Approaches to improve the ovulatory response and reproductive performance of ewes introduced to rams during seasonal anestrus

Three experiments were conducted to test hypotheses relative to the ability of gonadotropin releasing hormone (GnRH) and melatonin to improve responses of anestrous ewes to rams. Treatment with GnRH two days after treatment with progesterone at introduction of rams did not increase ovulation, pregnancy or lambing rates. Treatment with GnRH on days two, seven, or both after introduction of rams, resulted in ovulation, pregnancy, and lambing rates that did not differ. In another trial, GnRH four days before, or four days before and one day after introduction of rams did not improve a consistently high ovulatory response to introduction of rams without further treatment. Presence of corpora lutea in response to treatment was essential to synchronization of estrus with prostaglandin F2alpha. Treatment for 35 days with a melatonin implant increased the ability of anestrous ewes to respond to introduction of rams, more so in non-lactating than in lactating ewes.