C. Crowthbr and H. E. Woodman 43 



more marked with Cow B, and for this reason we were obliged to reject 

 the records of this cow for Periods VI and VIII. 



If the data for nitrogen-retention be examined, it will be seen that 

 with each cow up to Period IV there were distinct indications that the 

 daily retention of nitrogen was approaching a maximum value and, but 

 for the regrettable difficulty with the hay, it seems probable that this 

 limit would have been reached in Period V, since in the later periods, 

 despite increased intake of nitrogen, no higher retention than that of 

 Period IV was recorded. A slight increase is indicated with Cow B in 

 Period VII but it will be noted that the nitrogen intake in this case was 

 practically the same as for Cow A in Period IV, so that the maximum 

 retention of nitrogen under the conditions of experiment would seem to 

 be associated with a nitrogen intake of roughly 170 gm. per day {= 1060 

 gm. crude protein). Taking the average weight of the cows for Period V 

 this represents a daily intake of crude protein of 2-34 kg. per 1000 kg. 

 live-weight, or almost exactly the same figure as was dedxiced for the 

 sheep from the data obtained in the digestibility trials. This confirmation 

 is particularly interesting in that the diets in the different periods of the 

 sheep tests varied considerably in qualitative character, and the con- 

 clusion suggests itself that, when fed along with hay, the proteins of 

 maize, cottonseed, palm kernels and yeast are of equal value so far as 

 capacity to effect protein storage in the body is concerned. 



It would appear therefore from these experiments that for the last- 

 named purpose the optimum protein-supply in the food is in the neigh- 

 bourhood of 2-4 A-(/. crude protein, or say hi kg. digestible crude protein 

 per 1000 kg. live-weight. This, of course, appUes only to animals such as 

 those under test which were not subject to any losses of nitrogen other 

 than those voided in faeces and urine, and were free from any exceptional 

 internal needs for protein such as arise during pregnancy. 



It will be noted from Table I that the duration of the different periods 

 varied from 21 to 32 days. It was anticipated that a period of 21 days 

 would afford ample time for the re-establishment of nitrogen equilibrium 

 after the disturbance due to change of ration. It soon became evident, 

 however, that this was not the case, and the records of Periods IV to VI 

 bring this out clearly. For reasons already given the nitrogen-consump- 

 tion remained relatively constant throughout these three periods, which 

 may therefore be grouped together as one long period. Viewed in this 

 light it is seen that the establishment of nitrogen equilibrium is a much 

 slower process than we had assumed, the nitrogen-retention persisting, 

 though at a steadily diminishing rate, throughout the whole interval of 



