322 THE FEEDING OF ANIMALS 



milk which must have their origin exclusively in the 

 nitrogen compounds of the foods, for we have appar- 

 ently sound reasons for believing that milk-sugar and 

 the butter-fats are constructed, in part at least, from 

 carbohydrates. In an investigation at the New York 

 Agricultural Experiment Station as to the food sources 

 of milk-fat, two cows, both of which gained materially 

 in live weight during experiments continuing two months 

 or over, produced respectively nineteen pounds and forty 

 pounds more of butter-fat than could be accounted for 

 from the food fat and available proteins. The amount 

 of digestible food fat supplied was relatively insignificant 

 and the secretion of milk-fat seemed to be related in no 

 direct way to the protein exchange. These observations 

 led straight to the conclusion that carbohydrates are milk- 

 fat formers. The extent to which food fat assists in the 

 production of milk-fat is not yet determined. While the 

 ingested fats appear to pass directly into the milk to some 

 extent, it seems quite evident that the larger part of the 

 glycerides of milk have their origin in the animal. We 

 are not sure, either, whether protein is ever a source of 

 milk-fat, but that it is not a necessary source now seems 

 to be proved. 



422. The rate of formation of milk solids. A cow 

 yielding 6,000 pounds of average milk a year is not 

 regarded as an unusual animal. This means, however, 

 the annual production of not less than 780 pounds of 

 milk solids, an amount at least double the dry matter in 

 the body of a cow weighing 900 pounds. When we con- 

 sider that this manufacture of new material is carried 

 on not only during a single year, but through the entire 

 adult life of the animal, we begin to realize how exten- 

 sive are the demands upon the food-supply. Still more 



