Effect of Nutrition, Lactation and Gonadotropin Stimulation on Ram-Induced LH Surge and Subsequent Fertility in Early Post-Partum Ewes during Seasonal Anestrous

Effect of Nutrition, Lactation and Gonadotropin Stimulation on Ram-Induced LH Surge and Subsequent Fertility in Early Post-Partum Ewes during Seasonal Anestrous. Abiodun Emmanuel Adebiyi This study evaluated the effect of lactation, exogenous gonadotropin (GS), and nutritional supplementation (SUPP) on fertility of Katahdin ewes during seasonal anestrous. Experiment 1 evaluated effects of lactation and GS on ram-induced LH surge. Lactating (65 to 80 d postpartum; pp) and dry ewes were randomly assigned to be treated with an estrus induction protocol (EIP) consisting of a 5-d treatment with a CIDR device with an injection of a gonadotropin mixture (240 IU eCG and 120 IU hCG; P.G. 600®; 3 ml/animal i.m.; n = 16) or no further treatment (n=16) at removal of CIDRs. Experiment 2 was similar to experiment 1 and evaluated the specific effects of lactation and GS on the ram-induced LH surge and subsequent fertility. Lactating (approximately 68.7 ± 1.3 days pp) and dry ewes were treated with CIDR device with (n = 42) or without (n = 38) an injection of the gonadotropin mixture. In Experiment 3, effects of both levels of nutritional supplementation and GS on fertility of lactating ewes were evaluated. Lactating ewes (n = 87) on pasture, were assigned randomly and then balanced for litter size to receive either a low or high amount of grain supplementation (13.3% Crude Protein; CP; 0.68 kg or 1.14 kg/d/animal; equivalent to 30 and 50% of projected Dry matter intake; DMI, respectively) from 1.5 to 2.5 months postpartum and then treated with CIDR for 5 days with or without GS at CIDR removal. Analysis of Variance (ANOVA) was used to evaluate the main effects and their interactions on response variables. In experiment 1, the proportion of ewes with a detectable LH surge was greater (P = 0.0001) in ewes treated with EIP with an injection of gonadotropin mixture, but there was no effect of lactation status. In experiment 2, the mean proportion of ewes with detectable LH surge tended to be higher in ewes receiving GS than in ewes not receiving GS (P = 0.07; 87.1% vs 57.2%, respectively). Estrous response was higher in ewes receiving than in ewes not receiving GS (P = 0.0045; 63.9% vs 38.6%, respectively) with no effect of lactation status. GS tended to increase estrous response in lactating but not dry ewes (LACT x GS, P = 0.1). Conception and pregnancy rates were lower in lactating ewes compared to dry ewes (P = 0.0172, 0.0131). In Experiment 3, estrous response was greater in ewes receiving than in those not receiving GS (P = 0.009; 67.4 ± 7 and 33.6 ± 7%, respectively) and tended to be greater in ewes receiving the high vs. low level of supplementation (P = 0.10; 58.6 ± 8 and 42.3 ± 6%, respectively). The pregnancy rate (PR) was increased by GS (P = 0.003; 49 ± 7 and 19.9 ± 7%, for ewes receiving and not receiving GS, respectively) and by increasing the amount of supplement (P = 0.05; 43.8 ± 8 and 25.1 ± 6%, for ewes receiving the high and low level of supplementation, respectively). The interaction of GS and nutritional supplementation was not significant for any of the variables measured (P > 0.1). The mean peak concentration of LH and time to peak concentration in ewes with detectable LH surge were not affected by any treatment in this study. In conclusion, during anestrous, early postpartum fertility is reduced by lactation in Katahdin ewes, but fertility in lactating ewes can be restored to that of dry ewes with nutritional supplementation and by providing exogenous gonadotropin as part of an EIP.