If you have just a few sheep, then you may want to run the breeding females together, and I assume this is what you are asking about. For meat lambs, you would separate them at weaning then send them off for slaughter without going back with their dams. If you wish to retain some ewe lambs for breeding within your flock, then they can go back with their mothers a couple of weeks after separation for weaning - but of course they don't have to. You would need to take them back out at tupping time unless you are breeding them as hoggs, then they could go back in once the tup is out. This depends a bit on the breed, as primitives don't need much winter hard feed, so the lambs will not be overfed when the ewes get this. With a larger breed this may not be the case and they would be better kept apart so that the ewes can be fed sufficient for their unborn lambs.
There is some thought that young unbred ewes do benefit from remaining with the breeding ewes as they learn about lambs and lambing - we have found this.
Rosemary has just posted and says much the same as this - but I've typed it so I'll send it
