Most people use a rotational schedule involving 3 different classes of wormer. Spring thaw and early fall are normally Ivermectin. Around early summer, you'll want something for large Stongyles. There's plenty of good info on the various rotations if you just google it.
The best thing you can do is to have a fecal analysis of the horse done. That way you are only treating parasites that are currently a problem and not creating resistance by over using medications.