A single dose of bovine somatotropin 5 days before the end of progestin-based estrous synchronization increases prolificacy in sheep

Bovine somatotropin (bST) enhances ovarian follicular and embryonic development in sheep and cattle. In the present study, the objective was to assess whether bST given 5 days before the end of progestin-based estrous synchronization improves prolificacy and lambing rate in sheep. Pelibuey ewes (n = 92) exhibiting estrous cycles at regular intervals received an intravaginal sponge containing 45 mg of FGA for 12 days. Five days before sponge withdrawal, ewes were treated with either 125 mg of bST sc (bST group; n = 47) or saline solution (control; n = 45). After the sponge was removed, ewes were observed for estrus and subsequently mated twice. Lambing rate and prolificacy was determined at birth. Blood samples were taken from the time of treatment until day 15 after estrus in eight ewes from the bST group and nine from the control group. Concentrations of IGF-I were determined by immunoradiometric assay and progesterone by RIA. Treatment with bST increased (P < 0.01) the proportion of ewes with more than one lamb (bST, 56% compared with control, 26%) and prolificacy (bST, 1.6 compared with control, 1.3). Treatment with bST increased (P < 0.05) the lambing rate of multiparous (bST, 92% compared with control, 67%) but not in ewes at the first time they were mated (bST, 71% compared with control, 87%; P > 0.05). IGF-I concentrations were greater (P < 0.01) in ewes treated with bST than in control ewes from 2 days after treatment. Progesterone concentrations did not vary (P > 0.05) between groups. It is concluded that a single dose of bST 5 days before progestin withdrawal increases lambing rate and prolificacy in sheep. These effects are associated with an increase in circulating concentrations of IGF-I.