The effect of dietary protein level during the pre-pubertal period of growth on mammary gland development and subsequent milk production in Friesian heifers

Sixty-three Friesian heifers were assigned to three different diets (21 per group) of 11 MJ ME per kg DM containing low crude protein (CP, 142 g CP per kg DM) with high rumen protected protein (REP, 270 g REP per kg CP) (diet A), high-CP (183 g CP per kg DM) with low-REP (133 g REP per kg CP) (diet B) and high-CP (182 g CP per kg DM) with high-REP (267 g REP per kg CP) (diet C) to obtain liveweight gains (LWG) of greater than 900 g per day between five and ten months of age in order to study the influence of dietary CP and REP concentration on mammary gland development and subsequent milk production. Six heifers per group were slaughtered at 16 months of age for evaluation of mammary glands. Pre-pubertal LWG was influenced by dietary CP concentration, such that heifers consuming diets B and C gained more than those consuming diet A (918 vs. 952 vs. 990 g per d). Dietary REP concentration did not influence pre-pubertal LWG. At slaughter, heifers consuming pre-pubertal diets with high-CP concentrations had less mammary fat tissue area and a lower 2 ratio of fat to secretory tissue compared with those on the low-CP diet (74.9 vs. 42.7 vs. 24.1 m ; 1.6 vs. 0.69 vs. 0.61). Heifers that consumed diet B during the pre-pubertal period had heavier dry udder weights, and tended to have more mammary fat and more secretory tissue area in the dry udder at slaughter than those heifers that consumed diet C (820 vs. 2 519 g; 636 vs. 420 g; 64.2 vs. 39.9 m ). Age and LW at calving were not influenced by either dietary CP or REP concentration. Daily first lactation milk, protein and fat yields were not influenced by pre-pubertal dietary CP concentration. The REP concentration in the pre-pubertal diets did not influence daily milk and fat yields but heifers that consumed diet C produced 0.08 kg more daily protein than did heifers that had consumed diet B.  2000 Elsevier Science B.V. All rights reserved.