Known to withstand drought and high traffic, tall fescue is common forage in the southeastern United States. However, these resistant properties are partly associated with an endophytic fungus that infects the tall fescue plant. While the endophyte might offer benefits to the plant, it produces compounds that can have negative effects on animals. Cattle consuming endophyte-infected tall fescue have decreased heat tolerance and increased rectal temperatures during hot weather.

Grey Parks, a graduate student working with Laurie Lawrence, PhD, at the University of Kentucky, conducted a study to determine if feeding endophyte-infected fescue would affect exercising horses, particularly their ability to recover from exercise in the heat.

Twelve healthy horses were studied from June to September 2008. Horses were adapted for six weeks to diet, housing, and an exercise regimen. Horses were carefully paired by age, body weight, body condition, and heart rate response to an exercise test at the end of the adaptation period. Within each pair, horses were randomly assigned to a diet containing either endophyte-infected or endophyte-free tall fescue seed. The fescue seed was mixed with sweet feed, water, and a small amount of liquid molasses. Seed was used in the experiment instead of pasture so the amount of the endophyte consumed by the horses could be accurately assessed. Enough seed was given to each horse to mimic the amount of endophytic toxins that would be obtained from typical endophyte-infected tall fescue pasture or hay.

After receiving the test diets for 21 days, horses were exercised for approximately one hour. Rectal temperatures increased approximately 2 degrees Fahrenheit during exercise. Horses receiving the endophyte-infected tall fescue seed had slightly higher rectal temperatures in the recovery period following exercise. Heart rate responses during exercise and recovery were similar between the horses receiving the endophyte-infected seed and those receiving the endophyte-free seed. The horses receiving the endophyte-infected tall fescue seed had higher levels of T3, a hormone that helps regulate body metabolism, and they had lower levels of prolactin, a hormone that is commonly decreased in animals with fescue toxicosis