Endocrine control of energy metabolism in the cow: the effect on milk yield and levels of some blood constituents of injecting growth hormone and growth hormone fragments.

1. Circulating concentrations of some hormones and metabolites and nitrogen balance were measured in lactating beef and dairy cows given daily injections of growth hormone (GH) and were compared to values before injection. Changes in milk yield and composition were recorded in these cows and in additional cows injected with GH fragments. 2. GH, but not GH fragments, raised milk yield while milk composition did not change. GH injection caused a large, rapid increase in the level of the hormone in blood and slower, smaller increases in the concentrations of insulin, prolactin and thyroxine. Blood metabolite levels were unaltered except for a rise in glucose concentration in beef cows during GH injection and a fall in 3-hydroxybutyrate concentration in the same cows after the injections ceased. An increase in N output in milk was partly balanced by a reduction in urinary N so that N retention did not change significantly. 3. GH caused a reduction in food intake in the beef cows. 4. The results are discussed in relation to control of partition of nutrients in the lactating cow, where GH is considered to play an important role. It was concluded that this role may be in increasing the supply of energy metabolites for milk synthesis, rather than a direct effect on the activity of the mammary gland.