For you who breed your own rats; how long do you use a typical female rat for breeding before feeding her off? I have had two females for about half a year now and their litters have steadily decreased; they were having litters of twelve or more, and now each one has about 8 or 9 and it honestly seems like they are not growing as fast as the earlier babies. I am feeding them the same as I have in the past, but I think that maybe they are becoming "used up" after a lot of breedings, perhaps?