I don't know how true this is, but I watched a few documentaries that dealt with research into desertification and how to fight it. One of the claims is that to assist with prevention of desertification that various ruminants be cultivated and run through areas so that their dung and interaction with the ecosystem generates the kinds of processes necessary to grow the kinds of plants to stop desertification. Now, if this were absolutely true then Texas cattle ranches would be a rain forest by now, so I don't know. I used to buy into the whole "why feed 10 tonnes of vegetables to cows grow 10 pounds of meat when you can feed it directly to humans" thing. When you drill down into it, humans mostly cannot consume and subsist on vegetables alone and vegetarian diets are very harmful to humans because beyond steaming veggetables to eat them, you end up either adding sugars, vegetable fats or highly processing them into refined starches in order to be able to eat them, all things that are highly dangerous to human health. It's a complicated puzzle, there is no simple answer I think.