The effects of melengestrol acetate (MGA) and P.G. 600 on ewe fertility outside the natural breeding season were evaluated. Rambouillet ewes were assigned to one of four groups: (1) control (C; n = 92); (2) PG600 (n = 86); (3) MGA (n = 99); and (4) MGA + PG600 (n = 92). A pellet with or without MGA (0.3 mg/ewe/d) was fed at 0.15 kg/ewe/d for 7 d. On the last day of pellet feeding, ewes were given either saline or 5 mL of P.G. 600 i.m. (400 IU equine chorionic gonadotropin (eCG) and 200 IU human chorionic gonadotropin (hCG)). Ultrasonography was performed between Days 20 and 25 of gestation for ewes that were mated during the first 6 d of the breeding period from the MGA (n = 15) and MGA + PG600 (n = 8) groups, and the number of luteal structures and embryos were counted. During the first 6 d of the breeding period, MGA increased (P < 0.05) the percentage of ewes that mated and conceived when compared to C and PG600 (24.2% vs. 3.3% and 10.5%, respectively). Relative to MGA, the mean ( S.E.M.) number of luteal structures per ewe was enhanced (P < 0.03) in MGA + PG600 (1.53 0.13 vs. 2.38 0.42, respectively), however as pregnancy progressed, the number of embryos (1.5 0.13 vs. 1.8 0.16, respectively) and lambs born (1.3 0.15 vs. 1.5 0.27, respectively) did not differ. Treatment with MGA reduced (P < 0.01) the interval from ram introduction to lambing relative to groups that did not receive MGA (168 0.8 d vs. 171 0.6 d, respectively). In conclusion, treatment with MGA increased the percentage of ewes conceiving early in the breeding period. Although P.G. 600 increased the number of luteal structures present per ewe, it did not significantly enhance ewe prolificacy.
# 2008 Published by Elsevier Inc.