Using portable X-ray fluorescence (pXRF) to determine fecal concentrations of non-absorbable digesta kinetic and digestibility markers in sheep and cattle

Concentration of indigestible markers in feces is routinely used to estimate the digestibility or rate of passage of feed through the gut. Current procedure for measuring fecal marker concentrations is an acid digest before analysis using inductively coupled plasma optical emission spectroscopy (ICP). Portable X-ray fluorescence (pXRF) spectroscopy uses short wavelength X-rays to excite sample material and generate characteristic elemental emissions proportional to the concentration of the element present in ground fecal material. The aim of this study was to measure fecal concentrations of Co, Cr, Ti, and Yb digesta marker by both ICP and pXRF and determine any correlation between the two methods, thereby assessing the potential of using pXRF as a marker analysis method. Silica (Si) concentrations in fecal and feed samples were also measured using both methods. Cattle and sheep feces and various feed samples collect from four separate studies were analyzed for concentrations of Co (CoEDTA), Cr (Cr mordanted fibre), Ti (TiO2), Yb (Yb(III)acetate), and Si using ICP and pXRF. Regression analysis used to assess the relationship between ICP and pXRF determined fecal concentrations indicated strong linear relationships (P < 0.01) between pXRF and ICP estimates of Co (r2 = 1.00), Cr (r2 = 0.95), Ti (r2 = 0.97), Yb (r2 = 0.94), and Si (r2 = 0.88). All curves were validated using a second set of independent fecal samples. A full set of total collection fecal samples for six sheep were also analyzed by pXRF for Co and Cr (digesta kinetics), and Si (digestion) marker concentrations. These fecal Co and Cr concentrations were used to estimate digesta mean retention times (MRT) and compared to estimated digesta MRT from the same sheep using Co and Cr marker concentrations determined by ICP analysis. No differences in estimated MRT were found when using either analytical method. Silica concentrations from the same fecal samples as determined by pXRF were used to determine the rate of DMD for each animal. When compared to apparent DMD for the same animals as determined using feed DM intake and fecal DM output collected during the total collection, no differences were again found using either analytical method. The results indicate that pXRF can be used as an alternative to standard ICP procedure, obviating the need for acid digestion, reducing cost and time; and may also offer an alternative analytical method for determining livestock DM digestibility.