Evaluation of urea dilution as a technique for estimating body composition of beef steers in vivo: validation of published equations and comparison with chemical composition.

Urea dilution equations for prediction of empty body water in live cattle, developed by three separate groups of investigators, were evaluated by comparing empty body water calculated by these equations with that measured chemically in 6-, 12- and 18-mo-old crossbred beef steers (n = 10, 9 and 9, respectively). Of four equations for prediction of percent empty body water, one derived from mixed-breeds of steers overestimated empty body water in the 6-mo-old steers by 7.59% (P less than .05). For the 12- and 18-mo-old steers, calculated and measured percent empty body water did not differ (P greater than .05). Of seven equations for calculation of empty body water volume, two derived from Angus steers with an without live weight in the equation, and one derived from a combination of Angus and mixed-breeds of steers overestimated empty body water (P less than .05) in the 6-mo-old steers. No differences (P greater than .05) between calculated and measured empty body water volume were observed for either the 12- or 18-mo-old steers. When calculated empty body water values were regressed against that measured directly, all regression slopes were not different from 1 (P greater than .05). Intercepts from regressions involving percent empty body water (four equations) were not different from 0. Three of the seven equations for calculation of empty body water volume, one derived from bulls and the others from Angus steers had intercept estimates not different (P greater than .05) from 0. Validity required that these regressions have slopes not different from 1 and intercepts not different from 0. Empty body water calculated from equations that combined live weight and urea space were more highly correlated with directly measured empty body water than that calculated from equations derived only from urea space. Urea space correlations with body composition of our steers also were improved when live weight was included with urea space in multiple regression models. Results of this study suggest that before using any prediction equation for calculating body composition of cattle in vivo, equations should be tested with a sub-sample of cattle from the population for which its use is intended.