OK, I've been thinking about the best way to manage this myself and having done a bit of research on the topic.........I'm now totally confused!
Here's a typical roundworm lifecycle:
From this diagram,
and the SCOPS equivalent, let's say that it takes between four days (ideal conditions) and twelve weeks for the eggs your sheep dropped to then turn into the infective L3 larvae stage (
another source says approximately 3 weeks). Then, those L3 larvae are themselves infective for 3 months in the summer, and substantially longer in winter.
If I have three equal paddocks and rotate three weeks in each, that puts the sheep back into paddock one after nine weeks, whereupon they will be exposed to worms that are six to nine weeks old....... i.e. right at peak 'infectiveness'.
Help!! Anybody!?