Early Lactation Responses of Dairy Cows Administered Bovine Somatotropin and Fed Diets High in Energy and Protein

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Abstract
Early lactation dairy cows (n = 36) were used in a 2 .times. 2 factorial arrangement of treatments to evaluate responses to rbST when cows were fed a control diet or one simultaneously elevated in dietary energy (fat) and CP (primarily rumen undegradable) between 25 and 150 d postpartum. Supplemental dietary fat and undegradable intake protein had no effect on milk yield with or without rbST. Cows fed the higher density diet returned to a positive energy balance approximately 3 wk earlier than those fed the control diet when both received rbST. Change in BW was unaffected by treatments. Body condition scores at the end of the treatment period and reproductive performance were lower in cows administered rbST. Somatotropin significantly increased 3.5% FCM and milk fat yield and tended to increase milk fat percentage when combined with the high nutrient density diet. However, the high density diet significantly depressed both yield and percentage milk protein, which likely was related to the added fat. Intakes of DM, energy, and protein were not altered by rbST or the feeding of a high density diet. Somatotropin, but not diet, significantly improved all measures of efficiency. Feeding more nutrient dense diets to rbST-treated cows in negative energy balance did not affect adipose tissue responsiveness to .beta.-adrenergic agonist. The current NRC nutrient recommendations for high producing cows appear adequate for cows administered rbST.

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