Comparison of ytterbium chloride and chromium sesquioxide as fecal indicators.

To assess the potential of ytterbium chloride (Yb) as an indicator of fecal output, we dosed 13 mature beef cows with Yb and chromic oxide (Cr2O3) either once (0800) or twice (0800 and 1600 hr) daily. Fecal output and digestible dry matter were estimated from fecal marker concentration determined in grab samples collected at 0800, 1600 or 0800 and 1600 hr, and compared with values obtained by total fecal collection. With single daily dosings, fecal outputs estimated from Yb at all collection times did not differ from values obtained by total collection, whereas Cr2O3 underestimated (P greater than .05) fecal production at 0800 hr and in the composited (0800 and 1600 hr) collections. With twice-daily dosing, fecal outputs estimated from Yb differed to a greater extent from the total collection values for the 1600 hr collection; however, the Cr2O3 values more closely approximated those estimated by total fecal collection at 0800 hours. Relative marker concentration in the feces collected from grab samples at 4.hr intervals indicated that twice-daily dosing appeared to reduce diurnal variation. Diurnal fecal excretion patterns of Yb and Cr2O3 differed (P greater than .05); however, the variation observed for both markers within dosing schedule appeared to be as great for Yb as it was for Cr2O3. Although fecal output was accurately estimated from Yb with one simultaneous collection and dosing period, the response appeared to be due to time of collection rather than to an actual reduction in diurnal variation.