Simple SummaryProtein digestibility is important because protein consists of amino acids that are essential to dogs. As various feed ingredients, including plant or animal ingredients, are used in dog diets, protein digestibility values are variable among diets due to different diet formulations and nutrient compositions in the ingredients. In addition, protein digestibility may also be affected by feed processing procedures. However, animal experiments using dogs are expensive and time-consuming, and it has become a great concern for animal welfare and ethical issues. In this study, the nutrient digestibility of 18 commercial dog diets was measured using in vitro assays and prediction equations were developed for estimating in vitro nutrient disappearance of dog diets. The nutrient digestibility of dog diets was negatively correlated with fiber contents. Equations for estimating ileal protein digestibility of dog diets were developed: digestibility of crude protein (%) = 0.33 × crude protein (% as-is) − 0.49 × neutral detergent fiber (% as-is) + 81.25. Overall, in vitro ileal nutrient disappearance in a commercial dog diet can be estimated using the equations with protein and fiber.AbstractThe aims of this study were to determine in vitro ileal disappearance (IVID) of dry matter (DM) and crude protein (CP) in commercial dog diets and to develop equations for predicting the IVID of DM and CP in dog diets based on chemical composition. Eighteen commercial dog diets were analyzed for IVID of DM and CP using a two-step in vitro procedure for dogs. The diet samples in flasks with digestive enzymes were incubated for 2 h and 4 h to simulate digestion in the stomach and the small intestine, respectively. The contents of CP, ether extract, neutral detergent fiber (NDF), and ash in the diets ranged from 14.4 to 42.5%, 3.5 to 23.5%, 6.4 to 34.6%, and 4.9 to 10.0%, respectively, on an as-is basis. The NDF contents were negatively correlated with the IVID of DM and CP (r = −0.73 and r = −0.62, respectively;p< 0.05). The most suitable prediction equations for the IVID of DM and CP in the dog diets were: IVID of DM (%) = 81.33 + 0.46 × CP − 0.77 × NDF, R2= 0.78; IVID of CP (%) = 81.25 + 0.33 × CP − 0.49 × NDF, R2= 0.64, where all nutrients were in % on an as-is basis. In conclusion, dry matter and protein utilization of dog diets based on in vitro digestibility assays can be estimated fairly well using protein and fiber concentrations as independent variables.