Download How the Endocrine Glands Regulate the Productive Ability of Dairy

THE AMERICAN SOCIETY OF ANIMAL PRODUCTION
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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.
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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.
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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
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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.