Influence of selection for milk yield on endogenous hormones and metabolites in Holstein heifers and XXXX .

@entity8788 @entity1133 @entity1718 @entity291 @entity413
Plasma concentrations of prolactin (PRL), growth hormone (GH), @entity8788 , glucagon, @entity413 , @entity1133 and free @entity1718 (FFA) were measured in Holstein @entity291 , yearlings, bred heifers and primiparous @entity291 , either sired by bulls with high predicted differences (PD) for milk (selection group) or by bulls from an unselected random bred control population (control group; n = 6). Serial blood samples were collected before and after feeding for an 8-h period from 0900 to 1700 h. All animals were fed a complete feed at 1100 h and administered @entity8788 (.6 IU/100 kg body weight) at 1400 h. Mean plasma PRL was greater in control animals after feeding and @entity8788 administration, while GH was greater overall in selection @entity291 . @entity8788 remained elevated longer in selection animals after exogenous administration, and plasma glucagon was increased in the control group. While plasma @entity413 and @entity1133 were unaffected by genetic group, plasma FFA were elevated in selection group @entity291 and primiparous @entity291 compared with the control group. All hormones and metabolites differed among the pre- and post-feeding and @entity8788 administration periods and also with age. Mean PRL and GH increased after feeding, while glucagon decreased after exogenous @entity8788 . Plasma FFA declined after feeding, while @entity1133 and @entity413 were similar before and after feeding. Mean PRL increased and glucagon decreased with advancing age and plasma GH and @entity8788 showed inverse relationships at different ages. Plasma FFA changes closely followed GH changes with age, while plasma @entity413 more closely followed @entity8788 changes with age. Results indicate that all hormones measured and FFA responded to genetic selection for milk, and increases in GH are uniformly associated with increased genetic potential for milk yield.
