PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 57 



Overman's analyses for protein evidently are considerably higher than 

 Haecker's for milk of all grades/ and for milk containing 3.50 per cent 

 or more of fat, they are remarkably closely predictable by the simple 

 linear equation above given. For lower grades of milk, the equatioii 

 evidently over-estimates the protein content of milk from the fat content. 



A method of estimating the protein requirements for milk produc- 

 tion. — In view of the systematic method by which Overman's samples 

 were taken, assuring an equal representation of milk from all stages of 

 lactation, and in view of their number and the number of cows and of 

 breeds represented, it appears justifiable to use the Overman data as the 

 basis of estimates of the protein requirements of cows for milk produc- 

 tion. Since thus far protein requirements have been expressed in terms 

 of nitrogen, and since in practice a pound of milk is a convenient unit, 

 the prediction equation above given has been changed to 



N = 1.45 + 0.29f (38) 



in which N is the nitrogen in grams per pound of milk, and f is the per- 

 centage of fat in the milk. This equation is applicable only to milks 

 containing 3.50 or more per cent of fat. For milks of lower grade, the 

 nitrogen content may be computed roughly from the Overman averages 

 given ill Table 29. 



THE PROTEIN REQUIREMENT FOR MUSCULAR ACTIVITY 



The great variation in the muscular activity of farm animals, depend- 

 ing mainly upon their temperament and upon the nature of their con- 

 finement, offers a serious obstacle to the estimation of their require- 

 ments for those nutrients necessarily consumed in muscular metabolism. 

 It is a matter of considerable importance, therefore, to determine whether 

 protein necessarily serves as a source of muscular work, either directly 

 as the dietary amino acids coming to the muscles from the intestinal 

 tract, or indirectly as the result of an increased catabolism of the muscle 

 tissue itself. 



Does the muscle cell wear oait? — Since the days of Liebig, the relation 

 of muscular contraction to protein metabolism has been shown to be of 

 less and less importance. The classic experiments of Fick and Wis- 

 licenus, which have been abundantly confirmed, showed that muscular 

 work could be performed at the expense of carbohydrates and fats, and 

 the trend of modern investigation into the chemical and calorimetric 



•Perkins (Ohio Mo. Bull., 1:304 (1916)) has also published a summary of a 

 series of milk analyses (807), in which the protein to fat ratio is considerably 

 higher than that of Haecker's analyses. 



