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Circulating gonadotrophins during a period of restricted energy intake in relation to body condition in heifers

Summary. Beef heifers, 13 months old, were fed to achieve high (7·6 ± 0·2 units) or low (3·9 ± 0·1 units) body condition by feeding them one of two diets for 20 weeks. During week 17 of the growth phase, all heifers were ovariectomized. From week 20 to week 27 (restriction phase), all heifers were fed a daily diet containing 0·071 MJ metabolizable energy kg^–1 body weight. At weekly intervals throughout the restriction phase, blood samples were collected at 10-min intervals for 11 h to determine the pattern of secretion of luteinizing hormone (LH), the amount of LH released in response to 750 ng (pituitary responsiveness) and 50 µg LH-releasing hormone (LHRH, releasable stores) and mean concentrations of follicle-stimulating hormone (FSH) in the circulation. Body weight declined during the restriction phase in a similar fashion in heifers with high and low body condition and changes in body weight were unrelated to mean concentrations of LH and FSH and frequency of LH pulses. Amplitude of LH pulses and responsiveness to 750 ng LHRH increased in a linear fashion with weight loss in heifers with low but not in those with high body condition. The amount of LH released in response to 50 µg LHRH decreased with increasing weight loss in heifers with high but not with low body condition, indicating that releasable pools of LH declined with increased weight loss in heifers with high body condition. The results indicated that the pattern of LH in the circulation, responsiveness of the pituitary to LHRH and releasable stores of LH in heifers fed diets of low energy content are modulated by body condition.

Keywords: body condition; energy; luteinizing hormone; follicle-stimulating hormone; heifers