That's a dumb idea.
A tax will not reduce the amount of fart coming out of the back-end of a cow and I guarantee you the tax money will be used for something other than environmental uses. (i.e., Lotto money supposedly going toward education: The lotto money was directed to education; however the equivalent amount of other tax money was removed from education. Net Gain: ZERO.)
Instead of taxing cows, the farmers should adjust the cow's die. I read a study that shows that the "emissions" can be reduced by 50% by making some minor changes to the cow's diet.