A comparison of fecal output determined by total fecal collection or titanium dioxide in horses

Total fecal collection studies to determine digestibility of nutrients are costly and laborious. The use of externally dosed indigestible markers, such as titanium dioxide (TiO₂), to estimate digestibility using spot samples could be advantageous, but studies validating their use in horses are inadequate. Two experiments were conducted to determine if TiO₂ in fecal spot samples effectively estimated fecal output in horses. In Exp. 1, four mature horses were fed a forage-based diet (85:15 forage: concentrate) split into two equal meals with 1.75±0.03g TiO₂/kg DM (10g TiO₂) per day top-dressed on their daily ration. Horses were fed the marker for 10 d before a 4-d total fecal collection period. Fecal samples were collected every 4h to examine TiO₂ excretion and calculate fecal output (cFO), which was compared to actual total fecal output (aFO). In Exp. 2, 15 horses were split into three dietary treatments: low forage (LO, 45% forage), medium forage (MED, 73% forage), and high forage (HI, 95% forage) diets. Daily rations were divided into three equal meals with 2g TiO₂/kg dry matter per day thoroughly mixed in their feed. The marker was fed for at least 12 d before a 5-d total fecal collection period, with spot samples obtained similarly to Exp. 1. In Exp. 1, cFO overestimated aFO by 10% (P<0.05) across all horses and days. The aFO and cFO determined by fecal spot samples were poorly correlated (P>0.10; R² = 0.148). In Exp. 2, cFO was not different from aFO when averaged across all horses and days (P>0.10), but there were differences (P<0.05) between cFO and aFO for 10 out of 15 individual horses. In addition, compared to the MED and LO diets, mean fecal output calculated from TiO₂ underestimated aFO (P<0.05) for horses fed the HI diet. Due to a large amount of variation within horses, the use of TiO₂ in fecal spot samples to estimate fecal output needs to be better optimized before its use can be recommended as an alternative to total fecal collections.