EFFECT OF SELECTION FOR MILK-YIELD AND DIETARY ENERGY ON YIELD TRAITS, BOVINE SAMATOTROPIN, AND PLASMA UREA NITROGEN IN DAIRY-COWS

Effects of genetic merit on energy intake, milk yield, fat and protein percentages, BW, BW change, plasma concentration of bST, and plasma c oncentration of urea N were determined for 139 heifers. Heifers, daugh ters of bulls of high genetic merit (average +408 kg of PTA for milk) or of average genetic merit (average -153 kg of PTA for milk), were al lotted to either a high or low energy diet. Heifers of high genetic me rit yielded 8.1% more milk and had 7.7% higher bST concentration than did heifers of average genetic merit, which were 3% heavier than heife rs of high genetic merit, There was no significant effect of genetic m erit group on energy intake, plasma concentration of urea N, or percen tages of fat and protein. Heifers fed the high energy diet consumed 35 .1% more energy, yielded 11.8% more milk with a lower fat percentage, and weighed 3% more than did heifers fed the low energy diet. The high energy diet depressed bST concentration by 13.3% and plasma concentra tion of urea N by 14.2% compared with concentrations for heifers fed t he low energy diet. Correlations among bST, BW, and energy intake were negative and significant. Correlations of bST concentration with milk yield, fat percentage, and protein percentage were not significant. B ody weight, BW change, milk yield, and energy intake were negatively c orrelated with plasma concentration of urea N.