I live in Fresno, California, where we have the third largest SPCA in the nation. By largest, I don't mean most money to spend or kennels to fill, but the highest number of animals moving through. Over 108,000 a year, which breaks down to 9,000 animals a month, or 300 a day. We also have a 3% adoption rate. I'll let you do that math...
I worked there as a Euthanasia Technician for two years. All I did, all day every day, was euthanize shelter animals who’s four day adoption period had ran out. Very emotionally taxing.
Before anyone assumes the worst about why I did the job that I did, know this: The overcrowding in American shelters is real and terrible. Worse than most people in this field want people to know; it isn't exactly easy to talk about. The employees and volunteers who spend their time trying to make it better can only do so much. I did that job because I figured that the last few minutes of their lives should be kind and loving. I knew that I could do that for them.
Do not breed ANYTHING (not your rats, dogs, cats, rabbits, guinea pigs, etc) unless you are bettering the breed by using only registered, documented blood lines and breeding for health and temperament. It is the worst kind of selfish to assume that "a litter or two" won't hurt anything.