, potentially toxic plants. Being able to identify noxious plants may just save the life of your beloved horse.