COMPARISON OF GROWTH HORMONE-RELEASING FACTOR AND SOMATOTROPIN - BODYGROWTH AND LACTATION OF PRIMIPAROUS COWS

Growth hormone-releasing factor is the main endogenous stimulator of s omatotropin secretion. Our objective was to compare the effects of rec ombinant growth hormone-releasing factor and recombinant bST on somato tropin secretion, mammary function, and body composition of lactating, primiparous dairy cows. Cows (118 d of lactation) served as uninfused controls or were infused for 63 d with 12 mg/d of releasing factor or with 29 mg/d of bST. These doses elevated somatotropin in serum to co ncentrations of similar magnitude. The releasing factor and bST each s imilarly increased milk yield, yield of milk components, weight of mos t organs, mobilization of adipose tissue, accretion of lean tissue in the carcass, and metabolic activity (RNA) of mammary tissue. Relative to controls, neither releasing factor nor bST significantly affected c ell numbers (DNA) in mammary glands or concentrations of plasmin in mi lk. None of the variables measured provided evidence for galactopoieti c effects of releasing factor independent of somatotropin.