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 RIE per kg DM containing low crude protein (CP, 142 g CP p er kg DM) with high rumen protected protein (REP, 270 g REP per kg CP) (die t 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) (d iet C) to obtain liveweight gains (LWG) of greater than 900 g per day betwe en 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 pro duction. Six heifers per group were slaughtered at 16 months of age for eva luation of mammary glands. Pre-pubertal LWG was influenced by dietary CP co ncentration, such that heifers consuming diets B and C gained more than tho se consuming diet A (918 vs. 952 vs. 990 g pet d). Dietary REP concentratio n did not influence pre-pubertal LWG. At slaughter, heifers consuming pre-p ubertal diets with high-CP concentrations had less mammary fat tissue area and a lower ratio of fat to secretory tissue compared with those on the low -CP diet (74.9 vs. 42.7 vs. 24.1 m(2) 1.6 vs. 0.69 vs. 0.61). Heifers that consumed diet B during the pre-pubertal period had heavier dry udder weight s, and tended to have more mammary fat and more secretory tissue area in th e dry udder at slaughter than those heifers that consumed diet C (820 vs. 5 19 g, 636 vs, 420 g; 64.2 vs. 39.9 m(2)). Age and LW at calving were not in fluenced by either dietary CP or REP concentration. Daily first lactation m ilk, protein and fat yields were not influenced by pre-pubertal dietary CP concentration. The REP concentration in the pre-pubertal diets did not infl uence 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. (C) 20 00 Elsevier Science B.V. All rights reserved.