A review on the use of indigestible dietary markers to determine total tract apparent digestibility of nutrients in horses

Abstract

This review was written to summarise knowledge available on the use of markers to determine total tract apparent digestibility in horses, and to quantify differences in estimates obtained between marker techniques and total collection of faeces. Differences were quantified with a unitless standardised effect size (Hedges's g) and effect sizes within marker, diet (all-forage, forage and concentrate) and nutrients were combined with random effects models to account for unexplained heterogeneity among experiments. Digestibility of all-forage diets estimated by total faecal collection was not different to measurements obtained with acid detergent insoluble ash, 2N HCl acid insoluble ash, or the n-alkanes C27, C29 and C31. With diets containing forage and concentrate, acid detergent insoluble ash, chromic oxide, indigestible acid detergent fibre, indigestible cellulose or n-alkanes presented similar nutrient digestibility coefficients, and 2N HCl acid insoluble ash higher dry matter digestibility, compared to total faecal collection. Acid detergent lignin resulted in lower apparent digestibility coefficients with both types of diets. However, combined effect sizes for acid detergent insoluble ash and n-alkanes were based on 2–3 experiments conducted in few (1–2) studies, and should be interpreted accordingly. It is concluded that acid insoluble ash currently presents the most reliable marker that, with certain precautions, could be applied to determine apparent total tract apparent digestibility in horses.