Predicting bull growth performance and carcass composition from growth hormone response to growth hormone-releasing hormone

Development of practical, physiologically based methods that provide an ear ly, yet accurate, evaluation of a bull's genetic merit could benefit the be ef industry. The use of GH response to a single, acute dose of GHRH was eva luated as a predictor of future growth performance and carcass characterist ics of weanling bulls. Fifty-six Angus bulls averaging 229 d (SD = 27) of a ge were administered three doses i.v. (0, 1.5, and 4.5 mu g/100 kg BW) of h uman GHRH (1-29) analog in a Latin square design balanced for residual effe cts. Blood samples were collected via jugular catheter at -60, -45, -30, -1 5, 0, 5, 10, 15, 30, 45, 60, 90 and 120 min relative to GHRH injection. Ser um concentrations of GH were plotted over time. Response to GHRH was calcul ated as the area under the GH response curve (AUC-GH) using the trapezoidal approximation. Relationships between AUC-GH, weaning weight adjusted to 20 5 d of age (205-d WW), and direct weaning weight EPD (WWEPD) versus age-adj usted BW (BWadj), ADG, and carcass measurements from a 140-d growth perform ance test were evaluated using simple linear regression. A positive correla tion between AUC-GH and ADG and an inverse relationship between AUC-GH and carcass fat were observed. The present study provides evidence that AUC-GH is a better predictor of future growth performance in beef bulls than 205-d WW or WWEPD values. Thus, GH response to GHRH is associated with subsequen t growth and may be a useful tool for sire selection in beef production.