TABLE 45

on the strength of results reported elsewhere by the writer.1 It is fully recognized that this figure is uncertain and may be considerably in error in individual cases.

The results of these comparisons are summarized in Table 45, in which the protein available for growth after the estimated maintenance requirement is provided for is compared with the observed gain of protein and with the normal gain computed in the manner described on page 226.

Comparing first the results on the low-protein with those on the high-protein animals, it appears that except in a single instance out of fourteen (excluding the apparent negative gains in the Massachusetts experiments of 1919) the animals receiving the high-protein rations showed a greater gain of protein per 1000 than did their mates on the low-protein rations. Comparing the means for the several years, it appears that the gains of protein made by the low-protein groups were the following percentages of those made by the high-protein groups:

On the other hand, a comparison of the gains of protein by the low-protein animals with the estimated normal gain of protein presents a somewhat different picture. The individual results are strikingly variable but some indication of their general trend may perhaps be secured from the means for the several groups.

In the Massachusetts experiments, omitting, as in the discussion of digestibility, the doubtful periods in 1919, it appears that on the average of the remaining metabolism experiments the digested protein available for growth was but slightly greater than the estimated normal increase of body protein. On this amount in 1918 almost normal growth (96%) appears to have been made, which is equivalent to the utilization of 94% of the available protein. In 1919, on the contrary, the increase of body protein was much less than the assumed normal growth (60%, equivalent to 65% utilization). On the high-protein rations, on the other hand, the average increase of body protein was decidedly greater "Nutrition of Farm Animals," page 327.

than the assumed normal and this was also true in four out of the seven individual cases. Since, however, these rations obviously supplied a surplus of protein, thé percentage utilization is apparently low.

In the experiments at the Virginia Station, on the other hand, the estimated protein available for growth was on the average only about 70 per cent of the normal protein gain and the increase of body protein was only 39 per cent of the normal in 1918 and 49 per cent in 1919, or 45 per cent on the average of both years, while the energy supply of the low-protein animals was only slightly less (97%) than that of the high-protein group. It seems clear that the smaller gains were due, at least in greater part, to an actual deficiency of protein due to the reduced digestibility of the rations.

The following is a condensed comparison of the foregoing results:

A curious fact is that the deficient protein supply in the Virginia experiments seems to have so affected the power of growth that the percentage utilization of the protein available for growth was only 60 to 70 per cent whereas one would have expected that this diminished supply would have been used with the utmost economy.

It remains to be considered whether the inferior results on the low-protein as compared with the high-protein rations were due to lack of sufficient protein or to a deficient energy supply. It is difficult to answer this question very satisfactorily. In the Massachusetts experiments the average energy supply of the lowprotein animals in 1918 was 90 per cent of that of the highprotein animals (Table 44), yet the low-protein animals made an almost normal gain. In 1919 the energy supply of the low-protein group was 85 per cent of that of the high-protein, The difference

between the two years seems scarcely sufficient to account for the notably inferior results in 1919. These low results are substantially all on one animal, No. 22. In view of the fact that this animal made about the same gains of live weight as did the others (page 251), and in view of the seemingly erratic results of the nitrogen balances of 1919 the writer is inclined to interpret the Massachusetts results as indicating that a protein supply but slightly in excess of the total maintenance plus gain sufficed to secure nearly normal growth although insufficient to ensure maximum growth, i. e., that the particular protein mixture used was nearly 100 per cent efficient. In view of the small number of results, however, this opinion is held only tentatively.

In the Virginia experiments the energy supply of the low-protein animals was 93 per cent of that of the high-protein animals in 1918 and 97 per cent in 1919. It would seem, therefore, that the smaller gains of protein by the former as compared with the latter or with the estimated normal must have been due chiefly to lack of protein.

Are the metabolism experiments representative?-A metabolism trial necessarily involves more or less disturbance of the regular daily routine and if the animal is not accustomed to such experimentation it may interfere with securing normal results. This would seem to have been the case especially in the Massachusetts experiments of 1919, as is noted in the account of those trials, while the figures of the remaining experiments do not suggest any such influence. In all such experimentation, however, the preliminary training of the animals is a matter of much importance. In the experience of the writer, mature cattle, especially if selected with reference to their quietness, may be quite readily trained for such experiments and at least show no obvious effects of it upon their metabolism. In the case of young animals the proper selection and the subsequent training may very well offer greater difficulty and cause greater disturbance in their condition. This is a point which must be taken into consideration in the planning of experiments of this sort.

Aside from the foregoing considerations, a metabolism trial can cover only a small fraction of an entire feeding period and the question may be raised as to how far one is justified in assuming that a metabolism trial conducted under proper conditions represents such a period.