Effects of GnRH and hCG Injections on Lambing Rate of Estrus-Induced Ewes during the Non-breeding Season.

Four experiments were conducted to improve fertility using Suffolk ewes induced estrus by an intravaginal sponge impregnated with 60 mg medroxyprogesterone acetate (MAP) and PMSG treatments during the non-breeding season. Timing (Experiment 1) or dosage (Experiment 2) of a single injection of a synthetic GnRH after sponge removal or at the induced estrus, and effect of a 600 IU PMSG injection (one day before or at the time of sponge removal) combined with either 100 μg GnRH or 500 IU hCG injection at estrus (Experiment 3) were investigated. In Experiment 4, estrous ewes were treated with saline, 100 μg GnRH at estrus, 250 IU hCG injections or an insertion of MAP sponge on Days 10 to 12 after mating, or a combined treatment of GnRH at estrus and hCG after mating. Timing (36, 48 or 60 hr after sponge removal) and dosages (0, 25, 50 or 100μg) of a GnRH injection did not affect lambing rate and prolificacy. The timing of a PMSG injection also did not affect lambing rates, and neither GnRH nor hCG injection improved the fertility of ewes. Pregnancy and lambing rates were not significantly different between the treatments in Experiment 4. The present results indicate that a single GnRH or hCG injection at the induced estrus, successive injections of hCG on Days 10 to 12 after mating, or a combined treatment with a GnRH injection at estrus did not increase the fertility of ewes.