Somatotropin treatment for a second consecutive lactation

Twenty-five Holstein cows were used to examine effects of recombinant bST administration for a second successive lactation on milk yield and composition, BW change, body condition, and health. Treatments were 0 (physiological saline), 10.3, 20.6, and 33 mg bST/d (8, 8, 7, and 3 cows per treatment, respectively). Saline and bST were given daily as subcutaneous injections beginning 4 to 5 wk postpartum and continuing for 38 wk. Somatotropin increased 3.5% FCM yield linearly from 8 to 36% over controls (control FCM was 30.1 kg/d). Milk composition was not affected by bST except that milk from cows given the 10.3-mg dosage had slightly higher Ca and P contents than did milk from cows receiving 0 or 20.6 mg. Cows receiving bST were 9 to 30% more efficient than controls in converting feed to milk. Body weight gain was unaffected by treatment. Average condition score was unchanged for control cows during the first 84 d following initiation of treatment, but declined in cows given bST. Thereafter, condition score increased in a similar manner for all groups. No treatment-related patterns were found in incidence of mastitis, feet and leg problems, ketosis, or milk fever. Blood hormone, chemistry, and hematology revealed no effects of bST treatment other than to raise plasma bST concentrations. Under conditions of this experiment, use of bST during a second consecutive lactation did not change the normal physiology or productive capacity of lactating cows.