Shkkhy — The Comparative Values of Protein, Fat, and Carbohydrate. 217 



present in the ration in order that the maximum milk fat percentage may be 

 reached : beyond that level fat and carbohj'drate, and i^resumably fat and 

 protein, replace one another in definite proportions. In connexion with the 

 special role of fat in the food of a lactating animal, the foregoing results are 

 in exact agreement with the findings of Morgen, Beger, and Fingerling (1904-05). 

 Numerous conflicting statements as to the effect of food fat on the yield of 

 milk fat are found in the literature. Morgan and his collaborators, working 

 with goats and sheep, showed {a) that, when the fat in the ration of a milking 

 animal amounts to less than 05 gi-am daily per kilogram of animal weight, 

 an addition of fat to the ration, as a substitute for carbohydrate, has a marked 

 effect in increasing the percentage of fat in the milk as well as the total milk 

 yield; (b) that the milk yield and fat content of the milk continue to increase 

 with the addition of fat to the ration up to one gram per kilogram of animal 

 weight; (c) that further substitution of fat for carbohydrate has no effect 

 except in unusual cases. The role of fat in the ration of a lactating animal 

 having been definitely established, there remains to be worked out the practical 

 problem of the extent to which fats should be economically fed to animals like 

 the cow, whose mammary glands are regarded as machines, to be worked with 

 maximum efficiency for the purpose of milk fat production. It is generally 

 agreed, and the idea is borne out by the results of Jordan and his eo-workers 

 (1901), that the addition of large quantities of fat to the diet of the milking 

 cow does not produce a permanent increase in the yield of milk fat. Never- 

 theless the cow, in all probability, requires, like the goat, a certain minimum 

 of fat in the food to enable her milk fat producing organs to work with maximum 

 efficiency. 



Conclusions. 



1. The milk fat yield of lactating animals can be increased, within limits, by 

 feeding. 



2. Protein in the food may replace carbohydrate for the production of 

 milk fat : the protein, casein, when supplied in quantity in excess of the mini- 

 mum requirements for body metabolism and milk yield, replaced starch in equal 

 quantity for that purpose. 



3. Fat may replace starch or protein, i.e. protein in excess of the minimum 

 requirements of a lactating animal, in the proportion of 1 to 2^ : this proportion 

 holds only for rations already containing a certain quantity of fat. 



4. In a diet containing less than a certain quantity of fat a replacement of 

 some of the carbohydrate by fat gives results which credit fat with a much 

 higher value for milk fat production than that represented by the above 

 proportion : in this investigation fat has been shown to be at least fi'^'e times as 

 valuable as carbohydrates when fed under such conditions. 



5. If a diet is constituted so as to contain less than a certain quantity of 

 fat, the maximum yield of milk fat will not be returned no matter how liberal 

 the ration is in quantity. 



6. Fat in the ration of a lactating animal stimulates the secretion of milk 

 fat : this stimulative action required for maximum milk fat productioii is 

 induced by a comparatively small quantity of fat ; fat fed in excess of the 

 requirements for this purpose has no special value, but simply replaces the 

 carbohydrates in isodynamic proportion. 



