Since you do your own fecals (as I do) may I suggest that you try adding a scoop of Uckele Poly Copper to your horses feed. It would be a fairly cheap experiment. Use it for a month and see if you have any eggs. As long as I keep the copper bumped up, I tend to see zero parasite eggs. Copper sulphate was one of the go-to wormers prior to the introduction of anthelmintics. One mid-20th century researcher believed that copper sulphate didn't necessarily kill the parasites outright, but rather strengthened the gut lining, and also just simply caused the parasites to detach and pass through. I still worm at least once a year for bots and encysted strongyles.