I think all brassicas (cabbage family crops) cause a bit of methane generation, but it's usually the "country cousins" of the clan (cabbage, kale, collards) that are the worst offenders while their more "sophisticated" relatives (Brussels sprouts, broccoli, cauliflower) are a bit more civilized. Maybe it has to do with how long they're cooked. The more primitve varieties have to be cooked quite a bit longer to be equally tender. If I were to cook collards for as short a time as I cook broccoli, it'd be like trying to eat a shredded plastic milk jug, only greener.