We just went to a seminar on deworming. It is now recommended to NOT do a regular schedule deforming and not to rotate dewormers.
You should have fecal counts done on each horse. The ones with higher counts would get dewormed but not the lower ones.
Regular dosing and alternating causes a resistance by the worms to dewormer. Leaving the lower count horse untreated lessens the resistance.
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Just to clarify- was the seminar recommending "don't deworm every 8 weeks, rotating the class of dewormer each time" or were they recommending "don't deworm every 8 weeks, but when you do deworm always use the same class of drug"?
I agree that giving dewormer every 8 weeks regardless of worm load is unnecessary and in some cases harmful. But my understanding of the latest recommendations was that rotating (to some degree) is recommended. Ideally the FEC would give you an idea of what worms your horse is carrying, and you'd choose a product based on that.
Hairworms, stomach worms, gastric bots, etc. will only be killed by ivermectin/moxidectin, so clearly you wouldn't want to rotate to pyrantel or fenbendazole based dewormers if that's what your horse had.
OTOH, strongyles, roundworms, and pinworms will be killed off with any class of wormer (assuming they're not resistant), so there shouldn't be any harm in picking a non-ivermectin dewormer, and if I understand correctly, dosing them with a different class of drug helps to prevent resistance.
Another thing that tends to get overlooked is that there are very few products (ivermectin/moxidectin with praziquantel: Equimax, Zimectrin Gold, Quest Plus) that kill tapeworms, and tapeworms very rarely show up in FECs. You should use a product that kills tapeworms at least once a year.