In Vitro Digestibility Methodology Modification to Account for Horse Foregut Digestion Using Diets with Increased Soluble Carbohydrates and Protein

Ruminant in vitro methodologies use washing with neutral detergent solution (NDS) after incubation to mimic ruminant digestion, which is physiologically different compared to that of horses. Our objectives were to determine if washing feed samples with NDS before in vitro fermentation (PRE) would suppress fiber digestion versus a post-incubation wash (POST), and to compare in vitro digestibility of forage-based feed mixtures with added soluble carbohydrates (CARB), soluble protein (PROT), or soluble carbohydrates and soluble protein (C + P) to only-forage samples (CONT). Dried, ground feed mixtures sealed in ANKOM filter bags were placed in Daisy^II incubators for 48 h in a split–split-plot batch culture design. Digestibility was determined as in vitro neutral detergent fiber digestibility (IVNDFD), in vitro acid detergent fiber digestibility (IVADFD), in vitro hemicellulose digestibility (IVHD), and in vitro true digestibility (IVTD). The PRE treatment decreased IVHD for CARB versus POST (<i>p</i> = 0.007). Pooling all mixtures, PRE decreased IVTD (<i>p</i> = 0.001), IVADFD (<i>p</i> = 0.036), and IVHD (<i>p</i> = 0.001) and tended to decrease IVNDFD (<i>p</i> = 0.072). The CARB mixture increased IVTD versus all other mixtures (<i>p</i> < 0.001). Pre-washing with NDS suppressed in vitro fermentation by removing soluble carbohydrates. Without removal of soluble carbohydrates to mimic in vivo digestion, fiber digestibility is likely overestimated.