The answer varies with the management situation for each stable, however every 3 months is a good general rule. If you have a large number of horses in a small area and they are ground fed- more often may be required. If the horses are in individual paddocks that are regularly mucked, it may be required less often. The best way to tell if your worming program is effective is to do fecal checks for parasite eggs 1-2 times yearly.
