I loved the documentary Cowspiracy, and of course I 100% support the vegan message.
But here are a few question I was left with after watching Cowspiracy:
There are two main factors at work here for the environment and cows.
1. Greenhouse gas emissions
Cows in ideal conditions produce little greenhouse gas emissions. Properly cared for, free grazing, etc. When, force to fed on industrial quality corn, see King Corn documentary, and live in stressful cramped conditions, greenhouse gas emissions become a problem.
2. Land acreage and deforestation
This comes into play when mass areas of highly dense rain forest are cut down to make room for grass-fed cows. There is only so much space on this Earth and cows need a lot of land to roam. See slaughtering the amazon in the link below.
I got more insight into why wild cows would be better for the environment. If you watch the documentary, Beef State, you will learn that there is a lot of selective breeding of cattle. Wild cows tend to have less meat and can take care of their selves. Just like chickens have been selectively breed for additional meat. See the below ted video for more details.