The Nutrition of the Young Ayrshire Calf

The concept of an endogenous nitrogen metabolism characterized by the urinary N excretion on a diet free of N (Folin, 1905) has been criticized by Schoenheimer & Rittenberg (1940) on the basis of their demonstration, in experiments on the fate of labelled amino groups in the animal body, of the extraordinary lability of tissue N. Burroughs, Burroughs &Mitchell (1940) andMitchell (1948), however, have criticized the conclusion of Schoenheimer that no distinction is possible between the endogenous and exogenous metabolism of protein and have suggested that the endogenous metabolism of Folin represents the summation of those irreversible reactions involving N, exemplified by the reaction creatine + creatinine.Whatever interpretation of this N excretion is made, the determination of the endogenous N metabolism remains a measure of a basal and inevitable loss of N from the animal body.Palmer, Means & Gamble (1914) showed that the creatinine N excreted daily by men and women was proportional to their basal metabolic rates; in men the ratio, creatinine N (mg.):basalCalories of heat production, was 1.0 and in women 0.79-Terroine & Sorg-Matter (1927) extended these observations and showed that the ratio of total urinary N plus faecal N excretion (in mg.) on N-free diets to Calories of basal metabolism ranged from 2.2 to 2.6.This relationship was confirmed by Smuts (1935) who also showed that in mature animals varying in size from mice to pigs the ratio of endogenous urinary N to basal metabolism was 2.0 mg./Cal.In adult man much lower values have been obtained (Bricker, Mitchell & Kinsman, 1945 ;Murlin, Edwards, Hawley & Clark, 1946), some as low as 1.4 mg./basal Cal.In the young animal, the basal metabolism/kg.body-weight is approximately two to three times that at maturity.In the past it has been usual to express endogenous N metabolism in mg./kg.body-weight.It is logical to suppose that the endogenous N metabolism of the young animal expressed in this way would be considerably higher than that of the mature animal, provided the 'Terroine-Sorg-Matter law' applies during the growth period.Few studies have been made of the effect of age on this relationship.Treichler (1939) has shown that, in the rat, age has a marked effect on the ratio of endogenous N metabolism to basal metabolism, giving a maximum ratio in early maturity and low values for the young and mature animal.There are very few estimates of the relationship available for farm animals, for in cattle the only data are those which may be computed from the results of experiments