Hi S75,
You ask when is the best time to treat for Respiratory before the season starts. If they havn't got it, why treat ?
To help PREVENT it, make sure your not overcrowded and the loft has more than enough ventilation.
I aways thought Tylan to be the best for treating when it popped it's head up. I thought it had been taken off the market but reading some of these replies, it could still be available.