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, rang ing from 308 to 710 kg live weight after a fasting period of 16 h. Urea spa ce (US) was determined from the difference in plasma urea concentration bet ween samples taken before and 12, 18 and 24 min after mean infusion time. T he day after infusion, the bulls were slaughtered and subsequently the body composition was determined. Kg empty body water and empty body protein wer e quite accurately predicted by US. The precision of the prediction of kg e mpty body fat was less accurate. However, due to the large live weight inte rval and the small compositional variation, the fasted live weight (fLW) wa s a better parameter to estimate the weight of the body components. Predict ion 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 ext ra precision from using urea infusion was not large enough to justify the e xtra work of the infusion. The time for equilibration of the urea in the wa ter compartment was obviously longer in this study than in others, presumab ly due to the lower capillary density of the Belgian blue double-muscled an imals.