Galactopoietic effects of recombinant somatotropin and growth hormone-releasing factor in dairy cows

Eight Holsteins per group received 12 mg/d of recombinant growth hormone-releasing factor or 29 mg/d of recombinant bST or served as untreated controls for 60 d. Milk yield and composition were measured for 10 d before infusion, during infusion (d 0 to 59), and for 20 d after infusion ended. Compared with controls, bST and growth hormone-releasing factor increased SCM during infusion. The SCM yield of cows treated with growth hormone-releasing factor was greater than that of bST-treated cows during the final 20 d of infusion. Relative to controls, bST and growth hormone-releasing factor increased serum concentrations of somatotropin and IGF-I during infusion. Concentrations of somatotropin and IGF-I in serum of bST- and growth hormone-releasing factor-treated cows did not differ during infusion. In summary, growth hormone-releasing factor increased SCM yield more than bST, despite similar serum concentrations of somatotropin and IGF-I. Thus, the galactopoietic action of growth hormone-releasing factor was not explained solely by elevation of total radioimmunoassayable somatotropin and IGF-I in serum.