are the current horses really pawing, and rolling in that area? Is it an irrigated pasture ? When was the last time it was reseeded ? Depending on the grass type, it could just be reaching the natural die off . If the grass is to thin, they will overgraze that area causing it to erode. We will harrow our pastures to break up manure clumps, and mow them to control the weeds. And even though I have a large seperate fenced dirt area with the water and hay feeder, they have some spots they wear down to dirt.