Faecal particle size distribution in relation to forage type and digestibility in horses: preliminary results

The study aimed to explore the connection between horses’ total tract apparent dry matter digestibility (ttaDMD) of various roughages and faecal particle size (FPS), to develop a tool to rank roughage digestibility. Six horses, varying in size and breed, were fed six distinct roughages (four cuts of fescue, perennial ryegrass/clover mix, and straw). Each feeding period lasted 16 days, with a 14-day adaptation phase. Faecal samples underwent wet sieving analysis. Roughage chemical composition was evaluated using NIRs. Faecal and roughage acid insoluble ash content were used to estimate ttaDMD and FPS was determined by wet sieving using a set of 6 sieves of different mesh sizes (S1 to S6; 6.50, 4.50, 2.36, 1.18, 0.6 and 0.15 mm respectively). The effects of roughage ttaDMD on FPS across sieves were analysed using GLM. Linear regressions and correlations were performed between faecal residues on each sieve and mesh sizes of the sieves to create a mathematical tool to rank ttaDMD based on FPS distribution. Increased ttaDMD was associated with larger FPS, with perennial ryegrass/clover mix (ttaDMD 74.44%) and straw (ttaDMD 44.57%) resulting in smaller and larger faecal particles respectively. Correlations were observed between ttaDMD, S1 and S2 (r = 0.639 and 0.668; p ≤ 0.001). The angular coefficient of the linear function between faecal residues on each sieve support the idea to estimate roughages digestibility from FPS (R2 = 0.4153). Consequently, determining faecal residues across sieves of varying mesh sizes could serve as an indirect method for ranking roughages digestibility.