The antimethanogenic compound 3-nitrooxypropanol (3NOP) has been proven efficient to reduce methane (CH4) emissions from ruminants but it has been tested mainly on high-producing dairy cows fed high-concentrate diets. The aim of this work was to test the effect of 3NOP on CH4 emissions from medium-yielding cows fed a high-forage diet. Twenty-eight lactating dairy cows were recruited within 7-11 days after calving and distributed in 2 balanced groups in a randomized block design. The treated group received 3NOP (60 mg/kg DM basis) in a total mixed ration during 105 days and the control group received a placebo. The diet had 75% forage (52% corn silage, 17% hay, 6% straw) plus 25% concentrate and was distributed once a day after the morning milking. Individual daily kinetics of CH4 emissions were quantified using the GreenFeed system (C-Lock Inc., Rapid City, SD, USA). Intake and milk production were recorded daily, and milk composition (fat, protein, lactose, urea) twice a week. Intake was reduced with 3NOP (-1.6 kg DM on average; P = 0.01) whereas milk production was similar between groups (34.7 kg/d of energy corrected milk on average; P = 0.28). Milk composition did not vary between groups. Feed conversion efficiency in animals tended to increase with 3NOP (1.37 vs. 1.46 kg milk/kg DM intake on average; P = 0.07). Methane emissions, CH4 yield and CH4 intensity were lower in the treated group throughout the 105 days (on average -31% g/d, -25% g/kg DM intake, and -30% g/kg energy corrected milk, P < 0.0001). When monitoring daily kinetics of CH4 emissions, we observed that the major mitigation effect of the 3NOP occurred after feeding by preventing the post-prandial peak of CH4 emissions (P < 0.001). In conclusion, the CH4-reduction effect persisted for the duration of the study in early lactation dairy cows fed a 75% forage-based diet. The additive consistently prevented the CH4 production peak after the main meal of the day (P < 0.001). In systems without a total mixed ration, 3NOP should still have a significant antimethanogenic effect if provided throughout the main daily feeding period.