The efficiency of casein N utilization in dairy cows

Abstract Eight Friesian dairy cows, two in early, two in mid and two in late lactation and two in the dry period, was maintained by total intragastric nutrition. The average energy input from volatile fatty acids plus casein was 935 kJ kg −1 W 0.75 day −1 for lactating cows and 638 kJ kg −1 W 0.75 day −1 for dry cows. The efficiency of utilization of the nitrogen (N) of the test protein, casein, was established under N-limiting conditions from the regression of productive N (milk N plus retained N) on N input. Daily N inputs (g N kg −1 W 0.75 day −1 ) ranged from 1.15 to 1.81 in early lactation, from 0.45 to 0.95 in mid lactation, from 0.68 to 1.40 in late lactation and from 0.30 to 0.60 in the dry period. The efficiency of N utilization was 0.693, 0.683, 0.583 and 0.607 for early, mid and late lactation and the dry period, respectively. The slopes of these relationships did not differ significantly, indicating that under N-limiting conditions, the efficiency of N utilization was not affected by the stage of lactation. The overall efficiency with which casein N was converted to milk N and tissue N was 0.66.