At a guess cows that are grass fed and not given growth hormones grow much slower and thus take longer to reach 'slaughtering' weight.

Good point, thanks. The 60% number cited above does seem oddly specific, like it would come from a concrete source. Though as others in this thread have pointed out, methane seems to be less of an issue than it's sometimes made out to be.

The 60% is indeed what we studied for cows here in Switzerland. It is clearly related to the amount of time they live longer (18 instead of 11 month). You can check the source here: http://www.eaternity.org/blog/smart-chefs-research-results