Somatotropin and lactation.

Somatotropin, which can now be produced by biotechnology, could have an enormous impact on the dairy industry. Milk yield has been increased up to 40% with daily injections of somatotropin. Cows adjust their nutrient intake to support this increase. Somatotropin does not adversely affect cows' health, although all studies to date have been for less than one complete lactation. The search for a single biochemical or physiological event to account for the effects of somatotropin on milk production is elusive. Coordinated changes in many tissues and physiological processes occur to support the increases in the synthesis of lactose, fat, and protein in the mammary gland. Changes in the irreversible loss and oxidation rates of two key metabolites, glucose and free fatty acids, can quantitatively account for increases in lactose and milk fat during the short-term administration of somatotropin. Similarly, feed intake and live weight changes can account for increases in milk production in the longer experiments. Parallels between physiological changes that occur during somatotropin administration and differences between genetically high and lower yielding cows are highlighted, and the rates of improvement that can be expected from various new technologies are quantified. Existing data on the safety of somatotropin to both the consumer and the animal are evaluated.