Much of the country is experiencing drier than normal conditions this summer and, thus, some horses living on pasture might soon have limited forage choices. Owners must take care to ensure pastures do not contain certain plant and weed species capable of producing toxins during stress conditions such as drought.
Horses aren't as susceptible to nitrate toxicity or prussic acid poisoning from plants as are ruminant species, such as cattle and sheep. A rumen's microorganisms facilitate toxin release from the plants into the animal's digestive tract. Horses, however, are monogastric (one- stomached) animals and are more capable of breaking down prussic acid in the stomach, and convert very little nitrate to nitrite (wherein lies the problem in cows) in the large intestine.
Nitrate and prussic acid toxicity in horses are rare, but when they do occur, they typically have serious consequences.
During periods of low moisture, certain grass species can produce toxic products such as nitrates. Nitrates accumulate in plants--such as pigweed, smartweed, ragweed, nightshade, and goldenrod--that are heavily fertilized with nitrogen, or during certain weather conditions and after frost.
Should a horse consume excessive amounts of affected plants, the nitrates cause a series of reactions that prevent adequate oxygen transport in the blood cells. Clinical signs of poisoning might include difficulty breathing, weakness, tremors, ataxia, rapid heartbeat, grey/blue or brown discoloration of blood and tissues, seizures, and rapid death.
To prevent nitrate poisoning in horses, check pastures for plants that are most likely to produce nitrates (such as the aforementioned species) and remove them (for additional plants to watch for in specific geographic areas, contact your local extension agent). Additionally, have any hay produced during drought conditions analyzed by a laboratory to evaluate the nitrate concentration prior to feeding.
Prussic acid is another possible danger to horses on pasture during drought conditions.. This glycoside is normally present in plants but is produced in toxic levels during drought. The plant species of main concern include johnsongrass, sorghum, sudangrass, and wild cherry trees. Like nitrates, prussic acid affects the oxygen present in red blood cells.
Prussic acid levels decrease over time, and, therefore hay produced during drought will most likely be safe to feed horses if baled at 18-20% moisture, a common practice in hay production (hay baled above 20% moisture will often mold, ferment, or combust). If concerns arise, a hay analysis can determine whether prussic acid is present at toxic levels. Owners should fence off wild cherry trees in pastures so horses cannot chew on them, particularly the potentially toxic branches and leaves.
The most proactive approach to preventing your horse from ingesting drought-stressed plants that can produce nitrates or prussic acid is to avoid turning hungry animals out onto suspect pasture without an adequate, good-quality forage source such as hay. Additionally, evaluate pastures on a regular basis to determine if potentially dangerous species of grasses are present; if so, remove them from the field or fence them off to prevent consumption.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.