Farming industries account for roughly 20-50% of man-made emissions. The amount of grazing land is vast and If you think about it animals are another step in the food chain, meaning you lose a lot of calories and energy to grow a healthy cow. The numbers of retention of beef (3%), pork (9%), and chicken (13%). You could just as easily eat the vegetation that the land could provide instead of channeling it through an animal that will be of low production efficiency and lose about 90% percent of the nutrients.
Also as a lats note the farming industries can be very cruel, and it is very sad. The close proximity and nasty conditions some of these animals are kept in could contribute to the spread of disease in other livestock.