Effects of bovine somatotropin on milk yield and composition in Holstein cows in advanced lactation fed low- or high-energy diets

The objective of this study was to evaluate the response of Holstein cows t o bovine somatotropin (bST) during advanced lactation and its relationship to energy intake. Twenty-four lactating Holstein cows averaging 21 kg of mi lk/d, and 292 d in milk were assigned to one of three treatment groups in a randomized block design. Blocks were based on the 14 d of pretreatment mil k production, and treatment groups were balanced for days in milk. Treatmen t 1 was a low-energy diet (1.49 Mcal/kg of dry matter) without bST injectio n; treatment 2 was the low-energy diet plus injection of 500 mg of bST ever y 14 d; and treatment 3 was a high-energy diet (1.71 Mcal/kg of dry matter) with bST injections as in treatment 2. Treatment was divided into two peri ods (1 to 49 and 50 to 98 d) to determine if response to bST and energy cha nged with time on treatment. Results showed that bST significantly (P < 0.0 5) increased milk, fat-corrected milk, and fat and protein yields; and feed efficiency (fat-corrected milk per dry matter intake) for both periods. Mi lk yield responses to bST were greater for cows fed the low- than the high- energy diet in both periods. These data show that bST injections for cows i n advanced lactation increased performance, but excessive energy diminished the bST response.