i would reccommed a worm count every 6 -8 weeks throughout the grazing season, you only need worm when the sample comes back as over 200 eggs per gram, you are lucky having sheep as horse worms cannot survive in a ruminant and vice versa, all you need to treat for is tapeworm in march and october - or you can have a blood test done which will detect if tapeworm are present - or choose a wormer that is active against tapeworm, and treat for encysted small redworm (cyathastomes) in late nov and early feb, as these cannot be detected in a feaceal sample either, you may be interested in the equest pramox, treats all the above and in your situation a dose twice a year at the mentioned times would be quite sufficient! don't forget your sheep as well!!