I have been breeding meat rabbits for a few years. I cross New Zealands with Californias and the result is hybrid vigor, fast growth, and lots of meat. It works well.
However, I may have stumbled upon something to increase litter sizes even more, and I am wondering if any of you have knowledge about this.
I have two bucks. A New Zealand and a California. Typically the New Zealand does are crossed with the California buck, and vice versa. However, last year I made a goof, and put a NZ doe with the NZ buck. I noticed my mistake, but only after they had mated at least once or twice. I then moved her over to the CA buck's house, and he mounted her as well. So within about 10 minutes she had bred with both bucks, at least once each, but probably twice each. (if you breed rabbits, you know how quickly and eagerly they get it on...)
The result was 12 new kits a month later. Usually the litter size is about 8 or 9. I tried it again with a different doe, and got the same result. The mothers are able to nurse and I have not lost any kits.
I call it "double breeding," for lack of a better term for it.
Is there any possible harm in doing this?
Is it possible that this is increasing the litter size, or is it just luck?
This is a practice that, while 'stuff happens', is not something I'd believe would be good for the doe, on a regular basis. Each normal-size litter takes its toll on her, and double litters will, I'm sure take double or nearly double the toll - if not during gestation, then during lactation, effectively shortening her lifespan. Another thing to consider would be that if her body is much depleted, her ability to supply her offspring with sufficient/ good quality nourishment for their proper development could be drastically depleted, especially over the course of several such litters. So, doable? Likely. Good/ responsible/ sustainable husbandry? Maybe not so much.
The only thing...more expensive than education is ignorance.~Ben Franklin
Rabbits breeding works in that the eggs drop after the mating happens, rather than a cycle of available egg(s).
If you want to know who the dad is then obviously you can only breed her to that buck, but you can breed her more than once. For example if I can it's fine to bring the doe to the buck in the morning, then in the evening, then again the next morning. Now not all does will be receptive, so you have to kind of learn each animal. But frequently it's not a problem, just keep it to a 48hr period because you will want to know when she's due.
My theory would be that either for some reason she was already going to have that many no matter what you did. Has feed changed? The amount of kits is limited by how many eggs drop, which is determined in the does body. All you can do is feed as well as you can, then it's up to her.
The next reason could be the bucks. If one of the bucks consistently throws smaller litters with does that have larger litters to the other buck... Then his sperm motility may be impaired or he isn't having a high sperm count. Motility is genetic and the sperm will be malformed and keeps them from being able to get to an egg. Or the sperm is deformed enough that even though they may get to the egg, nature sorts things out and it isn't healthy enough to survive to term. If you want to know more about that there is plenty available on bull and ram testing. Sandi Brock on youtube has a video of the vet testing her rams to see which ones were causing problems. I don't know that there is specific testing like that available for rabbits but you should easily be able to figure it out by birth rates to different bucks and by each does history.