To meet their high energy requirements, trained racehorses are commonly fed high-cereal diets which have been correlated with increased health and behavior risks. Replacing part of cereal with dehydrated alfalfa could minimize these risks and provide higher protein levels for optimizing muscle protein synthesis in exercised horses. This study assessed the effect of alfalfa on muscle structure and exercise performance during an 8-week training period. After an initial maximal performance test, 10 trotters previously untrained for 4 mo were assigned in 2 homogenous groups. The control group (CON) received hay and oats (1.5 + 0.7 kg DM / 100kg BW), the alfalfa group (ALF) hay, oats and chopped alfalfa (1.5 + 0.2 + 0.8 kg DM / 100kg BW). Diets met 100% energy requirements, and 100% or 130% protein requirements for CON and ALF respectively. For 8 weeks, horses were ridden 3 times a week (1 interval-training session – IT, including 4 to 8 maximal accelerations from 5.0 m/s to Vpeak, 2 submaximal sessions at 60 to 80% MAV) and exercised 3 times in a walker (60 min at 1.7 m/s). Intensity and duration of ridden exercises were weekly and concomitantly adapted for both groups throughout the training period. During IT sessions, speed was recorded, and maximal power (Pmax) was calculated. Every third week, a 2400 m run performance test was carried out. Gluteus medius thickness (GMT) was measured at rest by ultrasonography every second week. Effects of diet, time (week of training) and their interaction were tested using MIXED procedure (SAS). Horses’ performance increased in both groups over time with no significant effect of the diet (Table 1). Training was associated with an increase in Pmax, Vpeak and Vmoy over 2400m that followed the same time course in both groups. There was no significant change in GMT over time for both groups. The present results suggest that replacing part of cereal by alfalfa in the diet of exercised racehorses could maintain performance level without altering muscle structure.