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There have been reductions in grazing cattle and corresponding increases in mixed diets across many regions. Mixed diets consist of silage, grains, legumes and other herbaceous plants (termed total mixed ration, TMR). TMR has been associated with increased milk yields but has also been linked to increased enteric methane production. We measured milk yields and methane production from high‐yielding Holstein‐Friesian cattle after substituting 29%–36% of a TMR diet with grass. Two feeding treatments were compared with a diet of TMR: grass grazed at pasture and grass cut in the field and delivered to housed cattle (termed cut‐and‐carry). Each feeding treatment was fed to 15 cattle, and the experiment was conducted in South‐west Scotland. Using a laser methane detector, we measured a twofold and fourfold decline in enteric methane production for the cut‐and‐carry and grazing groups, respectively, when the animals consumed grass. TMR was consumed by both grass‐fed groups overnight, so daily values were adjusted to include elevated methane production during this period. This revealed that methane production for the cut‐and‐carry and grazing groups was 17% and 39% lower than for the TMR‐fed group respectively. Milk yields were maintained for all three groups, and the efficiency of milk production per unit of methane was substantially greater for the two grass‐fed groups. A shift away from exclusively feeding TMR by adding fresh grass to the diets of cattle could contribute to meeting emissions targets and could also represent an economically sustainable climate change mitigation strategy.

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