66 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



ment by the blood, may ultimately be shown to stimulate muscle break- 

 down. Possibly, also, if the general body temperature, or the temperature 

 of the working muscles, increases above a certain point, the endogenous 

 catabolie processes, whatever they may be, are speeded up, with a result- 

 ing increase in the output of endogenous metabolites in the urine. Ex- 

 periments by Myers and Volovic and others (^^-^ g^^id by Linser and 

 Schmid(*') show that increases in body temperature artificially induced 

 in healthy subjects may increase the endogenous catabolism. 



The interesting results of Lee and Tashiro(^*) indicating an increased 

 production of ammonia in excised muscles during contraction may have 

 a bearing on the general problem of the effect of work on the endogenous 

 catabolism. On the other hand, this production of ammonia may be due 

 to the abnormal condition of the excised muscle, or to the lack of readily 

 available sources of energy, or to anoxemia (*^). 



The protein requirement of ivorh may he considered practically non- 

 existent. — Until evidence to the contrary is forthcoming, it appears 

 justifiable to assume, however, that ordinary work carried out under 

 favorable conditions does not accelerate the breakdown of muscle sub- 

 stance, and that, even with working animals, the slight increase that 

 might occur would be more than covered by the increased protein intake 

 necessarily accompanying the increased intake of total food. Hence, the 

 balancing of the rations of farm animals with respect to protein need not 

 involve any consideration of the amount of muscular activity that the 

 animal will undertake voluntarily, or that would be imposed upon it 

 under the usual conditions. 



ESTIMATING THE PROTEIN REQUIREMENTS OF CATTLE UNDER ANY 



SET OF CONDITIONS 



The problem factored. — The problem of the nutrient requirements of 

 animals is solved satisfactorily only when it is factored into its ultimate 

 and independent terms. The amount of protein required to nourish a 

 pregnant heifer in milk cannot be applied directly to another heifer dif- 

 fering either in size, stage of gestation, or rate of milk production, nor 

 can a satisfactory correction be made for such differences unless the 

 requirement for each independent function is known and some informa- 

 tion is at hand relative to the manner of its change with size, time, or 

 intensity of functioning. It has been the purpose of this paper to attempt 

 such factoring of the protein requirements of cattle in so far as available 

 experimental data will permit. 



The use of the information thus obtained may be readily illustrated 

 and an approximate method of putting the results in a form for practical 



