Growth hormone response to growth hormone-releasing hormone in beef cows divergently selected for milk production

In dairy cattle, increased circulating growth hormone has been associated w ith selection for greater milk yield. This study tested the hypothesis that beef cows divergently selected for milk production would have differing GH responses to a challenge dose of GHRH. Growth hormone response to a challe nge of GHRH was measured in 36 Angus-sired cows ranging from 6 to 10 yr of age. The cows were classified as high milking (n = 16) or low milking (n = 20), on the basis of their sired milk EPD. Mean milk EPD (in kilograms) wer e 16.6 and -14.4 for high and low milking cows, respectively. Milk producti on was estimated by the weigh-suckle-weigh procedure. Blood samples were ta ken immediately before and 10 min after a clearance dose of 4.5 mug of GHRH /100 kg BW (injected i.v.) and, 3 h later, immediately before and 10 min af ter a challenge dose of either 1.5 or 4.5 mug of GHRH/100 kg BW. Each anima l received both challenge doses, and the doses were randomly assigned acros s 2 d of blood collection. Serum concentrations of GH and IGF-I were measur ed by RIA. Serum IGF-I was measured in the baseline blood sample on d 1 of blood collection. A positive relationship (r = 0.35; P = 0.03) was observed between the cows' rankings for each dose of GHRH; that is, high responders to the low dose were high responders to the high dose. Growth hormone resp onse to the 4.5 mug/100 kg BW challenge dose of GHRH was positively related to sire milk EPD (R-2 = 0.09; P = 0.03). Response of GH to the 1.5 mug GHR H/100 kg BW challenge dose also tended to be related (P = 0.08) to sire mil k EPD of high milking cows. In addition, IGF-I concentrations of high milki ng cows were inversely related (R-2 = 0.24; P = 0.04) to sire milk EPD. Gro wth hormone response to GHRH challenge may have potential as an additional tool in the evaluation of milk production in beef cattle.