Effect of body weight and body condition score on follicular development and ovulation in mares treated with GnRH analogue

Anestrous mares (n=99) were assigned randomly to 1 of 5 treatments. Treatments 1, 2, 3, 4, and 5 consisted of implants containing 0, 0.9, 1.8, 3.6 and 5.4 mg of a GnRH analogue, respectively, placed subcutaneously in the neck. Data were collected on body condition score, body weight, number and size of follicles and time of ovulation. After an initial 6 wk acclimation period, all treatment groups were fed a diet of mixed hay and grain to maintain or increase body weight. Reproductive parameters were collected twice weekly or daily when in estrus. Palpation and ultrasonography were used to determine number of follicles >30 mm and ovulation. Blood samples were collected and measured by radioimmunoassay for serum concentration of proges terone. Mares were weighed and body condition scored twice during the acclimation period, at the time of implantation and at the end of the study. There was an effect (P<0.01) of GnRH treatments 4 and 5 and an effect (P<0.01) of change in body weight on incidence of ovulation. Additionally, there was an interaction between the final body condition score and treatment that had an effect (P≤0.05) on follicle number and an effect (P≤0.05) on the ovulation rate of the mares. However, a change in body weight did not effect (P>0.10) follicle number or size. Similarly the change in body condition scores over the 30 d period did not effect (P<0.10) follicular number, size or time of ovulation. The results suggest that GnRH (treatments 4 and 5) increased the incidence of ovulation and that a positive change in body weight increased the incidence of ovulation in mares.