If your dog is not treated it can defecate on pasture, and pass on thousands of eggs to sheep or lambs. It is the farmer’s livelihood, & so if you can prove you are dosing appropriately, he is more likely to allow you access to his land.
If you need to understand more about the problem, check out The Issue which covers: What is Sheep Measles, How do Dogs get Infected in the First Place, and How do Sheep & Goats get Infected