You're right, there is a multitude of factors that impact growth rates, including genetic line, time of year, temperature, light, feed type and amount, lodging, and any number of things that rabbits can perceive but we can't. However, growth rate is by definition a statistic, so you are getting a general idea of how you're doing, and whether you're making progress or not.
First, having all litters from a single buck isn't a bad thing - it reduces one of the variables. If you use only this buck over time, you can compare your does' productivity, across does and across time.
In my records, I don't use total litter weight but rather the weight gain of each bunny in the litter. This allows me to not only gauge the production of the sire and dam, but to look for consistency in both weight and growth rate across the bunnies. Whether you're producing meat or show bunnies, consistency is a thing worth moving toward.
This has something to say in answer to your question about whether large litters to take longer to grow out. In general, the answer is yes, at least a little, since there is more competition for milk among ten kits than among five, and there is also competition among bunnies in a growout cage even when pellets are unlimited, due to dominance behaviors. But what I see in my Satins is that bunnies from larger litters, (10-12 kits), are born slightly smaller and stay slightly lighter, until weaning at least, than kits born into a small litter (<6). However...when I look at growth rate charts for each bunny, I see that the
rate at which they grow is very similar among small and large litters, no matter the start and end weights. In fact when I swap bunnies around among nests to even out the litter sizes, the smaller ones from large litters tend to grow
faster for a while, and basically catch up. (Note that this is true of my Satins, with which I've been working for almost 15 years, so they are extremely consistent; the same cannot be said about my Champagnes which we started with a little more than a year ago.)
Litters of between 5-8 kits are pretty typical. You can probably push the average litter size up, but making rabbits that always have exactly six kits, for example, might be beyond what selection can do. There are so many factors that go into how many kits are conceived and born.
What is quite possible, though, is to standardize litter sizes to optimize growth rates yourself by moving the kits around. Usually it's a matter of breeding some/many does at the same time, and simply taking kits from an overly full box and plopping them in a box with a smaller litter. I do this when the litters are extremely divergent (one doe has 13 while one has 4), or when I am raising bunnies for meat pen competitions, where uniformity is a priority.
This is one reason I keep some older does as well as younger replacements. My Satins usually have very large litters (10-16) during their first season, then go down to more normally-sized litters of 6-8 in their second year, then 3-4 in their third year. So my practice is to breed at least one or two older does with every new doe. That way I can balance litter sizes, and also have fostering opportunities if something goes wrong with one of the mothers.
A pedigreed Californian, for instance, may come from a long history of production rabbits, bringing obvious benefits with it. And it's true that purebred meat rabbits will generally give you more bang for your buck (no pun intended!
) over random crossbreeds with who knows what in their background. But I've not really found that a pedigree in itself makes much difference. Several high-production meat rabbit types come from crossing two or more different breeds, e.g. Altex and TAMUK rabbits.
If a breeder keeps pedigrees, that
can be a sign that they pay attention to details, which is helpful. But not all breeders prioritize reproductive efficiency.
My new Champagne D'Argents, for example, are purebred and pedigreed. While they are beautiful, they have litter sizes and bunny weights all over the place. Litters have ranged from three to seven. In one litter, which was pretty typical, at one month the smallest bunny was 1 lb 12oz and the largest was 2lb 11oz. It's something I'll be working on by selecting the largest bunnies for replacement stock, and by crossing some of my Satins into the mix. So of course, for a few generations, they won't be pedigreed purebreds! But hopefully they'll come out the other side with a lot more meat rabbit to offer.
Another example: a friend of mine flew a quad of Blanc de Hotots up at great expense. Blancs are supposed to be meat rabbits, but when she tried to raise them for meat and show, she found that after their first litter, the does just couldn't pull off another one. When she contacted the original breeder, she was told that, oh yeah, the does could only have one litter a year.
Obviously not what you want in meat rabbits!
So, a good idea is to talk to the breeder from which you are thinking of buying stock. Ask them the questions you have and see what their experience with their rabbits has been. If you have the opportunity, it has been my observation that investing in good meat rabbit breeding stock pays off. If that's not possible, you can work with what you have and select for the characteristics you want. That's how they developed the good meat rabbit breeds to begin with!
