No, seriously. Global warming needs joined up thinking, and i think that picking a "target of the month" is not the best way to approach it. Yes cars and planes are contributing, but so, in larger scales, or other things, less mechanicially, and less obviously polluters, some of them, cows included, even natural.
Cows contribute 14% of total global methane emmissions, and methane is a far more aggresive gas than carbon dioxide. in Scotland, it 46% of methane emmissions.