The decline in milk production that occurs during the last several months of lactation is partly due to a gradual decline in the activity of the milk secreting tissue and to changes in the cells of the tissue. If milking is terminated suddenly, as is done when cows are being dried off, the milk left in the udder accelerates the changes until the tissue ceases to secrete milk. Even small amounts of milk left in the udder have a similar but less pronounced effect. The Minnesota workers showed that when cows are incompletely milked they decrease more rapidly in milk production than when they are milked dry at each milking. By properly stimulating cows to "let down" the milk and milking rapidly while the pressure in the udder is high, the rate at which cows drop off in production can, therefore, be decreased.

Valuable time can also be saved by taking advantage of the "let down" reflex. Most cows can be trained to let down their milk quickly and if they are then milked well all the milk in the udder can be removed by machine in 3 1/2 or 4 minutes and hand stripping can be dispensed with. Fresh cows are more easily trained than others. The saving in time to dairymen who follow the technique of rapid milking reaches sizable proportions during the year.

Rapid milking has another advantage. As the udder becomes empty, the teat cups tend to creep up the teat and the delicate inner teat structures can be damaged. The opening into the teat may be partly closed because of such damage, and it becomes harder completely to empty the udder. Rough hand milking can produce similar damage. The damaged structures also provide a point where mastitis organisms could easily gain entrance to the udder. Milking should therefore not be continued beyond Ate point where the cups begin to creep.

In summary: The research we have discussed. has brought a better understanding of the fundamental action of hormones in the processes of reproduction and lactation—facts that must be known before hormones can be used in a practical way.

The information has already been put to use with some success. Relief of certain types of sterility has been attained; reasonably good yields of milk have been induced in sterile cows and goats, and increased milk yields in poor-producing animals has been brought about by the use of hormones. The results with hormones in these regards, however, are far from being completely satisfactory. Apparently there are other factors that we must learn more about before best results will be attained. The present method of administering hormones is also not a practical procedure for the farmer, but if means can be found by which the secretion and activity of hormones may be controlled in a relatively simple manner, it may be possible to regulate reproduction and lactation more satisfactorily in farm herds and flocks.

In particular, more should be known of the interrelationships of hormones as they affect reproduction and lactation. Better methods of analysis for the various hormones concerned would be of an immense help in establishing such relationships. It should then be possible to determine at an early age which animals are potentially good producers and to develop better strains by breeding from such animals. Corrective measures, where desirable, could also be instituted more intelligently than is now possible.

The effect of various nutrients and the influence of environmental factors, such as light and temperature, on the secretion and activity of hormones needs further study. There is already some evidence that the actions of both vitamins and hormones are dependent on each other and that enzyme reactions in the body may be dependent on both. The level of nutrition may also be an important factor. Whether feed is efficiently used in making milk, meat, or wool depends finally on the proper functioning of all these processes. It has also been shown that the amount of light to which animals are exposed may, in certain species at least, affect hormone activity. Temperature may have similar effects. If such studies reveal that such factors importantly influence the activity of various glands, more practical means of regulating reproduction and stimulating lactation will be available.

Research of this type is already being undertaken in many laboratories, so that the future promises to yield information even more useful than that obtained in the past.

THE AUTHORS

S. R. Hall has been with the Bureau of Dairy Industry since 1936 and in that time has made important research contributions regarding physiology of reproduction and lactation. His work has included studies of the microscopic structure of the anterior pituitary gland, and the function, method of measuring, and distribution of the lactogenic hormone in cattle and other animals. Dr. Hall is a graduate of the University of Virginia.

J. F. Sykes is a physiologist in the Bureau of Dairy Industry. As a research associate in the Physiology Department of Michigan State College, Dr. Sykes studied the effects of hormones on lactation and reproduction.