LIMITS AND VARIATIONS 39 



amount of fat yielded by the various breeds is far less so and is 

 due to the quantity of milk being usually inversely propor- 

 tional to the fat percentage in the milk. The proportion, how- 

 ever, is not a direct one and it has been proved on many occa- 

 sions that the breeds giving the low fat percentages yield the 

 largest total weight of fat. For this reason the Dutch, Frisian, 

 and Holstein breeds are very popular for dairy purposes. 



Concerning the effect of food upon the composition of milk, 

 numerous investigations have been made but the results ob- 

 tained are apparently somewhat contradictory. This is prob- 

 ably partially due to the conditions under which the experi- 

 ments were conducted being not strictly comparable. Earlier 

 observers failed to appreciate the fact that a certain weight of 

 fat, proteid, and carbohydrates is necessary for providing body 

 heat and for the repair of waste tissue in the cow, and that this 

 amount is proportional, though not directly so, to the weight of 

 the animal. If the food ration is only slightly in excess of this 

 quantity, the effect of stimulants, such as oil cake, would be to 

 immediately increase both the percentage and total quantity 

 of butter fat secreted; on the other hand, if the ration is suf- 

 ficient for the body maintenance and milk secretion, additional 

 food would probably not increase either the percentage or the 

 quantity of butter fat, and it is conceivable that they may even 

 be somewhat reduced by this over-feeding process. 



Of the more reliable investigations, those of Morgen, Beger, 

 Fingerling, Doll, Hancke, Sieglin, and Zielstorff^ might be 

 mentioned. They found that food free from fat sufficed for the 

 maintenance of animals in a healthy condition and increased 

 the live weight of the animal, but was totally unsuitable for 

 milk production. The addition of food fat in quantities 

 equivalent to 0.5 to 1.0 gram per kilo of the animal weight 

 favoured the production of milk fat. Later, the first three 

 observers, in a series of experiments extending over six years, 

 obtained results which showed that of all foods, fat alone exerts 

 a specific action on the production of milk fat and that, within 

 certain limits, fat is the most suitable food for butter fat pro- 



