Effects of bovine somatotropin treatment and intermittent growth pattern on mammary gland development in heifers.
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Angus x Holstein heifers (initial age and BW: 6.0 +/- .4 mo; 159 +/- 12 kg) were assigned to one of four treatments (n = 10) in a 2 x 2 factorial arrangement to examine the effects of bovine somatotropin (bST) or vehicle treatment and continuous (CG) or intermittent growth (IG) pattern treatment on mammogenesis. Subcutaneous injections of bST (500 mg) or vehicle were administered at 14-d intervals. The CG heifers were limit-fed a diet to gain .8 kg/d throughout the experiment, whereas IG heifers underwent two successive growth restriction-compensation phases with each phase consisting of a 3-mo growth restriction period (.25 kg/d) followed by a growth compensation period. All heifers were individually fed during the experiment and slaughtered when average treatment BW reached 390 kg. Mammary gland development was measured by dissection, chemical analysis, and computed tomography scanning. Overall ADG were increased (P < .001) 20% by bST treatment of CG heifers, although ADG of IG-vehicle and IG-bST heifers were similar to ADG of CG-vehicle heifers. Dissectable extraparenchymal tissue (EPT) mass was reduced 23% (P < .05) and 36% (P < .001) by IG pattern and bST treatments, respectively, although the bST effect on dissectable EPT tended (P = .08) to be greater in CG than in IG heifers. Fat-free parenchymal tissue (PT) mass (P = .11) and PT volume (P = .08) tended to be greater in IG than in CG heifers. Fat-free PT was 60% greater (P = .01) and PT volume onefold higher (P = .01) in bST than in vehicle heifers. The bST and IG pattern treatments increased the proportional weight of fat-free PT mass, although the effect of bST was greater than IG pattern treatment. There was no evidence to suggest that the two treatments were additive in affecting mammogenesis.