High levels of feed and rapid growth before puberty can result in small functional mammary glands and lower milk yields in dairy heifers. One hypothesis suggests this effect may be mediated via changes in the circulating levels of certain metabolic hormones (in particular a reduction in GH), that are known to accompany high levels of food intake, and that may normally exert some mammogenic influence. Preliminary studies indicated that rapid rearing in sheep can result in similar metabolic changes, reductions in prepubertal mammary development and lower first lactation milk yields.