Actually none of the pour ons will 'affect' the wool.....however all the above mentioned WILL affect the environment and the shearer if not used with care before shearing!
It is not right practice to part the wool to apply crovect
It should be applied in a wide sweep along the back and from hock to hock in an ark including the top of the tail......this is how all of the flystrike preventers should be used.
Spot on should be just that...put on as a spot.
Some of these preparations are synthetic pyrethroids, which if they get into water courses are fatal to all aquatic life
....and the problem is if you wash these by hand and the washing water goes down the drain, it will end up one way or another in a water course! Specialist scouring plants have their own systems for effluent.
So.....to reiterate.....its not the wool that will be affected but the water it is washed in.....SO.....follow the withdrawl times.....please