Effect of bovine somatotropin injected as a sustained-release formulation at three injection intervals on the production and composition of milk from dairy cattle grazing pasture and supplemented with a grain concentrate

Abstract

Milk production and composition were measured, during spring, in 63 autumn-calving Holstein-Friesian dairy cattle either not injected or injected subcutaneously with a sustained-release bovine somatotropin formulation. Injections were given at 28-, 21- or 14-day intervals over a period of 84 days commencing in the sixth month of lactation. The cattle grazed pasture and were supplemented with a grain concentrate. Mean daily yields of milk (L), fat (kg) and protein (kg) were significantly higher in cattle injected at 28-day (17.6, 0.73, 0.56), 21-day (18.1, 0.76, 0.59), and 14-day (18.8, 0.80, 0.61) intervals, compared with yields of cows not injected (15.9, 0.68, 0.51). Yields of fat and protein were significantly different between all the injection interval treatments. Yield responses to injection were greater in cows than in heifers. There were no significant differences between treatments in milk fat content. The milk protein content (g/kg) of cows injected at 21-day intervals (33.2) was significantly greater than that of cows not injected (32.7) and those injected at the 28-day (32.6) and 14-day (32.6) intervals. It was concluded that bovine somatotropin injected subcutaneously as a sustained-release formulation was effective at all injection intervals in increasing the production of milk and milk components in mid to late lactation in dairy cattle grazing pasture and supplemented with a grain concentrate.