Effects of selection for milk yield on growth hormone response to growth hormone releasing factor in growing holstein calves

Bull and heifer calves (n = 81) from genetic lines of Holstein cows that differed by more than 4000 kg milk/ 305-d lactation were used to determine effects of selection for milk yield on growth hormone (GH) response to a GH releasing factor (GRF) analog. Calves received GRF (4 mug/100 kg BW) on 10, 56, 140, 196, 252, and 364 3 d of age. Jugular blood samples (n = 15) were obtained from -30 to 120 min relative to GRF administration. Area under the GH response curve (0 to 60 min, AUC60) was quantified after subtracting mean prechallenge GH concentrations. Data were analyzed for effects of line, age, gender, and their interactions with PROC MIXED of SAS for repeated measures and incorporated the spatial power law for unequally spaced data with age as the repeated effect. Means were considered different when P < 0.05. Prechallenge GH concentrations did not differ between lines, were greater in bulls than heifers (4.6 vs. 3.7 ng/ml), and decreased with age. The AUC60 decreased with age but did not differ between lines. Heifers responded more to GRF than bulls (1550 vs. 1336 ng x min/ml). Peak GH concentration decreased with age and was less in bulls than heifers (54.7 vs. 62.1 ng/ml) but did not differ between lines. Although plasma GH has been identified as an inheritable trait, we conclude the GH variables measured in this study were not useful in predicting genetic merit of calves from these substantially different lines of cows.