Abstract
Objectives of this study were to determine effects of exogenous progesterone (via controlled internal drug releasing devices; CIDR) in combination with exogenous gonadotropins (PMSG/hCG) use either at CIDR removal or 1 d before CIDR removal to induce estrus and cyclicity and subsequently enhance the proportion of ewes lambing, lambing rate, prolificacy, and days to lambing in ewes bred out of season. Multiparous ewes (n = 414) were randomly assigned to one of four treatments: untreated (U, n = 122), 7 d CIDR (C, n = 97), 7 d CIDR plus P.G. 600 (240 IU pregnant mare serum gonadotropin [PMSG] and 120 IU human chorionic gonadotropin [hCG]) at CIDR removal (CPG0, n = 97), and 7 d CIDR plus P.G. 600 (240 IU PMSG and 120 IU hCG) 1 d before CIDR removal (CPG-1, n = 98). Rams (n = 15) were joined with ewes immediately after CIDR removal and remained with ewes for a 21 d breeding period. Lambing data were summarized for the first 10 days of the lambing season and overall. Categorical data were analyzed using the GLIMMIX procedure of SAS whereas non-categorical data were analyzed using the mixed procedure. Proportion of ewes lambing in the first 10 d was greater (P < 0.05) for CPG0 and CPG-1 ewes compared with C ewes, which was greater (P < 0.0001) compared with U ewes. Overall proportion of ewes lambing was greater (P ≤ 0.0001) in all treatments utilizing CIDR compared with U ewes, but no differences (P ≥ 0.84) due to P.G. 600 were detected compared with C. Lambing rate in the first 10 d was greater (P < 0.05) for CPG-1 than C, with CPGO being intermediate, and all CIDR-treated ewes being greater than U (P < 0.0001). Overall lambing rate increased (P ≤ 0.0001) in all treatments utilizing CIDR compared with U ewes, but no differences (P ≥ 0.76) due to P.G. 600 were detected compared with C. Prolificacy was similar among all treatments both for the first 10 d of the lambing season (P = 0.86) and overall (P = 0.80). Day of lambing in the lambing season was reduced (P ≤ 0.03) for CPG0 and CPG-1 compared with CIDR-treated ewes, which was reduced (P < 0.0001) compared with U ewes (days 10.6, 9.0, 13.4, and 24.4 ± 0.9 for CPG0, CPG-1, CON, and U, respectively). Though timing of P.G. 600 did not influence results, the combination of CIDR and P.G. 600 enhanced the proportion of lambs born earlier in the lambing season, and incorporating a CIDR with or without P.G. 600 enhanced the overall proportion of ewes lambing and lambing rate in out-of-season breeding scenarios.