What to do if a horse eats fertilizer?

Laugh. It'll be fine

Additional Info: It is not recommended that you remove your horses when you fertilize your pasture with granular fertilizers. But it is strongly recommended that you check it often for clumping and spilling. You don't want to leave any amounts large enough for a horse to eat. If a horse eats enough it is toxic.

Also, if you use the liquid fertiziler you must remove your horses until you get a good strong rain that washes the application off the plants.

If your talking about chemical fertilizer, I recommend that you contact your Vet.