Long-term effects of recombinant bovine somatotropin (rBST) on dairy cow performances

Long-term (18-32 week) effects of recombinant BST (bovine somatotropin) injected daily (650 cows, 20 assays) or as prolonged release preparations (790 cows, injected each 14 or 28 d., 23 assays) are reviewed.Daily injections of 10-15, 20-27 or 31-50 mg BST increased milk yield of 3.9, 5.2 or 5.6 kg/d (respectively).Somidobove (320, 640 or 960 mg BST each 28 d.) increased milk yield 2.2, 3.1 or 4.2 kg/d, and Sometribove (500 mg/14 d.) 4.4.kg/d.Mean milk composition was unchanged, but cycled between injections in the case of prolonged release preparations.Dry matter intake increase for high BST doses was 1.5-1.7 kg/d in 32-week experiments, and 0.8 kg/d in 18 week-experiments.Energy balance and body condition score decreased during the first 3 months of BST treatment, due to delayed DM intake increase.In two experiments body lipids were sharply decreased in BST cows.Feed efficiency was increased (6-17 p. 100) according to dilution of maintenance requirement in total needs and to decreased body tissue deposition.There was no clear interaction of BST response with cow parity and milk yield potential, but a trend towards decreased response after several months of BST.High energy-high protein complete mixed rations tended to increase BST response, but inter-assay variability was high.Responses tended to be lower (2.5 ± 1.8 kg milk/d) at pasture in European conditions.