To optimize first lactation and lifetime milk yield, growth benchmarks were established to help nutritionists meet the appropriate growth objectives of breeding weight and age at an economically viable time and to achieve the optimum body size and composition at first calving (NRC, 2001). However, there are still concerns that mammary development is impaired when BW gain exceeds a certain threshold and this impairment negatively affects milk yield. The objective of this review will be to integrate concepts of body growth and composition, mammary development and milk yield to provide a systems based perspective on first lactation milk differences that have been associated with mammary development. The seminal work by Sejrsen et al. (1982; 1983) describing the effect of high energy intake on mammary development and the relationship with circulating growth hormone linked the relationship between pre-pubertal growth, mammary development and future milk yield. The primary outcome of Sejrsen et al. was to provide an intuitive mechanism to explain why rapid growth during the pre-pubertal phase resulted in reduced milk yield. The observation of reduced mammary development could be repeated in almost every experiment (Pritchard et al., 1972; Petitclerc et al., 1984; Mäntysaari et al., 1995; Capuco et al., 1995; Meyer et al., 2006). These repeatable observations lead to the conclusion that high energy intake and increased ADG reduced mammary development through altered hormone status or some signaling processes. However, Meyer et al. (2006) were the first to recognize that mammary development was not reduced by high energy intake, and instead a function of the time to reach puberty and the associated signals to change from allometric mammary growth. The mammary gland, like all reproductive organs, grows in proportion to the size of the body and not in proportion to nutrient intake during the post-weaning, pre-pubertal phase. First lactation milk yield, mammary development and body composition will be further discussed in the context of mechanisms and opportunities.