EFFECT OF BODY-WEIGHT AND BODY CONDITION SCORE ON FOLLICULAR DEVELOPMENT AND OVULATION IN MARES TREATED WITH GNRH ANALOG

Anestrous mares (n=99) were assigned randomly to 1 of 5 treatments. Tr eatments 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 subcutaneou sly in the neck. Data were collected on body condition score, body wei ght, number and size of follicles and time of ovulation. After an init ial 6 wk acclimation period, all treatment groups were fed a diet of m ixed hay and grain to maintain or increase body weight. Reproductive p arameters were collected twice weekly or daily when in estrus. Palpati on and ultrasonography were used to determine number of follicles >30 mm and ovulation. Blood samples were collected and measured by radioim munoassay for serum concentration of progesterone. 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 i n body weight on incidence of ovulation. Additionally, there was an in teraction between the final body condition score and treatment that ha d an effect (P less-than-or-equal-to 0.05) on follicle number and an e ffect (P less-than-or-equal-to 0.05) on the ovulation rate of the mare s. However, a change in body weight did not effect (P>0.10) follicle n umber 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) increas ed the incidence of ovulation and that a positive change in body weigh t increased the incidence of ovulation in mares.