4:8 Bulletin 173. 



solve the question as to whether or not the per cent of fat may be 

 permanently increased by feeding. But concerning this point we 

 have little information except theories based upon the results of 

 many experiments. One theory has been long and largely held that 

 milk-fat is produced from the protein in the food. If this were the 

 case a natural supposition would be that by increasing the amount 

 of protein in the food, the proportion of fat in the milk would be 

 thereby increased. Another theory is that milk-fat is produced 

 from the fat in the food. Then feeding an increased amount of fat 

 might be supposed to result in a higher per cent of fat in the milk ; 

 or, on the other hand, a decrease in the supply of food-fat would 

 likewise cause a decrease in the per cent of milk-fat. A third the- 

 ory, and the one which is most largely entertained, is that so long 

 as the animal is well nourished the per cent of fat in the milk is not 

 appreciably affected by even wide variations in the character of the 

 food Experiments supporting these three theories will be found 

 in subsequent pages. 



The question has a practical bearing in the economical manage- 

 ment of the dairy. For, if by food we may increase the richness 

 of the milkj then there is opportunity to enliance the value of all 

 our cows. Butter-fat is the most valuable constituent of milk, and 

 if the cow may be made to produce a milk richer in fat by giving 

 her certain foods, or foods containing a large proportion of a par- 

 ticular nutrient, then the dairyman may increase the value of his 

 cows to the extent that they may be made to respond to the particu- 

 lar foods by increased production of fat. In general this has not 

 been found to be the case, otherwise why should so many cows be 

 giving milk that is comparatively poor in fat ? And why should 

 they possess this same characteristic in common with their ancestors 

 as long ago as their history is known to man? 



Again, if feeding large amounts of protein tends to an increased 

 production of milk-fat, then the dairyman will need to purchase 

 foods containing a high proportion of protein, which foods usually 

 command higher prices than those containing less protein. If, on 

 the other hand, a large supply of protein is not essential to the pro- 

 duction of milk-fat ; if the per cent of milk-fat is dependent on the 

 supply of food-fat ; or, if it is not governed by the food so long as 



