Bovine somatotropin and feed interactions in dairy cows

Abstract

The influence of bovine somatotropin (bST) and two levels of rumen-degradable protein on milk production and composition was investigated in 42 cows. The treatments were given in a factorial design from approx. 9 to 25 weeks post-partum, following a 4-week pretreatment period. Three levels of recombinantly derived somatotropin in a slow-release formulation were used: 0 (control), 320 mg per 14 days and 640 mg per 28 days. The cows received forage ad lib. and a fixed amount (dependant on yield) of a concentrate, formulated to contain either a low (LD) or high (HD) level of rumen-degradable protein. The LD concentrate significantly ( P⩽ 0.05) increased milk fat and lactose by 0.18 and 0.07 percent units and tended to stimulate milk yield and protein by 0.7 kg and 0.08 percent units (NS), respectively. Somatotropin injections increased milk production by 1.5 and 0.7 kg per day and milk fat by 0.22 and −0.04 percent units on the LD and HD diet, respectively. The average response was rather low and significantly different between the two diets. The probable reason for these observations was the absence of a compensatory influence on forage intake. A cyclical pattern in the response of milk production and composition was observed, especially in the 28-day treatment groups. No indications were found that the bST treatments affected animal health or fertility.