Ninety male Holstein bulls were used in a complete randomized design to study the effects of a blend of plant extract (PE: cynarin, gingsen and fenugreek; Biostar®, Phytosynthese, France) supplementation on performance, rumen fermentation, and metabolism of Holstein bulls fed high-concentrate diets. Three treatments: control (CTR), supplementation with 32mg/kg DM sodium monensin (MON, positive control), and supplementation with 2.8g/kg DM of PE were tested. Animals were weighed (303±3.6kg of initial BW) and randomly distributed by BW in six pens. Concentrate and straw were both offered ad libitum. Animal body weight (BW), and group concentrate and straw consumptions were recorded every 3 weeks until the first animals reached the target slaughter weight of 460±30kg. Rumenocentesis was performed to all bulls at 63 days of study at 09:00h to determine rumen pH, ammonia nitrogen, and volatile fatty acid concentrations. Blood samples from all bulls were taken at 7, 35, and 71 days of study at 09:00h to determine cortisol, glucose, insulin, and leptin concentrations. At 84 days of study, when the first bulls reached the target BW, CTR bulls had lower (P