Differences in methane emission between cattle types and 273–4903-d-old cattle were evaluated. Rumen content from 59 beef and 32 dairy cattle, collected at an abattoir, was used for in vitro incubation with a forage-based diet alone or supplemented with extruded linseed. Incubating rumen fluid from dairy cows yielded lower methane per gram of diet dry matter (DM) incubated (?5.5%), methane/total gas ratios (?8.5%), and ammonia concentrations (?38%) than that from beef cattle. Linseed decreased methane yield (?6.5%), total gas production (?3.6%), and methane/total gas ratio (?2.7%). High emitters responded slightly more to linseed than low emitters. Lower methane yield (?15%), total gas (?11%), and ammonia concentrations (?20%), and a trend for lower methane/total gas ratios (?5.2%) were found in Limousin × Brown Swiss than in Limousin × Holstein. In beef cattle, methane yield increased (up to 40 mL g^-1 DM) with age in a linear manner, whereas in overall dairy and beef animals the relationship was curvilinear (high in young and old animals) concerning methane/total gas ratio and ammonia concentration (trend in beef). In conclusion, rumen fluid from slaughtered animals may be useful to study variations in methane emission from different cattle types.