Claire Shand answers:
The more horses grazing together, the higher the risk of parasite infection. The best way to control worm burdens within your yard is for all the horses to be managed simultaneously with an annual plan, rather than using an ad hoc approach, and it should be based on testing so that treatment can be targeted to slow the development of resistance in parasites.
This is especially important with larger yards to preserve the efficacy of the chemicals and to protect the grazing.