Effects of dietary protein percentage and beta-agonist administered to prepubertal ewes on mammary gland growth and hormone secretions.

The effects of dietary protein and beta-agonist, (L-644,969) on mammary growth from weaning to puberty, hormone secretions, and milk yield were studied. Twelve lambs were assigned to each treatment in a 2 x 2 factorial arrangement with dietary protein at 15 or 20% and beta-agonist at 0 or 1 ppm of concentrate. Lambs were fed pelleted concentrate to achieve a target weight gain of 260 g/d. The treatment period continued for 102 d, with protein starting on d 1 and beta-agonist on d 20. Five lambs in each group were slaughtered at the end of treatment to investigate mammary growth before puberty. The remainder were maintained, and, after lambing, daily milk yield was recorded for 7 wk and ewes were then slaughtered. In prepubertal lambs, compared with control, the 20% protein diet had no effect on any variable measured in the mammary glands, whereas beta-agonist increased (P < .05) RNA:DNA ratio. In lactating ewes, the 20% protein diet enhanced mammary gland weight, parenchyma, parenchymal dry fat-free tissue (P < .05), and parenchymal fat (P < .01). The beta-agonist reduced mammary gland weight (P < .01), parenchyma, parenchymal dry fat-free tissue (P < .05), and parenchymal fat (P < .01). There was a nonsignificant trend for increased milk yield for ewes fed the 20% protein diet and decreased milk yield for beta-agonist. The beta-agonist increased somatotropin concentration, but there was no evidence that increased somatotropin concentration induced mammogenesis in lambs. In conclusion, increased dietary protein may play a role in stimulating mammogenesis in rapidly growing lambs before puberty.