Effects of body weight and nutrition on histological mammary development in Holstein heifers.

Our objective was to determine the effects of rate of gain and body weight (BW) on development of the mammary parenchyma. Mammary tissue samples were collected from heifers (n = 72) reared on 1 of 2 dietary treatments (restricted, 650 g/d of daily gain; or elevated, 950 g/d of daily gain) and slaughtered at 100, 150, 200, 250, 300, or 350 kg of BW. Mammary samples were excised, preserved, prepared for histology, and stained with hematoxylin and eosin. Digital images of tissue sections were captured for analysis. Tissue areas occupied by the interlobular and intralobular stroma, epithelium, and lumen were measured (mum(2)). The numbers of epithelial and luminal structures per image were tabulated to measure the complexity of ductal development. Mean percentages of mammary parenchyma occupied by the interlobular stroma, epithelium, lumen, and intralobular stroma were 29, 20, 7, and 43%, respectively. Percentage of area occupied by the intralobular stroma was affected by BW and was lower for 100-kg heifers compared with heifers 200 kg and heavier (33 +/- 4 vs. 46 +/- 4), but the percentage of area occupied by other tissue elements did not differ by BW or treatment, nor was there an interaction. However, the numbers of both epithelial (8.3 +/- 4 vs. 47 +/- 4) and luminal-containing (6 +/- 4 vs. 38 +/- 4) structures per image increased markedly between 100 and 350 kg of BW, irrespective of diet. For heifers slaughtered between 100 and 350 kg of BW, alterations in the rate of gain between 650 and 950 g/d, accomplished by feeding varying amounts of the same diet, had no significant effect on tissue characteristics or the pattern of mammary parenchymal development. These data emphasize the importance of BW and age in determining developmental characteristics of the heifer mammary parenchyma and suggest that the rate of gain per se has a minimal impact on histological development, and thus do not support the hypothesis that rate of gain has a direct negative impact on ductal development.