Breeding Mice and Rats as Feeders
Mice can be kept in litter box or mixing tubs in racks, or any container of appropriate size. When buying mice to start your colony purchase young babies at least 4 weeks old. Some pet stores will sell babies barely weaned and these take much longer to establish, often dying. It is also best to make sure the mice are no older than 8 months old, by this age they are pretty much at the end of their breeding cycle and won't produce much. The ideal age to get is 5-8 weeks of age.
Try to get females from one location and males from another on the same day, to increase your change of genetic diversity. It is best to sex the mice yourself, as many pet storeowners have no clue how to sex the animals they sell. Male mice can bring their testicles up into the body so lack of these distinctive male organs does not guarantee a female. The distance between the anus and urethra opening is the best judge. This distance is much greater in the male than the female. In addition, female will have noticeable nipples.
Male mice have a potent musky scent to their urine, much like male cats. Vanilla added to their water supply will cut down the strength of this smell. About one drop of artificial or real vanilla to a quart (liter) of water is sufficient. Keep in mind that vanilla in the water can clog automatic watering systems used with racks.
Once you have your male and females put together, it is best to leave the colony alone. Mice mothers, when stressed, new mothers, or over crowded can eat their young and anybody else's young. Introducing a new female into a colony with soon to be or current mothers will result in the newcomer being killed. It is also best not to make any changes to the cage or to disturb the mice a lot when pregnant or nursing new babies. Unless you had previously taken the time to familiarize the mice with your scent (i.e. handling every day for short periods of time, feeding treats to get them used to you and no longer afraid) it is best to only disturb the mice when culling off young for feeding and cleaning cages.
If cleaning cages and your mice have babies, it is best to remove all the adults first, then the babies. You could even keep the babies separate from the mothers during cleaning then put the babies into the clean cage. Include some original bedding (from the actual nest is best) into the clean cage. This makes the move less stressful as the clean cage will smell familiar. After cleaning, put babies in first, then adults then leave them alone.
Food is very important and should be high protein and high fat for nursing moms and growing babies. Mom needs a lot of energy and food to nurse her young. If her milk is not good or thin, then her babies will suffer and die. Gestating mothers also need to feed their current brood plus the babies within her.
The best food should contain about 21% protein and 8% fat or close to those figures. A specially formulated breeding mouse diet is best (i.e. lab chow). A mixture of wild birdseed with all grain no dye dogwood is also a good mix for food. Just make sure the dog food has about 20% protein and 8% fat and no red dyes or preservatives. Offering cooked brown/whole rice, whole wheat cooked pasta, stale whole wheat bread (must be stale as fresh whole wheat or brown bread contains an enzyme that is harmful to male mice) are great treats. Thought it is easy to just use one kind of food and that is it, sometimes adding a little variety makes a difference.
Having a huge rack system breeding hundreds of mice would make it difficult and non cost effective to have exercise wheels, tunnels and other stimulation in every cage. However, an exercise wheel can do a lot to keep mice busy, relief stress and keep t hem from getting too fat, especially males. Toilet paper and paper towel tubes are great easy to get and disposable (and free) items to toss into a mouse's cage. They chew these, create tunnels, hide and even sleep in them. Other small tubes at least 1" in diameter are great. Christmas wrapping paper tubes could be cut to size and used as well. If you have small collection of mice, you could buy lengths of PCV tubes, elbows, T-junctions and other pieces instead.
The worse beddings to use are cedar and pine. Kiln-dried pine that carries no noticeable pine smell is ok. Shredded aspen is much better. Other good bedding to use is Care Fresh and other recycled paper products without any scent added in. These all have different levels of odor control. Keep in mind with mice it will never be odor free due to the male's musky scent.
Male mice are sexually mature between ages of 4-5 weeks old. Females
can be bred at this age too but it is better to wait until they are 6
weeks old at least. 8 weeks would be better. Mice go into heat every 3-4
days, take 19-21 days to gestate and can get pregnant again in the first
24 hours after giving birth. Though having back-to-back pregnancies is
hard on the female, with proper food they can keep this up for quite a
few litters. Eventually they will start having smaller litters and more
stillborns. At this point, you can feed her off and replace her. It is
best to let a couple of her daughters to grow up and mature, remaining
in the colony to replace the older females.
Mice give birth anywhere between 5-14 babies with 8-10 being average. The best way to ensure high yields of babies is to use daughters from mothers who consistently have large litters. There are special breeds of mice out there that have larger litters but the difficulty in obtaining them makes it nearly impractical for the average breeder to get. You also need special permits to get these mice as well. Starting with healthy young mice is the best way to go about ensuring large litters. Cull any females that consistently eat their young or have small litters.
Rats can be kept very similar to mice except much larger size containers. Unlike male mice, male rats do not have a strong scent to their urine. They do however have a mild oily scent but this is not as noticeable as male mouse urine. Rats take a little longer to be settled into a colony and start producing. Females should be 8 weeks old at least before being bred. Males are happy to do it as soon as they are able to.
Rats are less inclined to eat their young as mice are but they are more protective of their young, especially if they have not been socialized to humans. Care must be taken when trying to move baby rats with the parents still in the cage as rats can give quite a nasty bite. It is always best to remove the adult mice before trying to get at the young. With very nasty rats, it may be better to cull them off to prevent this aggression being passed on to the young. On the other hand, just use heavy gloves.
Rats are a lot less skittish than mice and very curious. Where mice would run and hide when their container is open, rats are more inclined to come out and check things out. In small groups of breeder rats, it certainly would not hurt to acclimate the rats to your presence. Offer treats like cheerios (works great with mice as well) and other non-sweetened grain based cereals.
Rats are omnivores like us. They eat a variety of foods, both meat, grains, greens and other foods. Like mice, they need a high protein and high fat diet to manage the demands of breeding. A specially designed lab chow for breeding rats with 21% protein and 8% fat or around there is best. Dog food containing similar ratios of protein and fat are fine, providing they have no dyes or preservatives.
A good mix of food is best, offering dog food and lab blocks as well as fresh food. Mixed greens, green peppers, cooked whole-wheat paste and brown rice, mealworms, silkworms and super worms as well as cooked chicken or turkey is great rat treats, as well as the grain based non-sweetened cereals mentioned above. Another great food to give rats is cooked liver, beef or chicken.
Like mice, rats need something to entertain them. Rats are not as inclined to use exercise wheels but love to climb and tunnel like mice. Large PCV tubes (4" in diameter) or paper rolls are great toys. An old drier vent pipe (the collapsible ones), ferret tunnels and similar toys are great. Being intelligent, some form of toy should be offered as this keeps the rats from being bored and depressed. Of course, practicalities come in when doing large breeding operations. Even a simple ball would be fine to give the rats something to move around the cage.
Bedding for rats is the same for mice. Cedars and non-kiln dried pine should be avoided due to the toxic effects of the aromatic oils.
Rats can be kept in colonies of one male to up to six or females. The larger the colony the larger space the rats will need. Too many rats in a too small cage will lead to stress, no or small litters, disease and fighting. If space is a problem, it is better to use a rack system with tubs with smaller colonies of one male to 2-3 females.
Unlike mice, rats are less inclined to eat their young unless over crowded or stressed. Once the colony is formed, it can take up to 2 months before the first litter is born. This is because rats take longer to settle down into a colony and get down to breeding. Once they start breeding, rats are great producers. Rats have 8-20 babies per litter with 14 being average. As with mice it is best to keep the high producing females and cull the small producing females. This ensures your colonies produce as much offspring as possible. Keep female offspring from large litter producing females to replace them once the mothers get too old to breed.
Female rats enter heat once every 5 days and gestation is 20-24 days. Like mice, female rats can get pregnant again within 24 hours of giving birth. The male if left with the females will most definitely take full advantage of this 24-hour period. Baby rats wean at about 3-4 weeks and it is always best to let them stay with the mother for the full 4 weeks to ensure they are eating and drinking on their own.
Male rats can be moved from one to another group of females without causing as much stress as mice would experience. However, it is best not to introduce a male to a colony of females if one or more have babies in the cage with them, even if they are newly weaned. The females may still view the male as an intruder even when their babies are no longer nursing. The best reason to introduce a new male to a colony of females is for genetic diversity.