Desertification

Why is desertification happening? Now I'm learning about this stuff in my environmental biology class... Overgrazing and cutting down plants/trees I think are the main causes of desertification. But how would pushing Midwestern farmers to switch part of their land to grazing pastures for cows help desertification, when overgrazing is part of the cause of these problems?

I don`t know about the use of land, but I thought that when you turn soil that is good for planting into pasture, you are downgrading it. I wish you would enlighting me with that. Besides, I think that people should learn to eat more veggies and less meat (I`m no vegetarian). I am worried about the change in the use of land if not enough research is being made.