Evaluation of equine rectal inoculum as representative of the microbial activities within the horse hindgut using a fully automated in vitro gas production technique system
Authors: Kujawa Theresa J, van Doorn David A, Wambacq Wendy A, Hesta Myriam, Pellikaan Wilbert F
Journal: Journal of Animal Science
Summary
# Editorial Summary: Rectal Inoculum as a Hindgut Fermentation Model in Horses Researchers have increasingly adopted the in vitro gas production technique (IVGPT) to study equine hindgut fermentation, but most studies have relied on faecal samples rather than rectal digesta, raising questions about whether results accurately reflect what occurs throughout the entire hindgut. This investigation compared fermentation kinetics across four anatomical sites (caecum, left ventral colon, right dorsal colon, and rectum) in eight horses fed a high non-structural carbohydrate (NSC) diet, with half receiving coated sodium butyrate supplementation and half a placebo, using collected digesta as inoculum in automated gas production chambers. Whilst the sodium butyrate supplement produced no measurable effect on fermentation parameters or volatile fatty acid (VFA) production, significant differences emerged between anatomical sites: rectal inoculum showed substantially higher cumulative gas production and maximal degradation rates compared with caecal and colonic samples, alongside variations in total VFA, butyrate, branched-chain fatty acids, pH and ammonia concentrations. The practical implication is that rectal digesta may not reliably represent hindgut fermentation when evaluating forage-based feeds in vitro, potentially leading to overestimation of total tract digestibility and misinterpretation of substrate fermentability—a critical consideration for nutritionists and researchers designing in vitro trials to assess novel supplements or feed ingredients for horses.
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Practical Takeaways
- •If using in vitro fermentation testing for forage evaluation, sample location matters: rectal content does not accurately represent fermentation occurring throughout the entire hindgut and may give misleading digestibility results
- •Rectal content as inoculum underestimates microbial activity compared to cecal/colonic samples, so research using fecal samples requires careful interpretation when applied to clinical or nutritional decisions
- •Coated sodium butyrate supplementation at this dose did not alter hindgut fermentation patterns in an NSC challenge scenario, questioning its efficacy under these conditions
Key Findings
- •Rectal content inoculum produced significantly different gas production and VFA profiles compared to cecal and colonic digesta (P < 0.0001 for A2 phase)
- •Sodium butyrate supplementation at 0.4 g/kg BW daily showed no significant effect on fermentation kinetics or gas production parameters (P ≥ 0.073)
- •Total tract digestibility may be overestimated when using rectal content inoculum for forage-based feeds in in vitro systems
- •Inoculum source significantly affected branched chain fatty acid production, pH, and ammonia levels (P < 0.0001 to P = 0.0024)