| dc.description.abstract | Twenty Angus X Holstein heifers were randomly assigned to one of four treatment groups (n=5) in a 2 X 2 factorial arrangement to evaluate the effects of recombinant bovine somatotropin (bST) and(or) nutritional manipulation on bioactive LH. The four treatment groups were arranged as follows: (I) continuous growth (CG) + excipient (EX), (II) CG + ST, (III) discontinuous growth (IG) + EX and (IV) IG + ST. CG heifers were fed a 68% concentrate diet formulated to provide for a growth rate of .8 kg/d throughout treatment. Two successive periods of intermittent growth were imposed on IG heifers by alternately feeding a 68% concentrate diet formulated for growth at .25 kg/d (restricted growth) for 3 months and then realimentation on a 90% concentrate diet fed ad libitum (compensatory growth) for 2 months. EX groups received excipient injections, while ST groups received injections (sc) of 500 mg bST every 14-d. Blood samples were collected at 20-min intervals for 6 h at ages 9, 11 and 14 mo (sample periods 1, 2 and 3, respectively). Bioactive LH was quantified by a validated in vitro rat Leydig cell testosterone assay. There was a sample period x treatment interaction (P<.05) for mean bioactive LH concentration. Mean bioactive LH concentration across sample period was greater (P<.05) for CG + EX heifers (.20 ng/ml) than for IG + ST heifers (.07 ng/ml). There was an effect of sample period (P<.05) on number of LH pulses and mean amplitude of LH pulses. Treatment affected (P<.05) mean amplitude of LH pulses but not number of LH pulses. Mean amplitude was greater for group CO + EX than for group 10 + ST (.36 and .09 ng/ml, respectively). These results suggest that nutritional manipulation designed to achieve intermittent growth patterns in combination with somatotropin administration reduces mean concentration and pulse amplitude of bioactive LH in heifers. | en |
