Prediction of body composition in Holstein steers using urea space.

Our objective was to evaluate urea space estimate of body composition in Holstein steers by examining prediction equations based on chemical composition and by comparison of these equations to those developed for beef steers. Urea space was determined in 38 Holstein steers ranging in full-fed live weight from 143 to 404 kg by the change in plasma urea N concentration before and 12 min after the midpoint of an intravenous infusion of 130 mg urea/kg live weight (20% solution in .9% saline). The relationship between empty body water (mean, 159.9 kg) and urea space in Holstein steers (Sy.x = 7.9 kg, R2 = .95) showed slope (.93) and intercept (20.6) similar to those of beef steers (.92 and 21.8). Predictions of empty body water, fat, and protein were slightly improved by including urea space in multiple regression equations with full-fed live weight over equations based on full-fed live weight alone. Predicted mean body protein in Holstein steers based on previous equations developed with beef steers was not different from chemically determined empty body protein. Beef steer equations underestimated mean empty body water (155.4 vs. 159.9 kg) and overestimated mean empty body fat (29.8 vs. 25.8 kg). These data suggest that urea space can be used to improve predictions of body composition on a mass basis in Holstein steers over that using of live weight measurements alone and that prediction equations may be different for different types of cattle.