Did you know that 20% of a flock of sheep or goats has 80% of the parasites?
There is a vet that does fecal samples on her 2 flocks and found that to be so. She culls the ones with high loads of worms. This is one way to reduce the use of dewormers. She also talked about drug resistance and the need to rotate dewormers.
If you only deworm the sheep/goats that need deworming the ones that aren't dewormed will have "weak"worms that will then cross with the drug resistant worms there for reducing the population of resistant worms. For more info on deworming programs go to http://www.scsrpc.org/