Well done, you spotted my crappy pun.
Over 75% of the gas causing the greenhouse effect is Carbon Dioxide. Methane is 13%
Animal flatulence accounts for 14% of global methane production.
QED: Less than 2% of greenhouse effect-causing Methane is produced by cows.
Yes, it's damned high and scientists are working on ways to reduce animal flatulence (no, really they are) but to come out with a stupid figure like 50% is so silly it actually detracts from the issue.