ABSTRACT Grasses and legumes are two groups of plants physic and chemically different (amount of crude protein and fiber, mostly), very used in animal feed, whose differences can determine variation in intake, digestibility and animal behavior. We aimed to evaluate performance, feeding behavior and the serum biochemical profile of sheep. The treatments were four levels of substitution of Bermuda grass (Cynodon dactylon [L. ] Pers.) hay for alfalfa (Medicago sativa L.) hay (0, 330, 660 and 1000 g kg-1 DM). The randomized block design was used being two blocks and four treatments with 10 replicates). There were no effects on the performance of animals. On the other hand, there was a positive linear effect (P < 0.05) for intake of DM, organic matter, crude protein (CP), total carbohydrates, and non-fibrous carbohydrates. However, the use of neutral detergent fiber (NDF) showed linear decrease with the increase in alfalfa hay (P < 0.05). CP and NDF decreased linearly (P < 0.05) with the replacement of Bermuda grass hay for alfalfa hay. There were decreasing linear effects (P < 0.05) when compared to the time of ingestion and chewing of feed, and increasing linear effects regarding total resting time. Estimated passage rate, density of particles and blood glucose showed a positive linear effect (P < 0.05), while weight of gastrointestinal contents decreased according to alfalfa levels (P < 0.05). The replacement of Bermuda grass hay by alfalfa hay to promote improvement in the performance is not recommended.