70 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



result of inaccuracies in the information upon which they are based, or 

 of an incomplete summation of the independent factors involved? 



In his discussion of the protein requirements of farm animals, Armsby 

 himself (^' p- 412-414^ j^^^g referred to a number of investigations, including 

 some of his own, that appear to show that animals can support what 

 seems a normal rate of growth upon a supply of protein little greater 

 than the maintenance requirement plus the amount actually stored. In 

 fact, in embodying the Kellner protein standards in his own feeding 

 standards for animals, Armsby says : 



On the whole, one can hardly fail of the impression that the requirements for 

 protein as such in growth have been over-estimated and that the organism may- 

 utilize its protein supply more economically than the current feeding standards 

 would indicate; in other words, that the actual protein supply may be made 

 considerably smaller than has been supposed before it becomes a limiting factor 

 in growth. Until this impression is confirmed by more extensive investigation, 

 however, it appears the safer course to adhere provisionally to the accepted 

 standards 



A comparison of the tentative estimates in Table 31 with the results of 

 the Armsby cooperative experiments. — On the initiative of Armsby, a 

 series of cooperative investigations on the protein requirements for the 

 growth of cattle was undertaken by eight agricultural experiment sta- 

 tions in this country under the auspices of the National Eesearch Coun- 

 cil (^''^). The plan of these investigations called for the testing of two 

 rations, identical in respect to the source of protein, but differing in the 

 content of this nutrient by reason of a variable inclusion of starch. The 

 low-protein ration was to contain 20 to 35 per cent more digestible true 

 protein than an estimate of the minimum requirements obtained by add- 

 ing a maintenance requirement of 0.5 pound daily per 1000 pounds live 

 weight and an estimate of the daily protein retention per 1000 pounds 

 from Armsby's equation given above (p. 41, Equation 21). The high- 

 protein ration was to conform to Haecker's standards, and contained from 

 50 to almost 100 per cent more protein than the low-protein ration. The 

 calves were to be fed in pairs, selected for similarity of age, sex, weight, 

 and breeding, both to receive the same intake of net energ}', one from the 

 low-protein ration and one from the high-protein ration. The growth of 

 the calves was to be followed by nitrogen balance studies and by deter- 

 minations of the increase in body v/eight and body measurements. 



For one reason or another, it was found impossible to conform to the 

 plan as outlined, mainly because the digestible nutrient and net energy 

 contents of the rations fed were found to be considerably lower than 

 were expected from average analyses and digestibilities, and because the 

 high-protein rations were generally more acceptable to the calves and were 



