So if a worm is in a less susceptible stage to a particular grug the first time he meets it, not only will he probably survive but he may begin building resistance to it. If he doesn't see that product again until 3 or 3 rotations later, by then his resistance may be so high as to protect him even in a stage where he'd normally be susceptible. Even if he survives again, he's only built resistance to the one product. If he meets another product next time, he won't know what hit him. Rotating doesn't just mean going from one brand to another.
Effective rotation requires rotating from one class to another.
Ivermectin is the only class effective against bots, lungworms, and threadworms-so most veterinarians give it in the fall, when cold has killed flies on the wing, and again in late winter or early spring, when fly populations are rising again. I include such a dose in the program of any horse who's showing or racing steadily, or who looks unthrifty. This application may give your horse a little extra bloom, but benzimidazole drugs were until recently classed as masking substances. Have a fecal egg count done once a year, either just before deworming or about 2 weeks afterward. Potomac fever requires semiannual boosters.