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Average daily gain, basal GH, and plasma GH kinetics following a bolus injection of GH were determined at birth and at 75, 150, 250, 350 and 450 kg BW in 17 Hereford and 19 Angus bull calves. Calves were offspring of two lines within each breed selected for six generations for rapid 168-d feedlot gain on either a roughage or a concentrate diet. These four lines were from a long-term selection study that were being evaluated to determine the effects of selection diet on factors that regulated their growth rate. Hereford calves weighed more (P   0.05) ADG between birth and 450 kg BW. Overall ADG was positively correlated with birth weight within both breeds (P   0.05) on ADG, basal GH, V SS , T 1/2 , MCR and SR. Compared with Hereford calves, Angus calves had higher MCR (P &lt; 0.05) and SR (P &lt; 0.1) for both diets, and lower basal GH (P &lt; 0.05) for the concentrate diet. Basal GH (P &lt; 0.01), V SS  (P &lt; 0.01), T 1/2  (P &lt; 0.05), MCR (P &lt; 0.01), and SR (P &lt; 0.01) generally decreased as calf weights (and age) increased. Basal GH and kinetic parameters were correlated with ADG in only a few instances mostly between birth and 250 kg BW. Several GH kinetic parameters obtained between 150 and 450 kg BW were correlated with calf birth weight. We conclude that ADG and GH kinetics are more closely related to calf birth weight than to diet fed during selection for rapid feedlot gain. Key words: selection for gain, Hereford and Angus cattle, bull calves, growth hormone, GH, kinetics

Effect of selection for feedlot gain, breed and age on growth hormone and growth hormone kinetics in bull calves