Effects of exogenous GnRH administration at insemination on pregnancy rates of estrus-synchronized seven ewe populations during the breeding season.

The objective of this study was to investigate the effects of GnRH at insemination on pregnancy and lambing in seven ewe populations during the breeding season. Estrus was synchronized in 1560 adult ewes using an intravaginal sponge impregnated with flurogestone acetate. The sponge was left in the vagina for 12 days followed by an injection of 330 IU of eCG at sponge removal. Each ewe was inseminated twice at 48 h and 60 h after sponge removal. The treatment group was intramuscularly injected at the first insemination with a dose of 16 μg GnRH and the control group with saline solution in each ewe population. The results showed that GnRH administration significantly decreased the pregnancy rate in three ewe populations, but had no effects in four ewe populations. Additionally, the litter size tended to increase in the treatment group compared to the control group in all seven ewe populations, but the difference was not significant. In conclusion, GnRH administration at insemination was not recommended for ewes undergoing a timed artificial insemination during the breeding season. The breed/population may be a critical determinant of the potential for exploiting GnRH application in sheep breeding programs.