The following changes appear to take place when thyroprotein is given for extended periods to cows fed at the usual level according to milk production : Milk production is markedly increased at the expense of body weight; after a period of 4 to 8 weeks, depending on the condition of the cow at the start of the feeding period and the amount of stimulation produced, milk production, and heart rate decrease, and when milk production drops to about 10 pounds a day, she will gain weight.

It would also seem probable that, if the cow were not able to make up the losses in body weight before the succeeding lactation started, milk production in the subsequent lactation might be adversely affected.

The English workers have therefore stated that for the best results, thyroprotein should be fed at a level that will raise milk yields about 20 percent, and that for this increase feed intake should be enlarged by about 20 percent. This practice largely eliminated the loss in body weight when thyroprotein was fed at the 15-gram level (one-half ounce). In one experiment these workers incorporated the thyroprotein into a grain mixture, which was cubed. Four pounds of these cubes contained sufficient thyroprotein to produce a 20-percent rise in production and at the same time supplied approximately the extra 20 percent of feed necessary to maintain body weight. The cubes were fed in addition to the regular allowance of grain.

In the Beltsville experiment, where the amount of thyroprotein fed for several months varied from one to one and one-half grams per 100 pounds of body weight, severe losses in body weight were noted. Because of the extremely poor condition of the cows and because it had been planned to continue the feeding of thyroprotein up to the last month of the lactation period, the total feed intake was increased 25 percent by grain feeding. As a result, the condition of the cows improved greatly. They gained weight, and their rapid decline in milk production was halted.

Obviously, if more feed must be fed to maintain body weight and milk production, the question arises as to whether the extra milk will pay for the extra feed. It is hard to give a definite answer on the basis of the data we now have. The Department's Technical Bulletin No. 815 points out that a 20-percent increase in the feed intake of a normally fed cow not receiving thyroprotein will increase milk production about 13 percent. If by feeding thyroprotein, 20 percent more milk could be produced with 20 percent more feed, that would leave an increase of 7 percent in milk production to pay for the thyroprotein and the extra trouble of feeding it.

A few men have studied the effect of thyroprotein on the composition of milk. One effect we have mentioned: The percentage of fat is raised. Some investigators have noted a slight increase in the percentage of solids not fat, although others have not. J. G. Archibald, at the Massachusetts station, reported a decrease in casein and a roughly proportional increase for lactalbumin and globulin, with no change in total solids, ash, or lactose. Workers at the West Virginia station found that the ascorbic acid content of the milk was lowered to 33 percent below normal. English workers reported a considerable decrease in the phosphatase content.

It will also be necessary to settle the question as to whether sufficient thyroxine is secreted in the milk of cows fed thyroprotein to produce possible harmful effects when the milk is used by humans. Data collected at the Missouri station have shown that when guinea pigs were fed such milk no detectable thyroid effects could be observed, but data on human subjects are needed before this point can be established.

Officials of purebred breed associations express concern over the use of thyroprotein in making records. Such a practice is now banned, because by its use the record would not be an expression of the cow's inherited milk-producing ability. To guard against an), possible unscrupulous use of thyroprotein to improve the official records of herds, methods of detection will be needed. Perhaps tests of the phosphatase content of the milk, which drops when thyroprotein is used, might be a tool in detecting the use of thyroprotein. Or, a study of some of the other enzyme systems in milk might be useful in this regard.

It seems doubtful that thyroprotein will ever be mixed into regular dairy grain mixtures, because it should not be fed except during certain periods of the lactation and because of the variation in effects produced in individual cows.

As previously stated, the effect of feeding thyroprotein for several lactation periods on the health and reproduction of cows has not been determined. Experiments are now in progress at Beltsville to answer these questions. Furthermore, when thyroprotein is fed for a long time it appears that extra feed should be fed. The amount of extra feed needed will probably vary with the condition of the cow, the amount of thyroprotein fed, and other factors. Until answers to these questions have been adequately settled, the general use of the material by dairymen is not recommended. If, however, it finally develops that feeding thyroprotein does not shorten the useful life of cows and does not adversely affect reproduction, it seems that this practice could be quite useful for increasing Milk and fat production in commercial herds and where milk is sold on a basis of percentage of fat.

THE AUTHORS

L. A. Moore is in charge of the section of Dairy Cattle Nutrition of the Bureau of Dairy Industry. Before joining the Department in 1945 he was successively associated with the dairy departments of Michigan State College and Maryland University. For outstanding research relating to the nutrition of dairy cows, Dr. Moore in 1943 received the Borden Award.

J. F. Sykes is a physiologist with the Bureau of Dairy Industry specializing on problems of reproduction and lactation in dairy cattle. Dr. Sykes is a graduate of the University of Toronto and has been research associate in the physiology department of Michigan State College.