Estimating total-tract digestibility of nutrients and their contribution to digestible energy supplies in equine diets

Our objectives were to use a quantitative literature review to explore dietary and feed factors influencing apparent total-tract digestibility of dry matter (DMD), crude protein (CPD), neutral detergent fiber (NDFD), ether extract (EED), non-structural carbohydrates (NSCD), non-fiber carbohydrates (NFCD), and residual organic matter (rOMD) in equine diets, and to assess their contributions to digestible energy (DE) supplies. Data from 54 studies were modeled using linear mixed-effect regressions, with publication as a random effect to account for study variability. For each nutrient, five models were derived with explanatory variables including: dry matter intake (DMI; % BW/day), DM (% as-fed), dietary components (CP, organic matter, EE, NDF, acid detergent fiber, NSC, starch, and NFC as % of DM), and feed types (forage, non-forage fiber, legumes, cereal, and oil proportions). Model selection was based on concordance correlation coefficient (CCC), sigma hat error, and corrected Akaike Information Criteria, with models chosen based on their performance in simulating digestibility on example diets. Two models, one from measured and one from calculated data, for each nutrient were used to estimate dietary DE content and compared to existing DE estimation methods. Selected models explained variation well, with CCCs ranging from 0.740 to 0.969 (CPD: 0.886; NDFD: 0.775; EED: 0.813; NSCD: 0.969; rOMD: 0.740). The models were evaluated against measured DE from 17 studies. For reference, the DE estimations were also evaluated relative to those from other current DE systems. Overall, this approach offers an additional, practical tool for estimating energy supplies in equine diets.