[Marches Farmer]

Vet administered 4 weeks before tups in (thought grass quality probably rubbish after two very wet winters and a dry summer).  No control group so couldn't be deemed scientific but so far:
Final stage of lambing very quick; more lambs than usual with one leg all or partly back (no other dystocia); lambs 2 to 4 days early; small but very vigorous; excellent mothering up; colostrum quality exceptional, all ewes in milk at birth (sometimes have a first-timer that takes a few hours to come in); lambs have very sticky mustard coloured faeces for 4 to 6 days.
Surmise therefore that: lambs don't have time to line up correctly; shorter gestation = smaller lambs; shorter final stage = lambs and ewes less tired therefore better mothering up and vigorous lambs; excellent colostrum = sticky faeces (must lay in more wipes if I do this again).
Another thing which may be entirely coincidental is that twins are evenly matched in size - generally have a couple of uneven pairs.  Only 8 left to lamb, no losses so far. 

[shep53]

Would maybe of been more information if you'd blood tested a sample few before giving bolus and then again before lambing

[Marches Farmer]

Had them blood tested a few years ago - some marginal on levels of some minerals but no clear pattern and no correlation with age or sex.  Quite stressful for the sheep and expensive for the farmer!