No size‐dependent net particle retention in the hindgut of horses

Sieve analyses of hindgut contents of horses as well as observations in horses where plastic markers had been applied to a caecal cannula suggested that there may be a discrimination by particle size in the passage or retention of digesta. Here, we performed a similar experiment with five caecum‐cannulated horses (562 ± 31 kg) fed a constant amount (6.81 kg dry matter/day) of grass hay. Passage markers representing the liquid (Co‐EDTA) as well as the particulate digesta phase (Yb—undefined; Cr mordanted fibre 1−2 mm; Ce‐mordanted fibre 8 mm) were given as a pulse‐dose into the cannula to measure their mean retention times (MRT). The MRTs were compared by repeated‐measurements analysis of variance. The MRT in the hindgut was 22.2 ± 2.4 h for Co, 25.0 ± 3.4 h for Yb, 26.2 ± 1.6 h for Cr and 26.3 ± 1.5 h for Ce. Whereas differences between the particle marker MRTs were not significant (p (adj. )> 0.05), significant differences were observed between the solute marker Co and each of the particle markers Cr and Ce (p (adj. )< 0.009). The results confirm the well‐known significant, albeit small, difference in MRT in horses between the fluid and the particle digesta phase, and corroborate another recent study that used a combination of whole, marked hay and individual marker analysis in different particle size fractions of the faeces, which also did not detect a selective retention of any particle size class.