We evaluated the impact of partially replacing a TMR with fresh alfalfa in lactating cows on intake, digestion, production, and CH4 emissions. In a crossover-design experiment (29-d experimental period, 21-d adaptation), 12 cows (150 DIM, 545 kg BW, previous lactation: 7000 kg milk), were randomly assigned to T100: 100% TMR, or T60: TMR + fresh alfalfa. The TMR was formulated for 30 kg milk and provided ad libitum in the T100. In the T60, cows were a.m. fed TMR at 60% of the predicted intake and p.m. grazed alfalfa without a limit of quantity. All animals were sampled for rumen environment and CH4 emissions (SF6 methodology). The intake of DM and nutrients was higher in T100 than in T60 (21 vs. 18 kg DM/d; P<0.001), but no differences were observed in milk production (25 kg/d), milk fat (4.2%), or milk protein (3.4%). Rumen pH was lower (6.2 vs 6.43; P=0.01) and propionic tended to be higher (20.6 vs. 19.8 %; P=0.06) in T100 than in T60, due to the higher starch content. Butyric was lower in T100 (10.5 vs 11.1%, P=0.03), probably due to the higher sugar content of fresh herbage. The cows in T100 emitted less CH4 related to intake (25 vs. 29 g CH4/kg DM, P=0.03; 48 vs. 54 g CH4/kg NDF, P=0.04), but there were no differences in CH4 emitted per day (523 g CH4/d), per kg of milk (22 g CH4/kg milk), milk protein (628 g CH4/kg protein) and milk fat (518 g CH4/kg fat). We concluded that the partial substitution of TMR by fresh alfalfa in dairy cows led to a rumen environment similar to that of cows fed only TMR, without increasing CH4 emissions or negatively affecting milk production.