Breeding ewes out-of-season using melengestrol acetate, one injection of progesterone, or a controlled internal drug releasing device

Abstract

A series of studies was designed to identify methods of improving out-of-season breeding success in ewes. In Experiment 1, 190 mature ewes were assigned to receive in April, either: (A) a control ration of 0.3 kg corn twice daily for 8 d before ram introduction (control ewes n=49), (B) the control ration containing 0.125 mg of melengestrol acetate (MGA) in 0.3 kg corn (MGA8a ewes n=46), (C) the control ration for 7.5 d followed by 1 feeding of 0.5 mg MGA in 0.3 kg corn (MGA 1 ewes n=48), or (D) the control ration plus a 20 mg im injection of progesterone on D 8 (P ewes n=47). Ewes were exposed to rams for 21 d. A greater percentage of MGA8a and P ewes lambed than did control ewes (P < 0.04). The lambing rate was greatest among MGA8a (P < 0.02 vs. control), intermediate among P ewes (P < 0.19 vs. control) and least among MGA1 and control ewes (P > 0.79). In Experiment 2, 70 mature ewes were assigned to receive in June, either: (A) a control ration of 0.3 kg of corn twice daily for 8 d before to ram introduction (control ewes n=25, (B) the control ration containing 0.125 mg of MGA per 0.3 kg corn (MGA8b ewes n=21), or (C) the control ration and simultaneous treatment of ewes with a progesterone-containing controlled internal-drug releasing device (CIDR ewes n=24). Ewes were exposed to rams for 21 d. Both CIDR and MGA8b ewes exhibited estrus earlier than did control ewes (P < 0.01). The CIDR ewes exhibited estrus earlier than did the MGA8b ewes (P < 0.05). A greater percentage of ewes treated with CIDR or MGA8b lambed than did control ewes (P < 0.01), with more CIDR ewes lambing than MGA8b ewes (P < 0.01). The lambing rate was greater in CIDR ewes than in control ewes (P < 0.04). These data provide evidence that several options exist to improve pregnancy success in ewes bred out of season and that success varies with method of treatment.