Applications of hormones in the metabolic regulation of growth and lactation in ruminants.

Exogenous natural and synthetic estrogenic and androgenic steroid hormones are used commercially to stimulate metabolic processes associated with increased rate and efficiency of body growth in ruminants. However, mechanisms of action of steroid hormone-induced effects on metabolism are relatively unknown. Application of peptide hormones to muscle growth, fat deposition, and lactation has lagged because of lack of sufficient quantities of the hormones. However, with recombinant DNA technology synthesis of large quantities of peptide hormones is now feasible. Most efforts have focused on growth hormone (GH), growth hormone-releasing factor (GRF), and prolactin (PRL) effects on lactation. For example, administration of GH or GRF stimulates yields of milk, milk fat, protein, and lactose as much as 41% in cattle. The mechanism of GH action probably involves somatomedin C acting at extramammary sites and (or) directly at the mammary cell. PRL is lactogenic but has no significant effect on established lactation in cattle. Daily exposure of cattle to 16 h light and 8 h of darkness stimulates milk yield and body growth and reduces fat accretion in the carcass, but the hormonal signals responsible for these photoperiod-induced responses are unknown. Photoperiod manipulations are relatively easy to apply to ruminants, but development of suitable delivery systems for animals will greatly enhance application of peptide hormones to further studies of metabolism as well as commercial livestock production systems.