Ok, you already have rats. How are you setting the breeding schedule? I run two male rats for three females at a time. I take the males out at day 19. Then the males get another three females an repeat till the first group is weaned then I give the females a week or two off. Most breeders don't give the females time off because the prime breeding year is short. I feed females off at two years an males it depends if they get fat and lazy. Young males do breed better. I've never had a male breeding after three, most are done by 30 months.

Keep the Norway male out of the pupping cage. He will back breed the female ten minutes after birth an that will drive the females crazy. Crazy moms equals bad moms.

Good luck!