The objective of this study was to determine the effect of feeding extruded linseed (EL) on milk yield and milk composition, including fatty acid profile, and enteric methane output on commercial settings. Thirty dairy herds averaging 55 cows and 30.1 kg of milk/d with 4.0% fat and 3.3% protein were recruited for this study. Twenty-eight herds had Holstein cows, while 2 herds had Jersey and Holstein cows, and all of them were located in the province of Quebec, Canada. A first group of 15 herds was randomly selected to be used as control (CTL). Cows from the remaining 15 herds were fed between 200 and 900 g of EL (mixture of linseed:wheat bran, 70:30; Valorex, Combourtillé, France) per cow per day (average 700 g/d) according to their lactation stage and their level of production. Diet compositions were then adjusted to cover the animals' nutrient requirements. The feeding trial was 6 mo in length, and data collected during a 2-mo interval before the experimental period were used as covariates. Actual milk yield was higher in herds fed EL as compared with CTL (28.5 vs. 27.4 L/d; P = 0.01). Milk fat content was similar between treatments (4.08 kg/hL; P = 0.20), but milk fat yield tended to be higher in herds fed EL as compared with CTL (1.15 vs. 1.12 kg/d; P = 0.09). Feeding EL decreased milk protein content (3.30 vs 3.37 kg/hL; P < 0.01), but had no effect on milk protein yield (0.94 kg/d; P = 0.17) when compared with CTL. Milk fat contents of cis-9,cis-12,cis-15 18:3 (6.72 vs 4.79 mg/g; P < 0.01), and other fatty acids of the n-3 family (18:4, 20:4, 20:5, 22:5, 22:6; P ≤ 0.05) were higher in herds fed EL as compared with CTL. Methane output, as estimated using a proprietary equation (WO 2009 156453 A1) based on milk yield and fatty acid profile, was 9.4% lower in dairy herds fed EL as compared with CTL (12.9 vs. 14.2 g/L milk; P < 0.01). In conclusion, feeding moderate amount of EL (700 g/d, providing 200 g of oil) was efficient to mitigate methane emission while maintaining animal performance and increasing the n-3 FA content in milk fat.