Growth hormone parameters at four ages in intact and castrated male and female cattle

Plasma Growth hormone (GH) profiles, secretion rates (SR), metabolic clearance rates (MCR) of intravenously infused GH, and response to 0.0165, 0.067, and 0.267 microgram of growth hormone-releasing factor (GHRF)/kg body weight (bwt) were studied in intact and castrated male and female cattle (4 of each) at 5, 8, 12, and 15 mo of age. As cattle aged, overall GH levels in the plasma decreased (P less than .01), GH baseline concentrations declined (P less than .05), SR decreased (P less than .01), half-life of GH decreased (P less than .01), and the GH response to intravenous injections of GHRF declined (P less than .05). Bulls had elevated overall and baseline GH concentrations (P less than .05), greater amplitude and number of GH secretory periods (P less than .01), and greater SR (P less than .05) than steers, heifers, or ovariectomized heifers. Males had more frequent GH spikes and secretory periods of higher amplitudes, greater SR, and greater GH responses to 0.0165 microgram GHRF kg bwt (P less than .05) than females. Castration had no impact on the GH status of heifers. It is suggested that testosterone or its metabolites enhanced GH secretion in bulls.