The objective of this study was to evaluate the effect of replacing (0 and 100%, DM basis) soybean meal with urea, in diets with two levels of concentrate (0.75 and 1.25% of BW), on intake, nutrient digestibility, and ruminal metabolism in cattle. It was also of particular interest to investigate the variation in the daily excretion of internal (iADF) and external (Cr2O3) markers. Four Holstein steers averaging 445 kg of BW and fitted with ruminal and abomasal cannulas were used in this trial. Markers excretion data were analyzed as a completely randomized block design in a split plot. The split plot model included the effects of treatment in the whole plot, markers in the subplot and the five days of total fecal collection as the sub-subplot. A 4 x 4 Latin square design was used to evaluate nutrient digestibility. The forage portion of the diet contained elephant grass silage and sorghum silage in an 80:20 ratio. No significant interaction was detected for any of the studied variables. Markers excretion was not affected by day of fecal collection and diets. Indigestible ADF underestimated nutrient digestibility while digestibility estimated by Cr2O3 did not differ from that measured by total collection of feces. Except for NDF, intake of all remaining nutrients increased when the dietary concentrate level increased from 0.75 to 1.25% of BW. The same was observed for nutrient digestibility with the exception of ether extract. Only ruminal digestibility of NFC was changed by the concentrate level in the diet. Ruminal NH3 and pH were both affected by inclusion of urea in the diet. It was concluded that there is no need of 5 days of total fecal collection because the markers used were able to satisfactorily estimate nutrient digestibility.
digestibility; feedlot; non-protein nitrogen
