EFFECTS OF BOVINE SOMATOTROPIN (BST) ON OVARIAN-FUNCTION IN POSTPARTUM BEEF-COWS

The effects of bovine somatotrophin (bST) on ovarian follicle developm ent and function and associated gonadotrophin profiles during the firs t nine weeks post partum were investigated in beef cows. Thirty-two co ws (Shorthom x Galloway) in moderately low body condition (BC) at calv ing were fed to maintain BC thereafter. At Weeks 2, 4, 6 and 8 post pa rtum, animals were injected with 320 mg bovine somatotrophin (bST) (T, treated; n = 17) or with the carrier oil only (C, control; n = 15). O vulation occurred in 4 of 17 T cows and 0 of 15 C cows (P = 0.10) by n ine week post partum. Treatment with bST did not affect the numbers of small (3-8 mm in diameter) or large (>8 mm in diameter) follicles or the granulosa cell populations but enhanced the oestradiol (P < 0.05) and insulin-like growth factor-I (IGF-I) content (P < 0.01) of large f ollicles by nine weeks post partum. It did not significantly affect th e testosterone concentrations of large follicles. Circulating concentr ations of growth hormone (GH) and IGF-I were higher in T cows than in C cows (P < 0.001) but were unrelated to gonadotrophin profiles or gon adotrophin receptor concentrations in the follicles. At Week 8, plasma insulin concentrations were higher in T cows than in C cows both befo re (P < 0.05) and after (P < 0.05) glucose injection. It is concluded that GH may play an important role in mediating the effects of nutriti onal state on ovarian function during the post-partum period, possibly through alteration of intrafollicular IGF-I concentrations.