Endocrine Factors Influencing the Intensity of Milk Secretion. A. Estrogen, Thyroxine, and Growth Hormone

Summary 1.A group of dairy heifers whose udders were grown and subsequently induced to secrete milk experimentally were allowed to reach their maximum production without further treatment. When they began to decline in milk yield, each heifer was treated with three hormones, one after the other, to determine their effect upon the intensity of milk secretion. 2.It was assumed that if the hormone administered either arrested the normal decline or increased the intensity of milk secretion it would indicate that the animal's endogenous secretion of the hormone was limiting its milk secretion capacity. 3.It was shown that the daily feeding of 10mg. of diethylstilbestrol for a period of 4 weeks tended to arrest the decline in milk yield and in some animals caused a slight increase in milk yield during the period. In two cows in late lactation the prolonged feeding of this hormone caused the maintenance of the level for periods of 9 to 10 weeks. 4.It was suggested that these data indicate that low levels of estrogen tend to prevent the decline in the secretion of the lactogenic hormone of the pituitary. 5.The injection of L-thyroxine at the level of 0.4mg. per 100lb. body weight for periods of 4 to 9 weeks in some heifers produced a marked rise in milk yield, whereas in others the effect was less or nil. 6.These data are believed to indicate the degree to which the endogenous rate of secretion of thyroxine was limiting the milk producing capacity of the individual heifer. 7.When the injection of thyroxine was stopped suddenly, the yield of milk declined rapidly for about 2 weeks then increased to the level which might have been expected at that stage of lactation. During periods of exogenous thyroxine administration, the secretion of enogenous thyroxine is prevented but returns rapidly to the normal rate when thyroxine injections are stopped. 8.The injection daily of 50mg. of growth hormone for a period of 1 week stimulated a marked increase in milk yield in five of eight cows injected, which increase was sustained for periods of 2 to 8 weeks. Three cows showed no response. 9.The variation in the response to growth hormone was interpreted as indicating the degree to which the intensity of milk secretion was limited by the rate of endogenous secretion of the growth hormone. 10.On the basis of the responsiveness of each animal to the three hormones administered, an analysis of the genetic-endocrine causes of their variability in milk production was suggested. Data similar to this in regard to the dams and to the progeny of dairy sires would make possible matings of animals, not on the basis of total production, but on the adequacy of the rate of secretion of the hormones that make possible high milk production.