Nutritional implications of bovine somatotropin for the lactating dairy cow

Historians of animal production will remember the 1980s in part for the application of recombinant DNA technology, which permits large-scale production of an almost infinite number of proteins. Bovine somatotropin represents the first such protein for animal agriculture. Since its introduction in 1981, extensive research demonstrated the galactopoietic actions in dairy cattle under diverse feeding, management, and environmental conditions. Milk production increases from cows receiving bST range from 0 to 11 kg/d. Magnitude of improvement is related to factors associated with rations, animals, and environment. Quantity, quality, and density of nutrients, especially energy and protein, are ration determinants for efficacy of bST. Factors such as parity, nutritional history, body condition, and stage of lactation also influence the response. High temperature or humidity, which affects feed intake, appears to reduce the magnitude of response to somatotropin. Relationships among factors affecting galactopoiesis by somatotropin are discussed. Recent studies describing body composition changes provide insight into the mechanism of action of bST in lactating cows. Nutritional guidelines for use in the lactating cow must emphasize current concepts for high production. Management of the cow when she is not receiving bST (dry period, early lactation) will be as critical for success as during bST administration.