I haven't added any new rats to my colony in over 5 years. I started out with two males and six females and have produced thousands of rats over the years. I currently have around a hundred adult breeders. The major problem I had was holding back the prettiest babies when I should have been holding back the ones from the largest litters because I did see a drop in production. But, it's been going back up again now that I don't hold back any breeders from litters of less then 12 no matter what they look like. In fact I usually just feed off small litters right away so that any larger litters in the same tub are better taken care of.