Evaluation of urinary creatinine excretion to estimate in vivo body composition of Belgian Blue double-muscled bulls

Urinary creatinine excretion (UCE) and chemical body composition of 18 Belgian Blue double-muscled bulls were determined. Partly due to the large fasted live weight (fLW) range ($308-710$ kg), kg water and protein in the empty body were quite accurately predicted (adjR$^2$ = 0.94 and 0.97) by UCE. The precision of the prediction of kg empty body fat was less good (adjR$^2$ = 0.77). However, the empty body composition of the bulls varied only slightly and linearly with increasing live weight so that the fLW was the best parameter to estimate empty body water and protein with an adjR$^2$ value of 0.99 for both. The empty body fat was estimated from fLW with an adjR$^2$ = 0.75. The equations predicting relative body composition from UCE had much smaller adjR$^2$ values, varying from 0.53 for % empty body water to 0.30 for % empty body protein. However, the coefficients of variation were much smaller. The prediction of the relative composition based on UCE was always somewhat better than based on fLW, but the improvement was not enough to justify the work involved in determining UCE. Conclusively, due to the small compositional changes, UCE did not significantly improve the estimation of the body composition from fLW. From the comparison of the results with other studies, it can be concluded that the double-muscled bulls have an extremely high urinary creatinine concentration, which cannot entirely be explained by their increased protein content. Évaluation de l'excrétion urinaire de créatinine pour estimer in vivo la composition corporelle de taurillons Blanc-Bleu-Belge culards. L'excrétion urinaire de créatinine (UCE) et la composition corporelle de 18 taurillons Blanc-Bleu-Belge culards, de poids au jeûne variant entre 308 à 710 kg ont été déterminées. Les quantités d'eau et de protéines (en kg) dans le poids vif vide ont été estimés assez précisément à partir de l'UCE (R$^2$ ajusté ou adjR$^2$ = 0,94 et 0,97), en raison de la grande variation de poids. L'estimation de la quantité de gras dans le poids vif vide a été moins précise (adjR$^2$ = 0,77). La composition corporelle des culards Blanc-Bleu-Belge a changé lentement mais linéairement avec le poids vif entre 300 à 700 kg. En conséquence, le meilleur paramètre pour estimer la quantité d'eau et de protéines (en kg) chez le culard est le poids vif au jeûne (valeur de adjR2 = 0,99 pour les deux). La précision pour estimer la quantité de gras (kg) à partir de ce même paramètre a été moins bonne (adjR$^2$ =