We start prevention at 10 days to 3 weeks old. Depends on how wet or humid things are. With toltrazuril....repeated doses are not usually needed. I do once..and only booster when I see a need or fecal or after a big stressor like banding or going to a new home, since stress can cause a bloom. If you are treating an active case it's dosed then booster in10 days. Some years I only need the one dose while others I had to do 3 or 4 times before weening age. Last few years the inevitable dose has been enough. We also have kids on herbs weekly which helps.