In this study, we analyzed the effects of including açaí kernel, in the experimental diets fed to horses on the intake and digestibility of their nutrient and the resultant effects on the behavior of horses. The experiments were conducted in Federal Rural University of Amazonia. Five mares with no defined breeds, were allocated to five treatments (the replacement of Mombaça grass (Megathyrsus maximus) with crushed açaí kernel at 0, 18.75, 37.50, 56.25, and 75% levels of dry matter), in a 5 × 5 Latin square design. The animals were housed in covered stalls with concrete floors, equipped with individual troughs and drinkers, and fed 80% roughage and 20% concentrate. The inclusion of açaí kernel altered the intake and digestibility of a few nutritional components and the feeding behavior of mares. Increasing levels of the substitution of Mombaça grass with crushed açaí kernel in the experimental diets increased the nutrient intake and digestibility of neutral detergent fiber, total carbohydrates, and non-fibrous carbohydrates (P < 0.05) while reducing the time spent on feeding by the animals. Crushed açaí kernel can, therefore, be included as a substitute for Mombaça grass at up to 37.5% in equine diets.