Abstract
To estimate the potential of GH response to administration of growth hormone (GH)-releasing factor (GRF) as a predictor of genetic merit, two experiments, utilizing proven AI dairy sires of differing genetic merit for milk production, were conducted. In a first experiment, bulls of superior genetic merit [S; n=4; genetic merit for economic value of product (MFP$)=$227] or inferior genetic merit (I; n=4; MFPS=$94) were administered, three dosages of GRF (0, 5.5 and 11 μg GRF · 100 kg BW −1). Each bull received one dosage of Ro23-7863, an hGRF (1–29) analog, on each of three consecutive days. Area under the GH response curve (AUC) from S bulls following 5.5 or 11 μg GRF were greater (P<0.01) than AUC from I bulls. In a second experiment, a single dosage of GRF (4 μg GRF · 100 kg BW −1) was administered to sires of superior (S; n=8; MFPS=$204), average (A; n=8; MFP$=$149) or inferior (I; n=8; MFPS=$74) genetic merit. There was no effect (P>0.05) of genetic group on preinjection AUC, however, AUCs following GRF were greater (P < 0.01) from S and A sires compared with I sires (1925 and 1676 vs. 549 ng min ml −1, respectively). Therefore, GH response to exogenous GRF administration may be a useful tool in identifying dairy bulls with superior transmitting ability for milk production traits.
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