PRE-PUBERTAL MAMMOGENESIS IN THE SHEEP .3. THE EFFECTS OF RESTRICTED FEEDING OR DAILY ADMINISTRATION OF BOVINE GROWTH-HORMONE AND BROMOCRIPTINE ON MAMMARY GROWTH AND MORPHOLOGY

In crossbred female lambs reared on a concentration diet between 8 and 20 weeks of age, a restriction in food intake to 40 g/kg body weight per day decreased live-weight gain (155 g/day) compared with that in control lambs fed ad libitum (284 g/day) and significantly reduced final live weight and the size of the mammary fat pad (P < 0.001). However, restricted lambs at 20 weeks had more total parenchymal deoxyribonucleic acid (DNA; 54.7 v. 47.5 mg) occupying a greater mass of mammary fat pad (17.0 v. 13.2 g) than control lambs (P > 0.10). Restricted feeding had little effect on mean plasma growth hormone (GH) concentrations at 12 and 18 weeks of age, but decreased plasma insulin and prolactin concentrations. In lambs fed ad libitum, daily subcutaneous (s.c.) injections of 0.1 mg bovine pituitary GH per kg live weight between 8 and 20 weeks of age significantly increased daily live-weight gain (347 g/day; P < 0.001) and also increased total mammary parenchymal DNA (71.2 mg; P < 0.10) and the mass of fat pad occupied by parenchymal tissue (20.6 g; P < 0.05), compared with the control treatment. Daily s.c. injection of bromocriptine (1 mg/day) had no effect on either body or mammary growth, but tended to reduce the effects of bovine GH when given in combination. Secretory activity was observed in the parenchymal tissue of the eight lambs receiving bovine GH alone, and also in five control lambs and three lambs receiving bromocriptine alone. Three lambs that had attained puberty by 20 weeks of age had the poorest mammary development of their respective treatment groups. Mammary gland development at 20 weeks of age was significantly correlated across treatment means with plasma GH concentrations estimated at 18 weeks of age (r = 0.95; P < 0.05), but no correlations of similar magnitude were found between individuals within treatments at 20 weeks. Strongest correlations were found among 8-week-old lambs, where mammary parenchymal DNA was positively correlated with mean plasma GH (r = 0.62) and the GH:insulin ratio (r = 0.79) and negatively correlated with the prolactin:GH ratio (r = -0.65).