It's far better to eat locally produced food from sustainable, integrated farming systems that use manure from the animals to fertilise the soil to grow vegetables, than it is to eat stuff grown half way around the world, requires tons of artificial (read: fossil fuel based) fertiliser and which degrade the soils and forests/eco-systems that were once there. Veganism is a one dimensional philosophy. It doesn't take into account the inputs and trophic flows of a functioning food system. Life is a circular system, not linear. It is impossible to have a farming system without, either animals, or humongous amounts of fossil fuel inputs.
Exactly this.
The Haber process, necessary to produce artificial fertiliser consumes vast amounts of enery and fossil fuel (something like 5% of the whole world's) natural gas resources each year, and has a huge carbon footprint.
Vegans say that it takes a large amount of grain, etc. to raise one beef cow - this is correct if it is grain fed, but increasingly farms are grass feeding their cattle, and many are carbon neutral in this respect. Also, the same argument does not apply to sheep, which frequently graze on land which cannot be used for anything else. Animals also, as mentioned, generate dung for fertiliser.
Where vegans are correct, IMO, is in the humanitarian aspect, and the whole ethos of factory farming, and the abuses in slaughterhouses are an utter disgrace.
I also think that as a whole, we need to eat less. I am in my 60s, and certainly remember that up until the 70s at least, people generally ate much less than we do now.