Well, apparently not. It doesn't look like it makes much difference at all. Instead, we need to stop consuming so much.
ScienceDaily (Apr. 1, 2010) — If everyone became vegan and so ate only fruit and vegetables, then the reduction in greenhouse emissions for the whole of food consumption would be a mere 7%. The widespread adoption of vegetarianism would have even less impact, while organic food production actually leads to a net increase in greenhouse gas emissions. Those are the conclusions of a research paper published in the journal Progress in Industrial Ecology.
So, eat meat, get less stuff - read the rest here.