Tall fescue is a common grass species that makes up more than 40 million acres of pasture in the United States. This grass is commonly infected with a fungus capable of producing the ergot alkaloid ergovaline, an agent responsible for late abortion, prolonged gestation, dystocia (difficult birth), and agalactia (poor milk let-down) in broodmares, reduced growth rates in young horses, and increased respiratory rates in equine athletes recovering from exercise. To better understand how infected tall fescue seed impacts equine athletes, a research team from Missouri State University (MSU) set out to determine if the consumption of infected tall fescue seed adversely affected the recovery of horses exercising under hot and humid conditions.
Lead researcher Gary W. Webb, PhD, PAS, a professor at the MSU William H. Darr School of Agriculture, and colleagues assigned 10 Quarter Horses to one of two different treatment groups; the horses consumed either endophyte-infected or endophyte-free tall fescue seed with a predetermined grain ration twice daily. The team also offered the horses alfalfa hay throughout the study period. The horses remained in treatment groups for 35 days before switching treatments for another 35 days. Researchers measured the horses' water consumption twice daily and collected urine during Weeks 4-5 and 9-10 to verify adequate alkaloid consumption.
All study horses exercised five days a week during the study period; exercise included cutting work with a mechanical cow. All horses performed two different standardized exercise tests (SET) during Weeks 3, 5, 8, and 10. The anaerobic SET consisted of 10 minutes of warm-up, then horses were required to stop and turn 40 times in a four minute period, simulating cutting work. The aerobic SET consisted of four minutes of walking, 10 minutes of trotting, and 11 minutes of loping in both directions. Researchers measured horses' rectal temperature, heart rate, respiration rate, sweat production, and whole blood lactate, as well as the arena temperature and humidity level, at the beginning and end of each SET.
Upon reviewing their data, Webb and colleagues found that:
- All horses on the endophyte-infected tall fescue seed treatment had minimal feed refusal.
- Urine analysis showed that horses consuming the endophyte-infected tall fescue seed excreted a higher amount of alkaloids compared to the endophyte-free treatment group.
- Horses' water consumption increased during periods of high ambient temperature, but was not affected by the animals' treatment group; Similarly, the different treatments had no effect on horses' sweat production or whole blood lactate levels.
- Higher humidity levels affected all horses' recovery after the aerobic SET, but did not have an effect on horses performing the anaerobic SET.
- Heart rate values were significantly lower in all horses during rest and the either anaerobic or aerobic SET when consuming the endophyte-infected tall fescue seed. Webb suggested this could be due to a baroreceptor (one of the body's mechanisms for maintaining blood pressure) response to a lower maximum blood pressure, resulting from vasoconstriction (narrowing of the blood vessels) caused by ergot alkaloid consumption. He also noted that during the aerobic SET, horses had to trot faster to maintain a specific heart rate level due to lowered heart rates caused by vasoconstriction.
- Rectal temperature was higher in horses consuming the infected seed at 60 seconds and 30 minutes post exercise.
- Respiration rates were increased in horses consuming the infected seed at 30 minutes post-aerobic SET and at 30 and 60 minutes post-anaerobic SET.
Webb noted that the exercise tests were performed under hot, humid conditions that likely also compromised horses' cooling ability. However, the team concluded that the apparent vasoconstriction caused by endophyte-infected tall fescue seed consumption could affect performance horses' recovery post-exercise by reducing their ability to dissipate core body heat.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.