Fall can be a beautiful time of year for horseback riding. However, falling leaves and frost can negatively impact horse health.
Ingestion of dried or wilted, but not fresh, maple leaves is associated with the toxicosis. Dried leaves are not generally believed to retain toxicity the following spring. Toxicosis normally occurs in the autumn when normal leaf fall occurs. Although studies indicate that leaves collected after September 15 are more toxic, cases of toxicosis in horses due to wilted leaves after summer storms have been observed. Horses are the only species for which maple leaf toxicity has been reported. Horses are often depressed, lethargic, and anorexic with dark red or brown urine after the first day of ingestion. They may progress to going down with labored breathing and increased heart rate before death. Horses should be fenced out of areas where wilted maple leaves are plentiful.
Prunus species (species in the cherry family) contain cyanide and should be removed from horse pastures. Cyanide is released after chewing the plant or seed, or when the plant material wilts (after a frost). Animals are most commonly found dead within minutes to a few hours of ingestion of the plant.
There are no reports of toxicity of horses grazing frost damaged pastures (includes grass and legume species). Frost damaged pastures can have higher concentrations of nonstructural carbohydrates, leading to an increase in potential for founder and colic, especially in horses diagnosed with or prone to obesity, laminitis and Equine Metabolic Syndrome. To reduce the chance of adverse health effects, it is recommended that horse owners wait up to a week before turning horses back onto a pasture after the first killing frost. Subsequent frosts are not a concern as the pasture plants were killed during the first frost.
This article is reprinted with permission from Krishona Martinson, University of Minnesota. This and other horse nutrition articles can be found at http://www.extension.umn.edu/agriculture/horse/nutrition/.