173 THE EFFECT OF EXERCISE ON FOLLICULOGENESIS IN MARES

Abstract

The effects of exercise on follicular development in mares have not been well established; however, a combination of exercise and heat stress can negatively affect embryo recovery rate in mares. The purpose of this research was to examine the effects of exercise, independent of heat stress, on follicular development in mares. One thoroughbred and 12 quarter horse mares, aged 3 to 15 years, were used in this study. Mares were assigned to the control group or exercised at 0700 h, 6 days a week, as follows: warmed up 5 min at a walk for 3 m s^–1 and then exercised 30 min, 9 m s^–1 for 10 min, 13 m s^–1 for 5 min, 9 m s^–1 for 5 min, 13 m s^–1 for 5 min, and 9 m s^–1 for 5 min. Exercised mares were given a 1-month conditioning period. Rectal temperatures were recorded immediately before and after exercise. The mean rectal temperature of mares increased by 0.85°C during exercise, whereas control mares under the same environmental conditions did not experience temperature increases. Individual mares underwent a daily transrectal ultrasound examination with a 7.5-MHz linear rectal probe to monitor follicular development. Estrous cycles were normalized to 17 days for linear follicular growth analysis. Exercise had a significant impact on follicular growth at deviation, with a greater diameter of the largest (P < 0.05) and second largest (P < 0.01) follicles in exercised v. control mares (Table 1). Additionally, growth rate of the second largest follicle was greater in the exercised group (P < 0.01). Deviation was observed in 13 of 14 cycles in control mares, whereas in exercised mares, it was clear in only 11 of 16 cycles (χ², P = 0.086). Time of deviation was defined as the day when a difference in the growth rate between the dominant and largest subordinate follicle of a wave was first observed. The largest subordinate follicle either declined in growth rate or regressed. At Day –5 (relative to ovulation), exercised mares had significantly greater follicle diameters in both the largest and second largest follicle (P < 0.05). Exercised mares had double ovulations in 4 of 16 cycles when compared with control mares, which had double ovulations in 1 of 14 cycles. No differences were found between groups in the length of the estrous cycle or ovulatory follicular diameter. Exercised mares exhibited larger diameter follicles and an increased growth rate of the largest subordinate follicle, suggesting that exercise delayed deviation. Further research is needed to assess the impact of exercise, independent of heat stress, on fertility.

Table 1.  Follicle dynamics in exercised v. non-exercised mares