Evaluation of urea infusion to estimate in vivo body composition of Belgian blue double-muscled bulls

Urea infusions were performed on 18 Belgian blue double muscled bulls, ranging from 308 to 710 kg live weight after a fasting period of 16 h. Urea space (US) was determined from the difference in plasma urea concentration between samples taken before and 12, 18 and 24 min after mean infusion time. The day after infusion, the bulls were slaughtered and subsequently the body composition was determined. Kg empty body water and empty body protein were quite accurately predicted by US. The precision of the prediction of kg empty body fat was less accurate. However, due to the large live weight interval and the small compositional variation, the fasted live weight (fLW) was a better parameter to estimate the weight of the body components. Prediction of relative body composition (as a percentage) instead of absolute (in kg) improved the standard error of the predictions. In estimating relative body composition, US has proven to be somewhat better than fLW, but the extra precision from using urea infusion was not large enough to justify the extra work of the infusion. The time for equilibration of the urea in the water compartment was obviously longer in this study than in others, presumably due to the lower capillary density of the Belgian blue double-muscled animals.