My vet also recommends using a different dewormer in spring and fall and none in between. I was going just by fecals for a while, but it is true not every parasite sheds out in the stool. People at my barn think I am nuts, but I pick up the manure off the field every three days. From what I've read, it takes at least four days in optimal conditions for worm eggs to turn into larvae that horses can ingest. If my horses were only in their own field, I might consider not deworming at all, but they have occasional turnout in fields with horses whose owners do not pick up manure and also may not deworm their horses. Also, strongyle eggs can live up to 7 years in a pasture and I've only had my horses on the field for 1.5 years. So I continue to do the twice yearly dewormer.