C. Crowtfier and H. E. Woodman 41 



the varying sources of protein-supply in the different periods, and the 

 fact that the order in which tlie diverse rations were fed was not that of 

 increasing nitrogen-content. It woukl be ei(ually legitimate, for example, 

 to account for the reduced nitrogen-retention in Periods III and VI, by 

 assuming the proteins of cottonseed cake and yeast respectively to be of 

 lower metabolic efRciency than those of palm kernel cake. 



Both alternatives engaged our interest, as we were at the time con- 

 templating a comprehensive study of the nutritional requirements of 

 the dairy cow, in which, as is well-known, protein-supply plays a part 

 of outstanding importance. It has been established that up to a certain 

 point increased protein consumption leads to increased secretion of 

 milk. Does this point coincide with that of maximum retention of nitro- 

 gen in the body? Does the amount of food-protein recj[uired to produce 

 the latter vary with the qualitative character of the ration? Is the 

 maximum of nitrogen-retention constant for any one individual or 

 variable according to the feeding? Before attempting to secure answers 

 to these questions we decided first to test whether, by feeding cows on 

 rations of the same qualitative composition, but of successively increasing 

 protein-content, and determining tlie nitrogen-retention, we could detect 

 an optimum of protein-supply, such as had been suggested by the records 

 of the sheep experiments. Further, in order to avoid the complications 

 that pregnancy and lactation would introduce, we selected as test animals 

 two fully grown Shorthorn cows, not in calf, and not producing milk. 



FIRST EXPERIMENT. 

 (196 days, November, 1916— June, 1917.) 



For reasons stated later this experiment cannot be regarded as en- 

 tirely satisfactory and consequently need only be dealt with in outhne. 



The two cows were first brought approximately into nitrogenous 

 equihbrium on a daily ration of 141b. "seeds" hay, supplying about 

 122 gm. nitrogen. After determining over a given period the daily 

 balance between the amounts of nitrogen consumed and excreted, the 

 ration was increased by adding 2 lb. maize meal per day (containing 

 about 13 gm. nitrogen) and the effect on the nitrogen balance was then 

 investigated. This was followed by several further experimental periods, 

 in each of which the ration was increased by 2 lb. maize meal. 



The general arrangements for the experiment and the methods of 

 analysis, etc., followed were similar to those described in detail later in 

 this paper. 



