The Initiation and Maintenance of Lactation

In general, the same anterior pituitary hormones that affect udder growth are concerned in milk secretion. If the pituitary gland is removed from pregnant animals, lactation does not commence when the young are born unless extracts or hormones from the pituitary gland are injected. This indicates that the pituitary hormones are necessary for the initiation of lactation. Prolactin is one of the hormones involved, but is not in itself capable of initiating lactation. The simultaneous administration of prolactin and adrenotropic hormone, however, is effective and it is possible that other pituitary hormones may also be involved.

The factors or hormone relationships that are responsible for the apparent sudden release of these hormones when pregnancy terminates are not fully known. It appears that the relative amounts of estrogen and progesterone present at that time are important in this regulation, because it has been shown that estrogen, at least, may alter the prolactin content of the pituitary gland. A solution of the interrelationship between estrogen and progesterone and the pituitary hormones as they occur before and after birth of the young may eventually be of considerable importance to the whole problem of the initiation of lactation.

Once lactation is established, the same group of hormones that are necessary for the initiation of lactation are responsible for its maintenance. Prolactin by itself cannot stimulate established lactation to any great extent. Other pituitary hormones when given singly are similarly incapable of markedly improving lactation, but if these hormones are given as a mixture substantial increases in milk yield can be obtained, particularly in the declining phase of lactation. In addition to the pituitary hormones, the synthetic estrogens and thyroxine, or materials containing thyroxine, will increase milk yield when given in suitable amounts.

The Use of Hormones to Stimulate Lactation

Several investigators have shown that it is possible to grow mammary glands and induce lactation in goats and cows by the use of hormones even though these animals were not pregnant. The synthetic estrogens, stilbestrol and hexestrol, have been most commonly used for this purpose. In virgin goats and heifers and in animals from which the ovaries have been removed, reasonably good udder growth and lactation, yielding up to 30 pounds of milk a day in cattle, has been obtained by implantation of these materials under the skin. Dry cows and freemartins do not respond so well to similar treatment and indeed the results are always quite variable. Several English workers have shown that the addition of pituitary extracts may in some instances improve the performance of estrogen in this regard.

Excessive amounts of stilbestrol or hexestrol will, however, lower or stop lactation, and the dosage necessary for best results must be carefully adjusted. While such animals are under treatment they are continuously in heat and may have to be segregated from other cattle; the English investigators found a high incidence of broken pelvises due to persistent mounting. Even though these animals are in heat they are sterile, and may remain so for long periods after treatment is stopped. We do not recommend the use of stilbestrol or hexestrol as a general practice, although it can be useful in salvaging some production from sterile cows.

Scientists have also investigated the use of anterior pituitary extracts for increasing lactation. Extracts containing a mixture of pituitary hormones will increase lactation when otherwise the cow would be drying Up- The extracts must be injected frequently, however, and the cost and scarcity of suitable materials make the use of such extracts impractical. Through further research, however, we may find some cheap, Practical means of stimulating the glands of cattle to produce hormones in increased quantities, and thus enable dairymen to increase production. Stilbestrol and hexestrol apparently partly exert their effects in this manner, and other similarly acting materials may be discovered.

When milk is secreted by the cells of the mammary gland, it is stored in the cisterns and ducts and finer spaces within the udder. The milk must be squeezed out of the spaces and forced toward the teat. At milking time, in response to such stimuli as washing the udder, the rattle of dairy utensils, or other barn practices that the cow ordinarily associates with milking, the posterior portion of the pituitary gland secretes a hormone that raises the pressure within the udder and forces the milk toward the teat and causes the cow to let down her milk. This hormone is destroyed relatively quickly in the body, and acts for only 3 to 5 minutes. As long as this pressure is maintained it is easy to obtain milk and the udder may be completely emptied.

Anything that disturbs the cow at milking time, such as unfamiliar noises in the barn, changes in milking or feeding routines, delay in milking after the udder is washed, or rough handling, may interfere with the secretion of this hormone or prevent it from being as effective as it should be. The result is that the cow does not let down her milk quickly, and adequate pressure may not be produced in the udder. The milk that has been formed since the last milking cannot be obtained under such circumstances.

Cows therefore can be milked more efficiently and completely if they are handled gently and quietly, if a regular routine is established, and if they are milked rapidly. Such facts are not new to good dairymen, but the explanation has only recently been brought to light, largely as. a result of investigations conducted at the Minnesota Agricultural Experiment Station. The investigations also led to new ideas concerning the practical advantages of milking rapidly during the time that the "let down" reflex is operating at its peak.