Four forage brassicas, kale (Brassica oleracea L. cv. Kestrel), turnip (B. campestris L. cv. Appin), rape (B. napus L. cv. Titan) and swede (B. napus L. cv. Dominion) were compared with perennial ryegrass (Lolium perenne L. cv. mixture of Delish and Banquet) to examine their potential to mitigate CH4 emissions from sheep. Sixty healthy, one-year-old Romney cryptorchids (liveweight 36±1.0kg) were randomly allocated to 5 groups of 12. The sheep were initially adapted to their diets for 2wks on paddocks and 1wk in indoor pens, then fed at 1.5 times their metabolisable energy (ME) maintenance requirement during measurements. Apparent total tract digestibility (n=5) and ME (n=5) of brassicas and ryegrass, and rumen fermentation parameters (n=10) were measured, and CH4 emissions (n=9) were determined using open circuit respiration chambers. Correlations between nitrate, sulphur, sulphate, glucosinolates and S-methyl cysteine sulphoxide (SMCO) contents of forages and CH4 emissions were determined. Methane emissions/unit feed dry matter intake (g CH4/kg) from sheep were kale 19.8, rape 16.4, swedes 16.9, turnips 20.6 and ryegrass 22.0. Compared with ryegrass, rape and swedes reduced CH4 yield by 23% and 25%, respectively. Total tract digestibilities were higher for brassicas than for ryegrass: 25% higher for DM, organic matter (OM) and crude protein, and 12–16% higher for neutral detergent fibre (aNDF) and acid detergent fibre (ADF). The ME values (MJ/kg DM) were kale 12.7, rape 13.2, swedes 14.1, turnips 12.1 and ryegrass 9.4. Digestibility (P