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Computer Graphics	Protein It is generally recognized that in a study of the utilization of food energy the biological requirements for protein must be met by the food. These biological requirements are conventionally indicated by the nitrogen content of the tissue constituents katabolized at maintenance and by the nitrogen content of deposits above this, as in flesh and milk. In evaluating feed protein on the basis of its worth to satisfy such biological demands the amount of nitrogen deposited as flesh or milk can be readily determined by a digestion balance experiment, from the results of which the nitrogen balance can be calculated. The critical problem arises in determining the amount and the constancy of the endogenous nitrogen katabolism or the so-called daily muscle wear and tear in body protein tissue that requires replacement. This has been the subject of careful research, the results of which tend to indicate that the necessary wastage of muscle due to daily activities is small. Obviously its absolute determination in cattle presents difficulties not encountered in similar attempts with small laboratory animals. For example, under conditions of feeding it is impossible to separate the endogenous nitrogen fraction from the feed nitrogen metabolized unless one fed a nitrogen- free food, and during fasting the nitrogen excretions contain variable fractions of nitrogen representing body protein that was katabolized for its heat, which latter could be supplied by food energy. In other words, although the nitrogen excretion during fasting is solely body nitrogen, it is much in excess of the requirements if sufficient calories in other nutrients for maintenance are available. The basal need for protein, as indicated by the minimum fasting nitrogen excretion to be about 40 gm. per 500 kg. of live body weight per 24 hours for Holstein cows and 47 gm. for Jersey cows, when fasting after hay rations. For steers the basal need is about 38 gm. per 500 kg. This corresponds to about 225 to 300 gm. of digestible crude protein as a daily requirement of the animal for replacement of body wear and tear. During feeding the excretion of nitrogen from body tissue would be considerably less than during fasting, because under the latter condition a large part of the heat production originates from the katabolism of stored body fat and to some extent from protein katabolism, whereas during feeding much of this heat production may be supported by calories from food. From the standpoint of production feeding (that is, above maintenance) the distinction whether the protein in the feed is for fattening and body deposit or whether it is for the support of lactation acquires particular significance, partly because of the manner in which excess protein in the feed is disposed of but even more because of the relative significance that may be attached to the known stimulating effect of protein ingestion on metabolism. In fattening the requirements for protein for deposit as body tissue are relatively small and apparently not materially in excess of basal requirements except in the early stages, when animals are in a depleted condition. In steers this protein storage has been shown to decrease to 125 or 175 gm. (i.e., 20 to 28 gm. of nitrogen per day) after the sixth week of surfeit feeding. As it is impracticable to feed these large animals a protein-free ration and thus determine directly the true endogenous nitrogen, the nearest approach to the excretion of endogenous nitrogen would be the minimum urinary nitrogen excretion found with the animals on a low protein feed, as is illustrated, for example, with cow H-V on late cut timothy hay, when her digestible nitrogen intake was 20 gm. daily and her nitrogen excretion 22 gm. Based on recorded determinations of the minimum endogenous urinary nitrogen of human subjects and on the observation that the minimum values (in urinary nitrogen) of pigs, sheep, cattle, and humans fall within the limits of 0.024 to 0.035 gm. per kilogram of body weight, Mitchell 15b has established that the minimum nitrogen requirement of cattle for maintenance is 0.030 gm. per kilogram of body weight per day. If this average value is applied to the body weight of cow H-V in her experiment on very late cut timothy hay, the result for endogenous 16 nitrogen thus calculated approximates the amount of urinary nitrogen (22 gm.) excreted when she was on her lowest level of digestible nitrogen intake (20 gm.), which, however, still maintained body balance. As the protein requirement for work has been shown to be so small as to be considered negligible, the assumption of constancy in the endogenous nitrogen excretion appears to have some basis of logic. Brody's conclusion that the endogenous nitrogen excretion is a linear function of the body weight raised to the 0.73 power tends to support this assumption of relative constancy that so generally prevails. On the assumption that this value of 0.030 gm. per kilogram of body weight (feed weight) represents the daily endogenous nitrogen excretion of cattle during normal life, calculations have been made of the biological utilization of protein and the protein wastage (that is, protein in excess of the amount the animal could make use of, not because of the character of the protein but because of the animal's own inherent limitations) by our cows on different feeds. As the indicated biological values obviously decrease with the increase in digestible nitrogen, the results are of interest from the point of view of the effect of the protein wastage on metabolism rather than for any definite information that they offer regarding biological values. Physiologically one of the striking facts is that the amount of protein fed has apparently no bearing on the amount that the animal can deposit above its own maintenance needs. The deposit of protein seems to be determined more by the particular state of the individual at the time and may conceivably even be hindered or become more restricted by increase in food protein when this increase itself is the cause of stimulating a greater amount of tissue activity or metabolism, as is suggested by the high protein diet. If the nitrogen excreted in urine less the endogenous nitrogen is taken as a measure of biologically unutilized food nitrogen, it appears that from the standpoint of nitrogen utilization for deposit the wastage (economically) may assume extraordinary proportions in fattening rations. Physiologically, however, this can not be regarded as an entire loss, for the oxidation of this protein resulted in the production of heat, the significance of which will be discussed in subsequent pages. In our cow experiments the amounts of digestible protein fed varied to a marked extent, largely owing to differences in the composition of the feeds. The impossibility of supplying a balance in both protein and energy from a single feedstuff and particularly from roughages was recognized at the outset and it was, therefore, decided that in order to rule out the effects of deficiency, it would be better to supply a surplus of energy and to maintain the nitrogen intake as nearly as possible at a level that would maintain zero nitrogen balance. This, however, was not always realized.