GALACTOPOIETIC EFFECTS OF RECOMBINANT SOMATOTROPIN AND GROWTH HORMONE-RELEASING FACTOR IN DAIRY-COWS

Eight Holsteins per group received 12 mg/d of recombinant growth hormo ne-releasing factor or 29 mg/d of recombinant bST or served as untreat ed 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 inf usion ended. Compared with controls, bST and growth hormone-releasing factor increased SCM during infusion. The SCM yield of cows treated wi th growth hormone-releasing factor was greater than that of bST-treate d cows during the final 20 d of infusion. Relative to controls, bST an d growth hormone-releasing factor increased serum concentrations of so matotropin and IGF-I during infusion. Concentrations of somatotropin a nd IGF-I in serum of bST- and growth hormone-releasing factor-treated cows did not differ during infusion. In summary, growth hormone-releas ing factor increased SCM yield more than bST, despite similar serum co ncentrations of somatotropin and IGF-I. Thus, the galactopoietic actio n of growth hormone-releasing factor was not explained solely by eleva tion of total radioimmnunoassayable somatotropin and IGF-I in serum.