We put our teaser out on Sept. 1 and he seems to be working pretty hard.  We actually had to vasectamize a new one this year, our old one got way too agressive last year and he had a full curl set of horns.  Actually battered a gate open after he was moved from the ewes to the ewe lambs.

I'm a firm believer in teasers, our conception rates went over 200% when we started using one, and our lambing season is much quicker.

For you non sheep people, we run a sterile ram with the ewes for 16 days before the real rams go out.  If a sheep is serviced and she doesn't conceive, the body thinks something was wrong and during the next cycle she sends down more eggs from the ovaries and thus you get more twins and triplets.  I know it sounds crazy, but it's been proven and recommended by every sheep ag institution in the country and in England.