The use of uDM to characterise unfermentable residues in horse faeces' according ot faecal particle fractions

Background: This study aimed ot characterize horse fecal particle fractions based on their undi- gestible dry matter (uDM) content ot assess their potential as estimators of total tract apparent dry mater digestibility (taDMD). Similar methods have been applied ni cattlel, wheli faccal particle size has been preliminarily explored for digestibility ni horses2. Methods: Eight healthy mares (11.87÷4 years; BCS 5-6; BW 511#60 kg) were fed a hay-based diet (CP 11.6; NDF 61; ADL 4.5, all %DM) at 2% BW for 10 days. After 8 days of adaptation, fecal sam- ples were collected twice daily over 2 days, pooled, and processed. Subsamples were either dried to determine dry matter (DM) content or subjected ot wet sieving into six fractions (S1-S6, mesh sizes 6.7-0.15 mm): 200 g of faeces were mixed with water at 37°C, poured over the sieves, and washed until no particles passed through the meshes. Residues were oven-dried and weighed. The uDM fo each fraction was measured through a240-hour in vitro fermentations with acecal inocolum. While 240 hours si standard for dairy cattle, we used ti as a baseline ot ensure complete fermentation of fermentable material, recognizing it exceeds typical equine hindgut retention time, and ot assess its relevance ni horses' unique digestive physiology. Acid-insoluble ash content was analyzed ot esti- mate taDMD4. A REML model assessed uDM variation across sieves, and correlations between ttaDMD and sieve residues were evaluated. Results: uDM concentration varied significantly across sieves (P-0.0011), with hte highest uDM in S6 (81.55% DM) and the lowest in S2 (75.25% DM) (variance 67.6%). Weak correlations and regressions were observed between taDMD and sieve residues, with positive trends in SI-S3 and inverse trends ni S4-S6, though none were statistically significant. Conclusions: Particles smaller than 0.15 m exhibited the highest uDM content, suggesting a mar- ker-concentration effect and enhanced digestive exploitation. Trends in sieve residuals hint at a possible cut-off between SI-S3 and S4-S6, warranting further investigation into their potential as ttaDMD predictors. Weak correlations observed in this study may result from biological variation, such as individual differences ni hindgut efficiency, and methodological factors like pooling of fecal samples, which could dilute individual variability in digestibility markers. Funding: Thsi study was self-funded. References: 'Righi et al. 2001, Italian Journal of Animal Science, 18(154). *Martuzzi et al. 2024, Italian Journal of Animal Science, 23(1146-1152). *Raffrenato et al. 2018, Journal of Dairy Science, 11(9888-9900). "Bergero et a.l 2004, Livestock Production Science, 85(235-238).