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Endocrine factors and ovarian follicles are influenced by body condition and somatotropin in postpartum beef cows^1,2

R. Flores^*, M. L. Looper^,3, R. W. Rorie^*, D. M. Hallford and C. F. Rosenkrans, Jr^*

^* 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: mike.looper{at}ars.usda.gov

Multiparous beef (1/4 to 3/8 Bos indicus; n = 99) 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 (b) ST on the number of follicles, diameter of largest follicle, and serum concentrations of IGF-I, triiodothy-ronine (T3), thyroxine (T4), and prolactin. 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 32, 46, and 60 postpartum. On d 60, all cows received a controlled internal drug-releasing (CIDR) device for 7 d and PGF₂ at CIDR removal (CIDR-PGF₂). Blood samples (7 mL) were collected at each bST treatment and d 39 and 67 postpartum. Ultrasound was performed 1 d after CIDR-PGF₂ to determine the number of small (2 to 9 mm) and large (10 mm) follicles and the diameter of largest follicle. Cows treated with bST in low BC had increased (P < 0.05) IGF-I vs. low-BC non-bST-treated cows on d 39, 46, 60, and 67 postpartum. Prolactin and T3 were greater (P < 0.05) in moderate-BC than in low-BC cows on all sample dates. Thyroxine was greater (P < 0.001) in moderate-BC cows on d 46, 60, and 67 compared with low-BC cows. On d 67, bST-treated cows had greater (P < 0.05) T4 compared with non-bST-treated cows. Diameter of the largest follicle 1 d after CIDR-PGF₂ was greater (P < 0.01) in anestrous cows treated with bST than for non-bST-treated anestrous cows. Diameter of the largest follicle was correlated with concentrations of IGF-I (r 0.18; P 0.08), T3 (r 0.17; P 0.10), and prolactin (r 0.20; P 0.06). Treatment with bST increased IGF-I in low-BC cows, and IGF-I was correlated with the diameter of the largest follicle 1 d after CIDR-PGF₂. Undernutrition of cattle may be communicated to the hypothalamic-pituitary-ovarian axis via metabolic hormones including IGF-I, thyroid hormones, or prolactin.

Key Words: beef cow • body condition • follicle • insulin-like growth factor-I • somatotropin • thyroid hormone

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