Experimental objective was to evaluate the effect of supplementation with plant extracts (Ruminolac, Phytosynthese, France) on lactation performance of dairy cows. Early lactation Holstein cows, from 15 to 75 d in milk, were housed in a free-stall barn, milked 3 times daily, fed a basal diet formulated to 16.5% CP and 6.8 MJ NEL (DM basis) as TMR. Secondary objectives were to investigate the effect on plasma parameters and body condition score. Throughout the trial period (6 mo), the experimental group (n = 117) received 20g of Ruminolac/cow/day, given as top dressing on TMR in the feed bunk with all cows of the experimental pen having an even access to it. The Control group (n = 110) received the basal TMR. The cows were evenly distributed by parity (primiparous and multiparous cows) and days in milk between the 2 pens. Measurements under study included: daily milk yield, milk values (fat %, protein %, SCC, milk urea), dung consistency estimation, serum lipomobilization indicators (NEFA, BHB), reproduction results (days open, insemination index), body condition scoring, health. Data were analyzed using the MIXED procedure of SAS, with treatment, parity and their interaction as fixed effects. The cow was an experimental unit. For multiparous cows, average daily milk yield between d 46 and 60 was significantly higher in the Ruminolac group (46.98 kg vs. 44.43 kg, P < 0.05). In primiparous cows statistical trend toward significance was achieved between d 46 and 60 (37.09 kg vs. 35.08 kg, P < 0.1). Serum NEFA levels were generally within the physiological standards, but slightly lower (P < 0.1) in the experimental groups on d 40 and 60. Moreover a significantly (P < 0.05) smaller average body condition decrease was observed for Experimental cows than Control cows (−0.32 vs. −0.43). In conclusion, the dietary supplementation with plant extracts resulted in 5.4% improvement of milk yield in peak lactation without a negative impact on energy metabolism and body condition of the cows.