1917] DAIRY FARMING DAIRYING. 671 



change. The effects of underfeeding on the fat constants appear to reach a 

 limit with a subnormal plane of nutrition of about — 40 per cent. Long con- 

 tinued underfeeding results in more or less recovery of the fat constants from 

 the most abnormal values, but the recovery is never complete. 



" The increase In the percentage of milk fat and the changes in the character 

 of the fat which accompany underfeeding are not satisfactorily explained by the 

 hypothesis that underfeeding causes a transfer of tissue fat to the mammary 

 glands. Blood fat analyses made during the height of physiological underfeed- 

 ing in the case of two cows failed to show any increase in the amount of fat 

 carried by the blood stream in comparison with normal conditions in the same 

 animals. 



" As a basis for the explanation of the effects of underfeeding on the percent- 

 age of fat in milk and on the composition of the fat, the hypothesis is advanced 

 that the synthesis of milk fat in normal amount and its synthesis with normal 

 composition are independent physiological functions. The production of the 

 normal amount of milk fat is controlled by the activity of the lipases and other 

 enzyms which accelerate this synthetic reaction in the mammary gland, and is 

 influenced greatly by changes in the general metabolic activity of the body, 

 particularly by the changes that affect the fat metabolism. By this hypothesis 

 it is possible to explain the normal variations in the percentage of fat in the 

 milk, the variations which occur with extreme weather conditions (which have 

 a particularly strong influence on the general energj' metaboUsm of the animal), 

 as well as the variations in the effects of underfeeding on the percentage of fat 

 in the milk, especially those that appear to be related to the fatness or thinness 

 of the cow. The synthesis of milk fat of normal composition is controlled by 

 variations in the amount and kind of material presented to the milk glands by 

 the blood stream, from which the normal constituents of milk fat are formed, 

 particularly those which especially characterize the milk fat, namely, the vola- 

 tile fatty acids. A further extension of this phase of the hypothesis is limited 

 by the lack of knowledge as to which constituents of the blood are utilized for 

 the formation of normal milk fat. 



" The effects of underfeeding on the composition and properties of milk and 

 butter fat show the importance of controlling this factor in feeding experi- 

 ments involving the effects of specific feeds on the composition of milk and 

 butter. The effects of underfeeding must be taken into account in the inter- 

 pretation of all data involving variations in the composition of milk and butter 

 fat due to specific conditions of the cow, specific environments of the cow, 

 changes in the feed of the cow, or to feeds of specific character. 



" The variations in the composition and properties of milk and butter fat with 

 underfeeding of the cow may have an important bearing on the use of such 

 milk for human food, particularly as food for infants. Lack of data on this 

 phase of the problem, however, makes it impossible to state how much bearing 

 the results have in this connection." 



Further studies of the relation of the quality of proteins to milk produc- 

 tion, E. B. Habt, G. C. HuMPHRirsf, and A. A. Schaal {Jour. Biol. Chem., 26 

 (1916), No. 2, pp. 457-4^1, fiffs. 4)- — Id continuation of previous studies (E. S. 

 R„ 33, p. 275), data are here presented on the comparative value for milk 

 production of the proteins of gluten feed, linseed meal, distillers' grains, casein, 

 and skim milk powder. Three pure-bred Holsteins of large milking capacity 

 and not with calf were used In the experiments. Each animal was placed on 

 one ration for a 4-week period with immediate change to another concentrate, 

 thus involving each individual cow in from 16 to 20 weeks of observation. Each 

 ration contained approximately 2 lbs. of digestible protein of which 50 per cent. 

 89684°— No. 7—17 6 



