Not only does grass absorb nutrients from the soil, it also acts as a physical barrier to prevent runoff. Take care of bare spots. Take horses off grass that has been grazed below three inches, and don't let them back on that grass until it is six inches high again. Keep horses off wet pastures as much as possible. In winter, always keep hay in the pastures to discourage horses from grazing grass that is dormant and can't rejuvenate.
Clip pastures regularly during the growing season to keep weeds under control.
Mowing also promotes more even grazing by cutting back older, less palatable grass. Drag pastures to break up and spread manure. This not only makes more of the nutrients available to grass, it also helps kill parasites. Pile manure and stall waste as far from water sources and neighboring houses as possible. Cover manure piles with a tarp during wet weather.