Well they would generally be fed coarse corn meal boiled, which they referred to as mush. What they were fed was enough to sustain grown men and women through their backbreaking labors in the field.
This what by any means considered a luxury meal and they often grew their own crops and stole just to get a good meal. I believe they were treated that way as a means of showing them how minuscule they were, how they were the equivalent of pigs. A way to establish where they stood and where their master stood, a way to oppress and subjugate them.