Effect of stimulation with a gonadotropin mixture on reproductive outcome in nulliparous ewes bred during seasonal anestrus and early breeding season

To evaluate the effect of gonadotropic stimulation on the reproductive response of nulliparous acyclic female sheep during mid-anestrous and early breeding season, fall-born ewe lambs of mixed breeding (N = 191; ∼7 months old; study 1) and yearlings (Dorset X Texel (DT), N = 120, study 2) were assigned randomly during May to July to be either pre-treated with progesterone using CIDR-g (0.3 g of progesterone) devices for 5 days or to also receive gonadotropins (240 IU eCG and 120 IU hCG, 3 mL i.m. injection of P.G. 600®) at CIDR-g removal. Study 3 was conducted with nulliparous acyclic DT yearling ewes (N = 104) in late August and was similar to study 2 except that a group of control ewes that were not treated was included. Gonadotropic stimulation increased estrous response (P = 0.0002), pregnancy rate to the first service period (P = 0.0007), proportion of ewes lambing (P = 0.03) and the lambing rate (P = 0.01) in fall-born ewe lambs (study 1), but did not modify reproductive outcome in yearling females (study 2) during mid-anestrus. During the transition into the breeding season (study 3), progesterone pre-treatment increased the pregnancy rate and the proportion of ewes lambing to first service after ram introduction (P = 0.003 and P = 0.02, respectively). However, gonadotropic stimulation of progesterone pre-treated yearlings did not modify reproductive performance variables. In conclusion, gonadotropic stimulation improved reproductive outcome in fall-born ewe lambs bred during the mid-anestrous period, but did not affect reproductive outcome in yearling females.