Lactation response and body composition of cows receiving somatotropin and three ratios of forage to concentrate.

Ninety multiparous Holstein cows were used to determine the effect of ration energy density and bST on lactation performance and whole body chemical composition. Seventy-eight cows, averaging 43.6 d postpartum, were assigned for 168 d to TMR with forage: concentrate ratios of 40:60, 50:50, and 60:40 (DM basis). Half of the cows on each ration received subcutaneously either 0 or 640 mg bST/28 d. Whole body chemical composition was determined by comparative slaughter in 12 cows (means = 35.9 d postpartum) prior to initiation of treatment and in 35 cows after either 84 or 168 d of treatment. Net energy intake was greater for 40:60 and 50:50 than for 60:40. Milk fat percentage was reduced in cows fed 40:60. Ration did not affect milk, 3.5% FCM, and DMI. There were no differences among rations for total body fat, protein, water, and calories. The bST increased milk and 3.5% FCM but had no effect on DM and net energy intakes. Yield of 3.5% FCM by cows receiving bST and fed 40:60 was 1.9 kg/d more and for 50:50 it was 2.7 kg/d more than for those fed 60:40. Administration of bST reduced total body fat and calories but did not affect protein and water. Partitioning of calories to milk at the expense of fat deposition is the primary mechanism for the galactopoietic action of bST.