The drench at weaning is the most important drench a lamb gets.

Why is this?

You have switched off the high protein milk supply.
Whatever worm burden the lamb has can check them significantly post weaning.
The worm population in the lambs comes from the ewe parasite population. So the weaning drench needs to be fully effective to ensure resistance is not multiplied through the autumn.

What about refugia in a lamb system?

Lambs on clean feed do best. Young grass, or fresh feed that was previously grazed by cattle or cut for baleage is ideal. But how do we slow the resistance in areas with low parasite challenge where a higher proportion of worms are in the guts of sheep and cattle?

Drench all lambs at weaning. Refugia can come from:

Ewes run with lambs. If you have tagged refugia/un-drenched ewes pre-lamb then these are the best to run with lambs. Otherwise light 2 tooths are good. 20- 50 is enough.
Run un-drenched ewes behind lambs to clean up 1-3 days is sufficient. This is my preference.
2nd and 3rd lamb drenches can mark and leave 5% undrenched.

What is an effective drench on your property?

Triple drench (matrix) has been the main stay for many years, and can still be >98% effective. But we cannot assume this. Do a 10 day FEC post weaning to ensure no worms are left behind. If you know from reduction tests or egg counts that triples are under pressure then use Zolvix plus or Startect, especially if you have had significant moxi leakage in ewes.
A Drench performance test on 50 lambs gives you the best insight into what drenches work or not. Get a pre-count. 3 drenches. Dose 10-15 lambs with each drench. Do 10 x post drench FEC on each group. Very useful data for the rest of the season.

All farmers should do at least one drench check a season. Pre and post drench FEC counts are required.