Abstract
Two trials were conducted to evaluate effects of injection time of pregnant mare'sserum gonadotropin (PMSG), gonadotropin releasing hormone (GnRH) and individual rams on fertility in seasonally anestrous Suffolk ewes. Trials 1 and 2 were carried out at 2-weeks apart during April to May. Hormonal treatments were; I) 60 mg 6-methyl-17-acetoxy-progesterone (MAP) vaginal sponge for 9 days insertion, 600 IU PMSG at the time of sponge removal and a 100 μg GnRH injection at estrus detection, II) MAP sponge for 9 days and 600 IU PMSG 2 days before sponge removal, and III) II)+ the GnRH injection. In Trials 1 and 2, 50 and 60 ewes were used, respectively. For natural mating, 5 mature Suffolk rams were used to compare the reproductive efficiency. In Trial 1, proportions of estrous ewes within 4 days after treatment and lambing rates were not significantly different among the groups (92.9% and 53.8%, 85.7% and 58.3%, 92.3% and 58.3% for groups I, II and III, respectively). However, there were significant differences in both proportions of estrous ewes (P< 0.05) and lambing rates (P<0.05) among the rams used. In Trial 2, group II showed a significantly higher lambing rate (57.1%) than group I (15.8%), although the proportion of estrous ewes in group II was significantly low (P<0.05). Lambing rates in Trial 2 significantly decreased as compared with Trial 1, but not for the proportion of estrous ewes and prolificacy. It appears that the repeated uses of the same 5 rams at 2-weeks apart lowered fertility in Trial 2. The present study indicates that a GnRH injection may be unnecessary if the injection of PMSG was given 2 days before sponge removal. The PMSG injection 2 days before the progestogen withdrawal and the use of rams with high libido and semen quality could induce improvement of fertility in hormonally treated ewes during the non-breeding season.
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