Body energy reserves and somatotropin influence estrous behavior, reproductive performance, and endocrine and follicular dynamics of Brahman-influenced cows

Ninety-nine, multiparous Brahman-influenced (1/4 to 3/8 Brahman) cows were managed to achieve low (BCS = 4.3 +/- 0.1; n = 50) or moderate (BCS = 6.1 +/- 0.1; n = 49) body condition (BC) to determine the influence of bovine somatotropin (bST) on estrous characteristics, reproductive performance, follicular dynamics, and concentrations of metabolic hormones and metabolites. Beginning 32 d postpartum, cows within each BC were assigned randomly to treatment with or without bST. Non-bST treated cows received no treatment, and treated cows were administered bST (Posilac, 500 mg, s.c.) on d -35, -21, and -7 before initiation of the breeding season. On d -7, all cows received an intravaginal, controlled internal drug-releasing (CIDR) device. On d 0 (initiation of 70-d breeding season), CIDR were removed and cows received prostaglandin F 2alpha (PGF2alpha). Blood samples were collected from the caudal vein of cows at each bST treatment and d -28 and 0. Estrous behavior was monitored by radiotelemetry during the first 30 d of the breeding season. Growth hormone was increased in low and moderate BC cows treated with bST. Treatment with bST increased IGF-I in low BC cows, and IGF-I was correlated with diameter of the largest follicle 1 day after CIDR-PGF2alpha. The percentage of cows detected in estrus during the first 30 d of the breeding season was decreased for low BC compared with moderate BC cows. The number of mounts received was reduced in bST-treated cows. More cows treated with bST became pregnant during the first 3 d of breeding season compared with non-bST treated cows. Low BC as well as bST reduced intensity of behavioral estrus in postpartum Brahman-influenced cows. However, bST increased first-service conception during the first 30 d of breeding and pregnancy rates during the first 3 d of breeding in postpartum Brahman-influenced cows.