Comparison of growth hormone-releasing factor and somatotropin: body growth and lactation of primiparous cows.

Growth hormone-releasing factor is the main endogenous stimulator of somatotropin secretion. Our objective was to compare the effects of recombinant growth hormone-releasing factor and recombinant bST on somatotropin 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 concentrations of similar magnitude. The releasing factor and bST each similarly increased milk yield, yield of milk components, weight of most 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 cell numbers (DNA) in mammary glands or concentrations of plasmin in milk. None of the variables measured provided evidence for galactopoietic effects of releasing factor independent of somatotropin.