A field full of dainty, bright yellow buttercups may look pretty, but those tiny flowers pose a threat to horses. When ingested, the leaves and stems of buttercups release a toxic oil called protoanemonin, which can cause excessive salivation, mouth blisters, diarrhea and mild colic. Toxicity varies with the plant's species and maturity, but to be on the safe side, consider all buttercups potentially harmful.
Buttercups tend to thrive in pastures where grass is overgrazed or patchy, because the weeds have little competition for space and nutrients. The proliferation of buttercups is further aided by their low palatability---horses tend to leave them alone because they aren't all that tasty.
But don't assume your horse won't ever eat them. If your pasture is sparse and you don't offer supplemental hay, your herd may begin to consume the yellow flowers out of hunger or even boredom.
Buttercups can be controlled chemically and/or mechanically. If a pasture has only moderate amounts of the weed, herbicides may be the easiest option. However, if buttercups are crowding out other plants in a pasture, more extensive measures, such as tilling and sowing new grass seed, may be necessary. Your local extension agent can help you devise the best buttercup eradication plan for your property.