Continuous infusion of gonadotrophin-releasing hormone (GnRH) advances the onset of oestrous cycles in thoroughbred mares on Australian studfarms.

Continuous infusion of gonadotrophin releasing hormone (GnRH) has been shown to induce oestrus and ovulation in experimental mares in seasonal anoestrus. The present study sought to apply this form of treatment to anoestrous mares subjected to normal management procedures on 15 commercial Thoroughbred studfarms in Australia. The GnRH was delivered via subcutaneously implanted osmotic minipumps (OMP) at dose rates of 100 ng/kg/h and 200 ng/kg/h. The ovaries of treated mares were palpated on the day of OMP insertion (Day 0) and blood samples were collected twice weekly from a proportion of mares for assay of plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone concentrations. The period from commencement of GnRH infusion to first ovulation was significantly shorter (P less than 0.001) in treated than control mares and was negatively correlated with the total ovarian volume (P less than 0.01) and the diameter of the largest follicle (P less than 0.05) at onset of treatment. GnRH infusion produced an immediate increase in plasma LH concentrations in the mares that ovulated during the infusion period and LH levels peaked at the time of ovulation. Plasma progesterone profiles indicated that the induced corpus luteum was of normal lifespan and progesterone secreting ability. The mean areas under the LH and FSH response curves in treated mares were significantly greater than in control mares (LH, P less than 0.001; FSH, P less than 0.05). However, there was no statistically significant difference in mean areas under the LH and FSH curves in the GnRH-treated groups. Continuous infusions of GnRH induced ovulation in a high proportion of seasonally anoestrous mares on commercial studfarms.(ABSTRACT TRUNCATED AT 250 WORDS)