The most common problem with horse pasture is that there is not enough of it.
People underestimate the needs of their horse and the size of their pasture.
Then the pasture ends up overgrazed. You can survey the potentially harmful plants in the pasture, but it always seems you can't discover the plant that the horse ate. Analytical procedures for plant agents from animal fluids or excreta are limited. Anything can cause colic if over consumed. However, we have desirable forages that we plant and try to propagate.