Modeling digesta retention in horses fed high or low neutral detergent fiber concentration forages

•Diets with different digestibility had similar total tract mean retention time.•Compartment retention times and transit time differed between diets.•Age-dependent compartment retention was longer than age-independent retention time.•Mathematical models can provide insight into equine digesta passage kinetics. One factor influencing the extent of feed degradation is the length of time feed resides at the site of digestion. Traditionally, mean retention time (MRT) is measured for the entire equine gastrointestinal tract (GIT), but retention time in specific organs may be more relevant to understanding digestion. Multi-compartment models theoretically partition the GIT to potentially represent MRT in individual segments. The objective of this study was to apply a multi-compartment model to digesta passage in horses fed forage-based diets. We hypothesized forage neutral detergent fiber concentration would alter total tract MRT, transit time (TT), and compartment retention time. Six geldings (4 ± 1.5 y, 545.0 ± 64.5 kg, mean ± SD) were randomly assigned a diet of timothy cubes (Hi NDF, n = 3) or alfalfa cubes (Lo NDF, n = 3) fed at 1.53% BW and concentrate at 0.22% BW (DM basis). After a 15-d dietary adaptation, geldings were orally dosed with Co-EDTA and Yb-labeled NDF residue to determine liquid and particulate retention time. A total fecal collection was conducted for 120 h after marker dosing. Samples were analyzed for Co and Yb concentrations by inductively coupled plasma spectrometry. Gamma distributed age-dependent two compartment models were fit to marker excretion curves for each gelding (MATLAB, vR2014a). Diet effects were analyzed by ANOVA (SAS, v9.3). There was no effect of diet on total tract MRT (liquid 21.78 ± 0.79 h, particulate 25.80 ± 0.79 h, mean ± SEM). However, liquid TT was longer (P = 0.041) in the Hi NDF diet (7.25 ± 0.91 h) than the Lo NDF diet (3.86 ± 0.91 h), and liquid age-dependent compartment retention time was shorter (P = 0.019) in the Hi NDF diet (8.40 ± 0.84 h) compared with Lo NDF diet (11.72 ± 0.84 h). Particulate TT was longer (P = 0.029) in the Hi NDF diet (8.24 ± 0.96 h) than the Lo NDF diet (4.54 ± 0.96 h). Although there was a difference in DM digestibility (Hi NDF: 50.1 ± 2.31%; Lo NDF: 60.0 ± 2.31%; P = 0.002), covariate analysis did not show a significant relationship between MRT or compartment parameters to diet digestibility. Additional research should pursue methods to ascribe physiological meaning to mathematical