THE EFFECT OF FEED-INTAKE AND GROWTH HORMONE-RELEASING FACTOR ON LACTATING DAIRY-COWS

Twenty-four lactating dairy cows, averaging 30.0 kg/d of milk and 159 d of lactation, were used to study the effect of feed intake and growt h hormone-releasing factor in a 2 x 2 factorial arrangement. For the f irst 10-d period, cows had free access to a TMR and received a fixed a mount of high moisture corn, protein supplement, and hay. In the secon d 10-d period, 12 cows were maintained on this high intake, and 12 cow s received 70% of their previous intake (low intake). During the follo wing 10-d period, each intake group was divided, and each of two subgr oups (n = 6) received twice daily s.c. injections of saline or growth hormone-releasing factor (10 mu g/kg of BW per injection). Feed restri ction decreased milk production by 24%. Milk production increase was n ot different following growth hormone-releasing factor treatment for c ows maintained at high intake (4.6 kg/d) or low intake (3.4 kg/d). Fee d restriction increased concentration of growth hormone but did not af fect IGF-I concentration. Growth hormone-releasing factor increased IG F-I concentration similarly for both intake groups but increased conce ntrations of insulin and IGF-binding proteins-1 and -3 only in the hig h intake group. Low intake did not affect growth hormone, IGF-I, or mi lk responses to growth hormone-releasing factor, but suppressed the in crease in concentrations of insulin and IGF-binding proteins-1 and -3 following treatment with growth hormone-releasing factor for cows on h igh intake.