Veterinarians generally assess whether a mare has ovulated and is ready to breed by rectal palpation. However, new research suggests that a mare’s blood plasma sample can tell the veterinarian just as accurately that ovulation has occurred. The Japan Racing Association (JRA)-sponsored study was presented at the 43rd Annual Meeting of Investigation and Research Concerning Racehorses, held Dec. 3, 2001, at the University of Tokyo in Japan.

The study confirms the significance of follicular fluid that is released during ovulation. According to Yasuo Nambo, DVM, PhD, of the JRA’s Equine Research Institute, plasma hormone concentrations before and after ovulation were carefully analyzed. Researchers found that the concentration of inhibin, which inhibits synthesis of follicle-stimulating hormone, and inhibin pro-aC (which is a precursor of inhibin) more than doubled within a few hours after ovulation. They hypothesized that this increase was due to absorption of inhibin (from the follicular fluid released by the ruptured follicle) via the abdominal cavity, rather than an increase in the mare’s secretion of inhibin.

Nambo and his colleagues placed 50 mL (the typical amount a follicle releases) of equine follicular fluid (eFF) in the abdominal cavity. One hour after administration of the fluid, plasma levels of inhibin were measured. “Levels of inhibin were significantly higher in eFF-treated mares than in control mares,” explained Nambo. “The hormone profiles in eFF-treated mares were similar to those in mares with spontaneous or induced ovulations.

“Knowing the exact time of ovulation in both the donor mare and recipient mare is important in making embryo transfer or artificial insemination with frozen semen successful,” said Nambo