Endocrine factors and ovarian follicles are influenced by body condition and somatotropin in postpartum beef cows

¹ Department of Animal Science, University of Arkansas, Fayetteville, 72701
² USDA-ARS, Dale Bumpers Small Farms Research Center, Booneville, AR 72927
³ Department of Animal and Range Sciences, New Mexico State University, Las Cruces 88003

^* To whom correspondence should be addressed. E-mail: mike.looper{at}ars.usda.gov.

	Abstract

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 number of follicles, diameter of largest follicle, and serum concentrations of IGF-I, triiodothyronine (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 number of small (2 to 9 mm) and large ( 10 mm) follicles, and 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 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 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 0.05). Treatment with bST increased IGF-I in low BC cows, and IGF-I was correlated with 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 that include IGF-I, thyroid hormones and (or) prolactin.

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

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