CALCULATION OF FECAL KINETICS IN HORSES FED HAY OR HAY AND CONCENTRATE

Marker methods are needed for estimating fecal output by grazing anima ls in studies of nutrition and environmental impact. In addition, esti mates of prefecal mass and turnover time are relevant to exercise perf ormance and certain digestive disorders. As a first step in developing marker methods for field use, a chromic oxide model of fecal kinetics was developed and tested in the context of a digestion balance experi ment with stall-fed horses. The model consists of removal of feces at a constant rate from a single compartment, the prefecal mass. Four hor ses were fed hay, and another four were fed hay and concentrate. Balan ce-marker experiments were conducted for 10 d, following 7 d of adapta tion. A dose of chromic oxide mixed in chopped hay and molasses was ad ministered from a nose bag at 0700 daily for 10 d. Dry matter and Cr w ere measured in feeds and feces. Fecal Cr concentration (C, mg/kg DM) varied during the day, so data from total daily collections were used for model development. These fecal Cr data (C-t) at time t (days) were fitted to a single exponential, with one rate constant (k), rising to an asymptote (C-a): C-t = C-a - C-a.e(-kt). Superior fits were obtain ed when a delay (d) was introduced between the pulse oral dose and the entry of marker into the prefecal pool: C-t = C-a - C-a.e(-k(t-d)). U sing pooled data, delays of 2.7 and 2.0 h gave best fits (highest esti mates of R-2) for pooled data from horses fed hay or hay plus concentr ate, respectively. The model generated estimates of 3.4 and 3.8 kg/d o f DM for fecal outputs (dose/C-a) of horses fed hay or hay and concent rate, respectively. The rate constants yielded turnover times (1/k) of 33 and 18 h, and prefecal masses [(dose/C-a)/k] of 4.6 and 2.9 kg of DM for hay or hay and concentrate groups, respectively. Using data fro m individual horses, mean estimates for each diet were similar to corr esponding values for the pooled data. In balance experiments, feces co llected were 3.7 and 4.4 kg/d, and Cr recoveries were 108 and 115% dos e for the hay and hay plus concentrate diets, respectively. Marker est imates (M) were correlated with total collection estimates (T) of feca l output [M = T(.890 +/-.045); r =.70, P =.041]. Adjusting for recover y improved the regression coefficient to 1.009 +/-.028 (r =.87, P =.00 2). The findings suggest that if Cr doses are more frequent than daily and if Cr inputs other than dose can be eliminated this method should give accurate and precise estimates of fecal output.