The warming effect of ruminant greenhouse gases is an issue of increasing global concern. In this experiment the abatement of ruminant gases by offering three levels of grain-based concentrate (GBC) to dairy cows of high and low rumination frequency was tested in a pasture-based, automatic milking system. Two groups of 30 cows (milk yield = 35 ± 2 kg/d), with consistently high (HR; 619.9 ± 8.9 min/d) or low (LR; 472.5 ± 10.2 min/d) rumination frequency were selected and assigned to either a low (LGBC), medium (MGBC) or high (HGBC) grain-based concentrate feeding treatment (n = 10 cows/treatment) that included a standard grain-based pellet ration of 7 kg DM/cow/d plus 0, 1.5 or 3 kg DM/cow/d of shelled corn, respectively. Cows on all treatments were also offered the same temperate pasture at 30 kg DM/cow/d that was split evenly between two fresh pasture breaks. Milk production, rumination frequency, activity, mass flux of enteric methane (QCH4) and expired carbon dioxide (QCO2), and dry matter intake (DMI), determined empirically by use of QCO2 as a biomarker, were recorded for 12 d and analysed with mixed models for a completely randomised design. No effect of rumination on QCH4 or QCO2 was detected. Offering HGBC decreased (P = 0.02) DMI and QCO2. Offering HGBC also reduced (P < 0.001) pasture DMI, and this decrease was associated with a lower (P = 0.03) DMI/activity. Both the intensity of QCH4/milk and QCO2/milk decreased exponentially with increasing milk yield, supporting a dilution effect as more feed energy was sequestered in milk. Feeding a composite GBC between 7 to 10 kg altered the activity and grazing patterns of cows, but had only a marginal change on total QCH4, probably because the effects of GBC plateaued beyond the feeding of 7 kg DM/cow/d of GBC. A broader life cycle analysis including wider ranges for GBC feeding levels at different lactation stages must be considered before providing conclusive recommendations on ruminant dietary manipulations for greenhouse gas abatement.