Do you presume that because you have a strict and regular worming regime that your horses have no issue with internal parasites?
Resistance to chemical dewormers is a threat to your horses health. Monitor your horses Worm egg burden by simply performing regular Faecal Egg Counts. This will allow you as a horse owner to take control of your deworming programme safe in the knowledge that you are targeting each horse appropriately. This will allow you to see the effectiveness of your worming regime. You may find that you are unnecessarily worming horses i.e. horses with a low parasitic burden or using products that are resistant.
By monitoring Faecal Egg Counts you can decide when and what to dose for which in the long run will save you money and avoid running into resistance problems which unfortunately can be seen with overuse / unnecessary use of dewormers.