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THE AMERICAN SOCIETY OF ANIMAL PRODUCTION 55 HOW THE ENDOCRINE GLANDS REGULATE THE P R O D U C T I V E A B I L I T Y OF D A I R Y C A T T L E 1 C. W. T U R N E R Missouri Agric~.Itural Expe~me~t Statio~ The search for measurable indications of the productive abihty of dairy cattle goes back 2000 years. How to select and breed the superior animal has been the objective of progressive breeders during this long period. Physical appearance as measured by the judging score card has been of some help but all too often type and production have not gone together, indicating that conformation is not a ver y reliable guide to productive ability. During the past 50 years records of production have gradually come to the fore as the basis of selection. It has been found in many cases that animals of about the same body weight and general conformation differ greatly in their ability to secrete milk. What are the causes of these differences? No great difference has been observed in the maintenance requirement or coefficient of digestion of low and high producers but it was found that the higher producers had the ability to consume large amounts of food and convert it into milk r a t he r than body fat. About 15 years ago there was a great revival of interest in research in sex physiology due to the discovery of the sex hormone by Allen and Doisy. It was soon discovered that this hormone, now called estrogen, induced the growth of the mamm ar y gland. It seemed to the speaker that knowledge of the action of the endocrine glands and the hormones which they produce might be the key that would unlock the door to the physiological processes which regulate or limit milk secretion. At the 1933 meeting of the Society I gave a report of our progress u n d er the title, "The Relation of Endocrinology to the Inheritance of Milk Secretion." During the past four years great progress has been made. Today it is possible to state with assurance that the secretions of the pituitary and the glands and physiological processes regulated by this tiny gland, control directly and indirectly the growth of the udder, the stimulation of milk secretion, and the persistency of lactation. The G r o w t h of the Udz~er.--During the past year Gomez and T u r n e r 2 showed that the growth of the udder is controUed by ~The work reported has been aided in part by grants from the Mount Hope Farm, the General Education Board and the National Research Council. Contribution from the Department of Dairy Husbandry, Missouri Agricultural Experiment Station, Journal Series No. 541. " Gomez, E. T. and Turner, C. W. 1937. Hypophysectomy and Replacement Therapy in Relation to the Growth and Secretory Activity of the Mammary Gland. Mo. Agr. Exp. Ste. ires. Bul. 259. 56 THE AMERICAN SOCIETY OF ANIMAL PRODUCTION hormones secreted by the pituitary. The action of the ovarian hormones is to stimulate the secretion of the pituitary factors during recurring estrus cycles and pregnancy. By the use of pituitary extracts coming from animals which have been injected with estrogen, extensive growth of the mammary duct system has been produced. Further, the pituitaries of pregnant cattle have stimulated the complete growth of the gland. That this mammary growth hormone is not identical with the lactogenic hormone which stimulates the cells to active milk secretion has been demonstrated. It has been named the mammogenic hormone. T h e discovery of the udder growth promoting hormones in the pituitary opens up an entirely new field of study. It will be possible to determine whether the differences in the ability of the pituitary to secrete large amounts of these hormones determine the size of the udder. Could the size of the udder be increased by supplementing the normal pituitary secretions? Are there nutritional factors which affect either the secretion of the pituitary or limit the growth of the udder? Assay methods for the determination of these hormones are now in process of development as well as methods of extraction and purification. Stimulating Milk Secretion.--The pituitary secretes a second hormone, the lactogenic factor, which initiates the secretory process after the udder growth hormones have acted. This hormone not only starts the process but is constantly required to maintain lactation. In experimental animals, Reece and Turner 3 observed that the pituitary content of this hormone was rather uniform during the recurring estrus cycles and during pregnancy, but more than doubled following parturition. It was shown further that the stimulation of nursing caused the rapid discharge of the hormone into the blood, presumably to stimulate secretion during the interval between milking. Extensive comparisons were made on the lactogen content of the pituitaries of dairy and beef cattle. In all comparisons the pituitaries of the dairy cattle contained much greater amounts of this hormone. Preliminary experiments with dairy cattle both in England and the United States indicates that the injection of lactogen may increase lactation in some animals during the declining phase of the period as much as 50 percent. Cannot this be interpreted as indicating that the secretory activity of the pituitary for this factor is one of the limiting factors in her productive ability? This hormone has been detected in the urine :~Reeee, R. P., and Turner, C. W. 1937 The Lactogenic and Thyrotropie Hormone Content of the Anterior Lobe of the Pituitary Gland. Mo. Agr. Exp. Sta. Res. Bul. 266. THE AMERICAN SOCIETY OF ANIMAL PRODUCTION 57 of lactating women and preliminary studies indicate a low level of excretion from women deficient in milk secretion. In addition to these pituitary factors which directly influence the m a m m a r y gland, there have been discovered several pituitary hormones which influence the yield of milk through their action on the amount and composition of the blood. It is only within the past few years that the importance of these factors has been recognized. The Thyroid Stimulating Hormone.uThe pituitary secretes a factor called the thyrotropic hormone which stimulates the thyroid gland to the secretion of increased amounts of thyroxin. Thyroxin is a regulator of general metabolism. Animals with t h y roid deficiencies tend to fatten, whereas those with an over-active gland are thin and of dairy temperament. Milk secretion will continue in the absence of the thyroid glands but at a considerably reduced rate. On the other hand, the injection of thyroxin or the feeding of thyroid tissue has been shown in several experiments to definitely increase milk and fat production during the declining period of the lactation curve. Individual cows differ in the extent of response as would be expected for it can only be effective in cows whose pituitary or thyroid is secreting less than optimum amounts of these hormones. In a study of the thyrotropic hormone content of cattle pituitaries there was a definite tendency for the pituitaries from lactating cows to contain more of the hormone than those from dry cows and an equal weight of pituitary tissue from the dairy cows contained decidedly more than that from beef cows. In experimental animals there has been observed an increase in the pituitary content of this hormone following parturition. The Parathyroid Gland.~While only a few studies have been reported, there is evidence for a pituitary factor which stimulates the parathyroid gland to the increased secretion of parathormon. As milk secretion requires considerable amounts of calcium and this hormone plays a role in calcium metabolism, it will probably soon be shown to play a role in milk secretion. The Adrenotropic Hormone.--The adrenal is composed of a central medulla which secretes adrenaline, and an outer shell called the cortex, which secretes cortin. The pituitary secretes a hormone which stimulates the growth and secretory activity of the cortex. Cortin has been shown to be essential for the maintenance of lactation in two ways. If the adrenals are removed, lactation ceases in the course of several days but may be renewed by the injection of cortin. If the pituitary is removed, lactation ceases but may be again stimulated by the use of the lactogenic and the adrenotropic hormones with frequent glucose 58 THE AMERICAN SOCIETY OF ANIMAL PRODUCTION injections. In the absence of cortin, salt and water is lost from the body with a resulting dehydration of the tissues. This changes the course of the normal flow of water and milk precursors from the blood to the udder to that of withdrawal of water from the udder to the blood, thus limiting or causing the cessation of milk secretion. Excessive salt feeding will help to maintain lactation but it is a poor substitute for cortin. The Carbohydrate Hormone.--That the pituitary secretes a hormone which influences blood sugar and carbohydrate metabolism has been shown by the reduction in blood sugar to about one-half the normal level following pituitary removal and the marked increase in the sugar level induced by the injection of pituitary extracts in normal animals..This is of special significance due to the importance of carbohydrate metabolism in relation to milk secretion. During active secretion from one-third to onefourth of all sugar and lactic acid present in the arterial blood coming to the udder is removed by the mammary glands. It has been claimed by a prominent investigator that the lactogenic and the carbohydrate metabolism hormones are identical. During the past year it has been proven by work in our laboratory that this is not the case for we have been able to separate the carbohydrate factor from the lactogenic hormone. Fat and Protein Metabolism.--There is increasing evidence that the pituitary secretes hormones which play an important role in the metabolism of fat and protein. The possible significance of these hormones in relation to lactation has not yet been investigated. As milk secretion requires the mobilization of large quantities of fat and protein for its synthesis, it is probable that these principles will be shown to have an important place in maintaining the lactation of high producing cows. Summary.--While much further work is required, it is believed that the evidence available indicates that the hormones of the pituitary have a profound effect upon the milk secretion of cattle. while part of the inheritance for milk secretion is of a general nature, the differences in the production of cattle are believed to be due in large part to the interaction of the hormones of the pituitary and the other glands in stimulating the growth of the udder, in stimulating the secretory epithelium of the gland, in stimulating the metabolism of the various precursors of milk in the blood, in increasing the speed of circulation of the blood, etc.