74 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



heifers was under experimental feeding for periods ranging from 90 to 

 840 days. Different levels of protein feeding were tested on these ani- 

 mals in periods ranging in length from 30 to 750 days. The level of 

 protein intake was varied by varying the proportions of timothy and 

 alfalfa hay, and of corn, oats, bran, cottonseed meal, dried skim milk, 

 starch and sugar in the rations. The lower protein rations thus contained 

 more timothy hay, corn, oats, starch, and sugar than did the higher pro- 

 tein rations. The number of different protein levels tested on each animal 

 as well as the order in which they were tested was irregular for most of 

 the animals. The feeds were analyzed during the course of the experi- 

 ments, but average digestion coefficients were used in computing the 

 intakes of digestible crude protein. The energy intakes were computed by 

 the use of Armsby's tables. The measurements of growth included body 

 weights and body dimensions. 



The results are summarized according to a number of different plans, 

 and upon these summaries the final conclusions are based. It was found 

 that dairy heifers could make normal growth, in accordance with Eckles' 

 standards, upon amounts of protein far smaller (56 to 77 per cent for 

 Holstein and Jerseys, respectively) than those called for by the Armsby 

 standards; in fact, with an increase in energy intake, normal growth 

 was attained on still smaller intakes of protein. However, an increase in 

 protein intake above that requisite for normal growth increased the rate 

 above the expected. 



It is also concluded that the Holstein heifers were able to make dis- 

 tinctly better growth with reference to their normal, on lower levels of 

 protein, than were the Jersey heifers. However, this unexplainable dis- 

 tinction between the breeds is not clearly shown in all of the summary 

 tables (Nos. 42, 43 and 44). 



The lowest estimated levels of protein intake in these Missouri ex- 

 periments were equal to or lower than the intended low protein intakes 

 in the Armsby cooperative experiments, and on the average were about 

 40 per cent higher than the estimated requirements as given in Table 31 

 of this report. However, the actual intake of digestible crude protein in 

 the low-protein periods of the Missouri studies probably approximated 

 closely in the average to the estimated requirements in Table 31 in the 

 same manner and for the same reasons as obtained in the Armsby co- 

 operative experiments. The use of average digestion coefficients for pro- 

 tein with rations containing considerable amounts of starch and sugar 

 will always overestimate the content of digestible crude protein, and for 

 large amounts of these carbohydrate materials, such as must have been 

 used in the low-protein Missouri rations, the overestimation might well 

 amount to 40 per cent or more. 



