Joint Alpharma-Beef Species Symposium: interactions of feed efficiency with beef heifer reproductive development.

The influence of nutrition on puberty in beef heifers is complex and under neuroendocrine control. The stores of body fat in mammals are a determinant of the onset and maintenance of puberty. Body fat stores are greater in heifers with greater residual feed intake than in their more efficient herdmates. A 1 unit increase in residual feed intake resulted in a reduction of 7.54 d in age at puberty in Bos taurus beef heifers. However, Bos indicus-influenced heifers, which reach puberty at older ages, were not found to have sexual maturity influenced by selection for residual feed intake. The strong influence of body fat stores on return to estrus after calving does indicate that selection for leaner beef heifers could affect reproductive performance relative to puberty and postpartum rebreeding of first calf heifers. The influence of intermediary metabolism, through signals at the central nervous system, regulates the GnRH pulse generator, thereby influencing pituitary and ovarian function culminating with puberty and return to ovarian cyclicity after calving. Tropically adapted cattle (i.e., Santa Gertrudis and Brahman) selected for low residual feed intake had a lesser response of insulin to a glucose challenge than their less efficient herdmates. These studies indicate the possibility that animals with differing residual feed intake (efficiencies) may have differing intermediary metabolism and, therefore, differing rates of reaching puberty. Selection for low residual feed intake results in selection of leaner heifers that reach puberty at older ages. These leaner heifers calve later in their first and subsequent calving seasons. Selection for residual average daily gain has no negative influence on age at puberty or calving interval. Selection for residual ADG has a negative influence on age at first calving but not calving interval. There may be no acceptable method to improve feed efficiency without harming reproductive efficiency.