Comparative selective retention of particle size classes in the gastrointestinal tract of ponies and goats.
Authors: Hummel J, Scheurich F, Ortmann S, Crompton L A, Gerken M, Clauss M
Journal: Journal of animal physiology and animal nutrition
Summary
# Editorial Summary Understanding how horses process forage requires clarifying fundamental differences in their digestive mechanics compared with other herbivores, particularly regarding whether the equine gastrointestinal tract selectively retains particles of different sizes to maximise nutrient extraction. Hummel and colleagues fed ponies and goats a single dose of whole hay alongside solute markers, then tracked the passage of large (2.0–16 mm), medium (0.5–1.0 mm) and small (0.063–0.25 mm) particle fractions through faecal collections over time, calculating mean retention times (MRTs) for each. Whilst goats demonstrated the expected separation of digesta phases—with solutes moving through in 35 hours versus particles remaining 51 hours, and large particles clearing significantly faster (8.6 hours faster) than fine material—ponies showed a fundamentally different pattern: solutes and particles both transited in approximately 24 hours with negligible separation between large and small particle fractions. The practical implication is substantial: the common assumption that horses employ selective particle retention in the large intestine to optimise digestion appears unsupported, suggesting instead that equine hindgut function relies on different principles than previously theorised, and that nutritional strategies based on particle size manipulation may warrant reconsideration in light of this more efficient, homogeneous transit model.
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Practical Takeaways
- •Horses digest particle size classes relatively uniformly without significant selective retention, unlike ruminants; this has implications for feed processing decisions and understanding why whole forage is well-tolerated
- •The absence of digesta separation mechanisms in horses means feed quality and nutrient availability depend more on initial mastication and passage rate than on selective retention of fine particles in the hindgut
- •Feeding strategies for horses need not account for mechanisms that preferentially retain fine or large particles, unlike in ruminant nutrition where such separation is physiologically important
Key Findings
- •In ponies, mean retention time of solute (24 hr) did not differ significantly from mean retention time of particles (24 hr), unlike in goats where solute MRT (35 hr) was substantially shorter than particle MRT (51 hr)
- •Ponies excreted a higher fraction of large particles (5%) compared to goats (2%), but with minimal difference in MRT between large and small particles (<1 hr)
- •No evidence of relevant net digesta phase separation in the equine gastrointestinal tract, suggesting selective particle retention mechanisms in the large intestine are not important characteristics of equine digestive physiology