My breeding was totally out of control up until about six months ago. I would get wild surges of tons of rats and then nothing at all. I finally got organized and started pairing up a certain number of females each week on a regular schedule, it gave me a consistent supply of offspring. Then pretty much all at once my whole colony grew old and production slowed way down. I had a surge of babies at the end and held back six tubs of six females to grow up. Then once I cycled through a few older females and my young females matured I started by putting a new male in the tub with six new mature females. I have two more tubs left, two weeks of putting new males with tubs of six new females. I didn't start until my females were mature so I didn't have a lag time. I like the idea of replacing females and males on a regular basis. It may be good to have a female grow out tub and every couple weeks add a couple weaned rats. Once Then pull out the mature ones as you go and add them to the rotation. I find keeping a rat colony is amazingly complicated and you have to be really organized.