To get the highest production, you have to balance 2 things. You don't want adult rats sitting idle, because obviously that means less production. But if the females are bred constantly, they'll tend to produce smaller litters and they'll get "used up" and you may have to replace your females more often. So you want to breed your females as often as possible without it over straining their bodies. Unfortunately, there is no magic answer of exactly how often that is. Most people seem to separate pregnant females, let them have their litter and raise them to weaning size, and then either give them a little bit of rest (maybe a week or so) or just put them right back in with the male as soon as the litter is weaned. If you do not separate pregnant females, they should still raise the babies just fine (the male shouldn't hurt the babies), but the females can become pregnant again only hours after giving birth, and being pregnant while nursing a litter can really take a toll on them. Some rats handle it better than others, depending on their genetics I guess. I have experimented some with allowing a female to have 2 back-to-back litters, then giving her a rest, and for most of my females it has worked out ok. Several didn't get pregnant or reabsorbed the 2nd litter (possibly a sign there was too much strain on their bodies). None of them looked really poor after the second litter, but they didn't look quite as fat and happy as normal, either. However, with many of them, the second litters were noticably smaller than the first litters. I guess that's nature's way of giving the momma rat a sort of "rest", but it slows down production, and in some cases the litters were as small as only 3 babies, so it was barely any better than giving the mother a break. So really the bottom line is if you want to be sure you are maximizing production, you'll have to experiment to see what works best with your rats as far as how often the females can be bred without compromising the mother's health and the litter size, and also work hard on improving your lines by only holding back babies from the mommas who do best with back to back production. But if you just want a safe formula that should get you a good steady production, then I'd suggest separating pregnant females, and breeding them again as soon as the litter is weaned.