Abstract
Beef cows (n = 64) were slaughtered to evaluate effects of dietary energy and calf removal (CR) on hypothalamic and adenohypophysial endocrine characteristics. From d 190 of gestation until parturition, cows received maintenance (ME; n = 32) or low (LE; n = 32) energy diets (ME = 100%, LE = 70% NRC recommendations). After parturition, half (n = 16) of each prepartum diet group received low (LE; n = 32) or high (HE = 130% NRC; n = 32) energy diets. At 30 d postpartum, cows were slaughtered 0 or 48 hr after CR. Hypothalami [preoptic area (POA), hypothalamus (HYP), stalk-median eminence (SME)] and pituitaries were collected. Basal and K +-induced release of GnRH from SME, and pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) did not differ among groups (P>.05). Hypophyseal LH was correlated (P<.01) with body condition score (BCS) at parturition and slaughter (r = .36 and .47, respectively). Prepartum LE diet increased (P<.05) met-enkephalin in POA compared to prepartum ME (.59 ± .05 vs. .44 ± .04 pmol/mg) regardless of postpartum diet or suckling status. Concentrations of β-endorphin in combined HYP + POA were decreased (P<.05) by 48 hr CR (15.1 ± 1.1 vs. 18.1 ± 0.7 fmol/mg). Content of GnRH in HYP was decreased (P<.05) by 48 hr CR in animals on ME prepartum diets (17.0 ± 2.4 vs. 9.4 ± 0.7 ng). Cows on LE-HE diets had lower (P<.01) content of GnRH in POA than cows on LE-LE diets (7.8 ± 0.8 vs. 13.1 ± 0.7 ng, respectively). Overall, GnRH in HYP was decreased (P<.05) by 48 hr CR (11.3 ± 1.0 vs. 14.8 ± 1.4 ng) and content of GnRH in POA was lower (P<.05) in cows on HE postpartum diets compared to LE postpartum (8.9 ± 0.7 vs. 11.2 ± 0.7 ng, respectively). In summary, prepartum/postpartum effects of dietary energy/body condition on reproductive responses to CR were associated with changes in hypothalamic and preoptic levels of GnRH, met-enkephalin and β-endorphin.
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