Effects of treatment of dairy cows with recombinant bovine somatotropin over three or four lactations.

Jersey, Dutch Red and White, and Friesian cows were subcutaneously injected with 640 mg of recombinant bST at 28-d intervals from 87 to 115 d after calving through four successive lactations. A TMR (6.72 MJ of NEL) and 168 g of CP/kg of DM) was fed for ad libitum consumption. The bST effects per day were 3.3 kg for milk yield, 189 g for fat yield, 109 g for protein yield, 157 g for lactose yield, 4 MJ of NEL for feed intake, and -4 kg for body weight. Responses in blood parameters measured 7 d after injection were -.007 mmol/L for glucose, -1.3 mg of N/100 ml for urea, 221 mumol/L for 3-hydroxybutyrate, 59 mumol/L for NEFA, 65 ng/L for insulin, 2.8 micrograms/L for thyroxine, and 26.7 micrograms/L for somatotropin. Somatic cell count in milk was 75,000 cells/ml higher in treated cows. Concentrations of NEFA, Ca, Mg, and phosphorus were unaffected. Repeatability of the maximum response in milk yield after bST treatment was low: .2 within and .5 between lactations. Cows treated in the previous lactation had slightly more retained placentas, and birth weight of their calves was 2 kg less. No differences were observed between treated and control cows in disease incidence. Six treated cows were culled in third and fourth lactations. No indications for tissue damage, inflammation, or stress after bST injections were detected.