Technical Note: Estimation of Fecal Output and Dry Matter Digestibility Using Various Chromic Oxide Marker Methods in the Horse1

Abstract Eight Quarter Horse yearlings were used in a split-plot designed experiment to estimate fecal output (FO) and dry matter digestibility (DMD) using different methods of Cr 2 O 3 administration. The two methods of Cr 2 O 3 application were 1) mixed with the concentrate during pelleting (mixed diet) and 2) top-dressed on the concentrate prior to feeding (top-dressed diet). Total FO was measured to compare with calculated values. Diurnal grab samples were also taken at 6-h intervals for 24 h to measure variation in Cr concentration over time. Dry matter intake did not differ between treatments. Calculated FO was greater for horses fed the mixed diet versus those fed the top-dressed diet. Consequently, DMD was lesser for the mixed diet versus the top-dressed diet. However, measured FO did not differ between treatments. Because calculated FO and DMD differed significantly between treatments, all subsequent data comparing calculated FO with measured FO will be reported within treatment. For horses fed the mixed diet, calculated FO was greater than the measured FO. Similar results were observed for horses fed the top-dressed diet. Concerning changes in Cr concentration over time, there was no significant diurnal variation in fecal Cr excretion within treatments. However, fecal Cr content was significantly greater in the top- dressed versus the mixed treatment at all times. It appears from these data that the use of Cr as an external marker may result in an overestimation of FO and subsequent underestimation of DMD.