12 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



THE PROTEIN REQUIREMENT FOR GROWTH 



The protein requirement for growth is measured, in terms referable to 

 the animal rather than to its feed, by the rate of deposition of protein 

 (nitrogen) in the tissues of the growing animal. The determination of 

 the rate of deposition of protein involves determinations of the normal 

 rate of growth in body weight, the protein content of animals of different 

 weights and, by combining the two, the normal rate of growth in protein 

 content. 



Unfortunately, in the case of cattle these two determinations have not 

 been done with any great thoroughness on the same breeds. The growth 

 of the dairy breeds of cattle has been measured quite extensively by a 

 number of investigators, but it appears that the chemical composition of 

 dairy calves at different Aveights has not been studied, although a few 

 determinations of the composition of mature dairy cows have been 

 made. On the other hand, no comprehensive investigation of the growth 

 of beef cattle, involving a large number of individuals of known age, 

 has been discovered in the literature, although two extensive studies of 

 the composition of beef cattle at different weights have been conducted 

 in this country. The situation, therefore, must be approached with the 

 expectation of making not infrequent use of somewhat debatable assump- 

 tions and estimations. 



The use of mathematical methods in the study of growth. — Animal 

 growth, in any of its numerous aspects, is a dynamic phenomenon which 

 may be supposed to proceed in a smooth and definite manner when the 

 influence of disturbing factors is removed. Growth is ordinarily studied 

 in piecemeal fashion by attempting to determine the change with time 

 of some animal measurement, such as body weight or the weight of some 

 definite organ. If this change is depicted graphically on coordinate 

 paper, it will under ideal conditions move along a smooth curve, often 

 a relatively simple curve, the shape of which is defined by a simple math- 

 ematical function (equation) relating age (time) to the variable in 

 question. More often, however, a simple mathematical equation will 

 not describe the entire growth change, but only a fraction of it. » However, 

 for the range over which it describes the growth change, the mathematical 

 equation is a complete expression of it. 



Quantitative observations of growth changes can only rarely be made 

 under the ideal conditions just considered, this being particularly true of 

 the growth changes occurring in the farm animals. The confinement of 

 large numbers of these animals under uniform environmental conditions 

 is quite impracticable. Hence, disturbances in growth, due to weather 

 changes, feed changes and digestive and other minor pathological affec- 

 tions of the animal, occur and they occasion irregularities in the mea- 



