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,3
* Department of Animal Science, University of Arkansas, Fayetteville 72701;
and
USDA-ARS, Dale Bumpers Small Farms Research Center, Booneville, AR 72927; and
and
Department of Animal and Range Sciences, New Mexico State University, Las Cruces 88003
3 Corresponding author: mlooper{at}spa.ars.usda.gov
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, and concentrations of serum GH and plasma NEFA. 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 the 70-d breeding season), the CIDR were removed and cows received prostaglandin F2
(PGF2). Blood samples were collected from the median caudal vein of the cows at each bST treatment and at d –28 and 0. Estrous behavior was monitored by radiotelemetry during the first 30 d of the breeding season. Growth hormone was increased (P < 0.05) in low and moderate BC cows treated with bST. The percentage of cows detected in estrus during the first 30 d of the breeding season was decreased (P = 0.05) for low BC (64%) compared with moderate BC (82%) cows. The interval to first estrus tended (P = 0.07) to be shorter in low BC-bST-treated cows (3.7 ± 1.9 d) than in moderate BC-bST-treated cows (9.6 ± 1.8 d). During the first 30 d of the breeding season, cows in low BC had a decreased (P = 0.02) number of mounts received and increased (P = 0.001) quiescence between mounts compared with cows in moderate BC. The number of mounts received was reduced (P = 0.04) in bST-treated cows. More (P = 0.02) cows treated with bST became pregnant during the first 3 d of the breeding season compared with non-bST-treated cows. The cumulative first-service conception rate tended (P = 0.07) to be greater for bST-low BC cows than non-bST-treated cows in low or moderate BC. On d 0, NEFA were greater (P < 0.05) in bST-treated vs. non-bST-treated cows. Low BC and bST reduced the intensity of behavioral estrus in postpartum Brahman-influenced cows. However, bST increased the first-service conception rate during the first 30 d of breeding and pregnancy rates during the first 3 d of breeding in postpartum Brahman-influenced cows.
Key Words: beef cow • body condition • conception rate • estrous behavior • somatotropin
2 This study was supported in part by USDA, Agricultural Research Service cooperative agreement #58-6227-8-040. The authors gratefully acknowledge L. Huddleston, W. Jackson, G. Robson, and D. Jones, USDA-ARS, for technical assistance and daily animal management.
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