When it's time to use a chemical wormer, make sure you have the right one for the worms you need to treat. Always calculate the weight of your horse with a weigh tape and give him the recommended amount in a single dose.
Under-dosing aids chemical resistance!
Keep your horse in long enough to clear the wormer from his system, check the droppings and dispose of on the midden.
Be careful with your empty worming syringes - don't let your dogs chew them. Ivermectin is poisonous to dogs.
Make a note of when your horse is wormed and which wormer you used.
If you suspect chemical resistance test again a few days after worming - otherwise, leave it for 6-8 weeks. You will soon be able to predict which horses are likely to need worming by keeping track of your test results.
If you're getting consistantly clear results, you might consider worming once or twice a year for encysted Strongyles and Tapeworm.