Excretion of creatinine and creatine by beef steers.

In the classical theory of protein metabolism proposed by Folin (1) two types of protein metabolism are recognized, one a constant or endogenous metabolism, and the other a variable or exogenous metabolism. According to this theory the endogenous metabolism is represented by the excretion of creatinine at a relatively constant rate which is independent of protein intake. More recently, as pointed out by Friedemann and associates (2), there has been increased emphasis on the variation rather than on the uniformity of creatinine excretion, and the independence of creatinine excretion of protein intake has been questioned. Most of the research relating to this problem has been conducted with human subjects and laboratory animals. The creatinine and creatine excretion of dairy cattle on standard dairy rations was studied by Ashworth and Brody (3) ; however, no data were presented to show the effect of protein intake on the excretion of these two urinary constituents. Carpenter (4) determined the creatinine and creatine excretion of beef steers on pasture and on maintenance rations. The protein intake of the animals was not calculated. Although similar values for creatinine excretion were obtained in these two investigations, slightly higher creatine values were reported for the dairy cows than for the steers. In a comparison of the results, Ashworth and Brody (3) suggest that creatine excretion in cattle is influenced by the level of protein intake. The present study was undertaken to determine the creatinine and creatine excretion of beef steers on rations containing different amounts of protein and different amounts of dietary urea added as a protein substitute (5).