New York Agrigultukal Experiment Station. 521 



The extent of protein metabolism seems to be influenced mainly 

 by the protein supply rather than by the quantity of milk solids 

 secreted. When in the first periods the digestible food protein 

 varied between 1.42 and 1.85 lbs. daily and there was an increase 

 of body protein, the milk solids were from two and six-tenths to 

 three times the protein equivalent of the urine nitrogen, but when 

 the available protein in the food fell to 0.41 lbs. daily, so that the 

 animal's needs forced a body loss of 0.44 lbs. of protein daily, the 

 milk solids were four and two-tenths greater than the protein 

 broken down. In view of these figures, it is not easy to avoid the 

 conclusion that in some way the abundant metabolism induced 

 by a generous supply of protein in the ration had a stimulative 

 rather than a constructive (building) function in its relation to 

 milk secretion. 



The Relation Between the Food and Quality of the Milk. 



The evidence on this point is incidental to the main purpovse of 

 the experiment, but is none the less emphatic. 



Changes were made in the rations in three ways: (1) By de- 

 creasing the fat in the food from about the usual quantity to 

 practically none; (2) by producing wide variations in the protein 

 supply and nutritive ratio; and (3) by producing wide variation.-? 

 in the supply of total digestible material. 



Were there changes in the constitution of the milk correspond- 

 ing to any or all of the variations in the kind and quantity 

 of the food supply? A careful examination of the accompanying 

 graphic display (Fig. 19) of the amounts and kind of food eaten 

 and of the composition of the milk, during sixty-six days, does 

 not reveal any such relation. To be sure, when the ration was 

 changed from the normal to the extracted foods there was quite 

 a marked drop in the percentage of milk solids, but in a few days 

 the milk recovered its former richness. Neither a deficiency in 

 the protein of the ration nor a depression of the digestible nutri- 

 ents to about 5.5 lbs. per day caused the cow to produce poore:* 

 milk. The only apparent effect was in changing the quantity 

 of product. The percentages of milk solids and fat varied greatlv 



