Erosion happens when the horses’ grass consumption rate is greater than the grass’ growth rate. "It’s called getting ahead of the pasture," says Glen Aiken, MS, PhD, a research animal scientist at the USDA Agricultural Research Service’s Forage Animal Production Research Unit in Lexington, Ky. Think of him as the grass guy.

When desirable grass species cannot keep up with demand, the plant population in the pasture changes and undesirable (i.e., weed) species grow up. Many weeds produce stems that grow more upright than grasses. This can contribute to erosion, as weeds leave bare patches of ground around their stems and have less of a root network. The more horses graze, the more grass root death you will have. Less root density means less stable soil. This becomes important if the patch of ground is on a slope, and, explains Aiken, not much of a slope is required. "If we didn’t have grasslands, we wouldn’t have topsoil. It would all be in the rivers," he says.

Therefore, one element to preventing erosion is to maintain a good stand of grass.

Fixing pre-existing erosion will probably entail a visit by a county extension agent or a pasture management consultant. Options would be to reduce the number of horses, lime and fertilize, or, in cases of major deterioration, to start over. A pasture with a high encroachment of weeds might need to have the weeds killed, the ground cultivated, and grass replanted. Aiken explains that a frequent mistake is to put horses back onto replanted pasture too soon. This is one of the considerations when you decide which grass to plant. The grass needs to have time to get firmly established or you waste your effort