A two-sample method for assessing growth hormone response to growth hormone-releasing hormone challenge: use as a predictor of gain in beef bulls.

In two experiments, Black Angus bulls were challenged at weaning with GHRH analog and evaluated for their GH response to determine whether GH response can predict subsequent growth characteristics. The GH response was determined by measuring GH in blood serum collected 0 and 10 min after GHRH injection (Exp. 1: 1.5 μg/100 kg BW human GHRH, n = 34; Exp. 2: 1.5 and 4.5 μg/100 kg BW bovine GHRH [treatments LGHRH and HGHRH, respectively] administered 3 h after a 4.5 μg/100 kg BW clearance dose of GHRH, n = 38]. In Exp. 1, GH response did not predict growth or carcass measurements. In Exp. 2, GH response to LGHRH was positively related to ADG (R 2 =.18; P =.007) during a 112-d controlled feeding trial. In addition, there was a tendency for bulls with a greater GH response to HGHRH to exhibit greater ADG than animals with a low response. However, GH response to GHRH was not related to changes in hip height (HH) or carcass ultrasound measurements at d 112 of the growth performance trial. Response of GH to repeated GHRH challenges was consistent within animal over time (r =.47; P =.003). The use of a clearance dose 3 h prior to GHRH challenge improved the relationship between GH response and ADG. Results of this study suggest that GH response to GHRH challenge is a useful tool for identifying beef bulls with superior growth potential.