Effect of selection for milk yield and dietary energy on yield traits, bovine somatotropin, 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 concentration of urea N were determined for 139 heifers. Heifers, daughters 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 allotted to either a high or low energy diet. Heifers of high genetic merit yielded 8.1% more milk and had 7.7% higher bST concentration than did heifers of average genetic merit, which were 3% heavier than heifers of high genetic merit. There was no significant effect of genetic merit group on energy intake, plasma concentration of urea N, or percentages 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 concentration of urea N by 14.2% compared with concentrations for heifers fed the 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. Body weight, BW change, milk yield, and energy intake were negatively correlated with plasma concentration of urea N.