What age to breed a doe?

You will often see written that there is an upper age limit in breeding a doe, but there are certainly no increased risks of breeding a rat under a year old – and realistically if the rat is in peak health and fitness at any age, a litter isn’t going to be an increased risk no matter what the calendar says (and if she wasn’t in peak┬ácondition why are you breeding from her anyway?).

Your main issue with breeding older does tends to be that if the line has been bred from at the lower end of the breeding window for multiple generations, you may have accidentally selected for early onset infertility, and so find it difficult to get older does pregnant. I always select for later fertility as it makes my life easier as a breeder.