The objective of the present study was to verify the seasonal luteal activity of racehorses in training in Japan from March to August. We allocated 102 horses into a luteal activity group and non-luteal activity group. The luteal activity group included horses with serum progesterone levels that were consistently >1 ng/ml and changed by ± 1 ng/ml. In contrast, the progesterone levels of the non-luteal activity group were consistently <1 ng/ml. In late spring (from May 1 to June 30) and summer (from July 1 to August 31), the percentage of horses in the luteal activity group was significantly higher than in early spring (from March 1 to April 30, P<0.01). These findings demonstrate clear seasonal variations in ovarian activity. The present study also suggest that training for a race may not affect ovarian activity in female racehorses.
Keywords: progesterone; racehorse; seasonal change.