A REVIEW OF BOVINE GROWTH-HORMONE

Unprecedented numbers of technical papers, abstracts, and short commun ications have been published in the past decade regarding the effects of exogenous bovine growth hormone on milk production, health, and rep roductive efficiency of treated dairy cows. In well-managed dairy herd s, exogenous growth hormone increases milk production without altering normal variability in milk composition. This has held true regardless of dairy breed tested, geographical location studied, or feeding mana gement system used. Also consistent across studies is the rapidity of the galactopoietic effect of administered bovine growth hormone, which arises from altered partitioning and use of post-absorptive nutrients and increased synthetic capacity of the mammary gland. Growth hormone and its associated peptide, insulin-like growth factor-1, are now kno wn to provide chronic lipolytic, diabetogenic, and gluconeogenic signa ls to target tissues culminating in increased mammary gland availabili ty of glucose and nonesterified fatty acids. Together with yet ill-def ined effects on mammary secretory tissue, this homeorhetic control of metabolism elicited by exogenous growth hormone is so efficient that t reated cows are not more susceptible to metabolic disorders than untre ated cows. However, some studies have reported an increased frequency of mastitis in groups of treated cows. This has been attributed mainly to increased milk volume in the mammary glands of treated cows and no convincing data are available that show decreased mammary gland immun ity as a result of growth hormone treatments. On the contrary, an expa nding body of evidence implicates growth hormone as a key neuroendocri ne factor that is required for immunological competence. Trends of dec reased reproductive efficiency in cows treated with growth hormone hav e also been reported, but available data imply that this is probably a n indirect effect via prolonged negative energy balance in cows treate d in early lactation rather than a direct negative effect on estrous c ycling via altered reproductive hormone profiles. The objectives of th e present review are to bring into focus and summarize pertinent biolo gical discoveries regarding the treatment of dairy cows with recombina nt bovine growth hormone, and to explore areas where additional growth hormone research is needed or warranted.