Most breeding recommendations seem to refer back eventually to the Food and Agriculture Organization of the United Nations's book The Rabbit: Husbandry, health, and production.
The first follicles appear on the 13th day after birth, and the first
antrum follicles at about 65 to 70 days. Does are able to mate first
at 10 to 12 weeks, but as a rule this will not produce ovulation. The
onset of puberty varies greatly with:
- the breed: sexual precocity is
more de-veloped in small or medium breeds (four to six months) than in
large breeds (five to eight months). In Europe does are now mated at
120 to 130 days and fertility performance is good;
- body development:
precocity goes hand in hand with rapid growth. Does fed ad lib reach
puberty three weeks earlier than other does of the same strain
receiving only 75 percent of the same daily feed. The body development
of the latter is also delayed by three weeks.
Does generally reach
puberty when they have grown to 70 to 75 percent of their mature
weight. However, it is usually preferable to wait until they reach 80
percent of their mature weight before breeding them. These relative
weights should not be considered absolute thresholds for all rabbits,
but rather limits applicable to the population as a whole. Sexual
behaviour (acceptance of mating) appears long before the ability to
ovulate and bear a litter. Such behaviour should not be regarded by
the breeder as a sign of puberty, but as prepuberty play.
Emphasis is mine. I'm not sure what research this number comes from (there is a bibliography, but no indication of what statements are sourced from which references, and many of them aren't in English so I can't even guess). This book appears to have been in wide circulation for at least 30 years in different forms, because I keep finding it in references with different dates.
I did find one study that looked at age and body weight at first insemination (and the interaction of those two factors). The ages they looked at were 14.5 and 17.4 weeks, so it doesn't explain why some sources are preferring as many as 6 months. This study found:
Does fed restrictively and inseminated at
14.5 weeks of age were too immature for reproduction. In these does, body weight was low (3.2 kg), protein development was not completed,
and puberty characteristics were poor.
At 14.5 weeks of age and ad libitum feeding during rearing, over 70%
of the does did not reach optimum body weight of 4 kg. Litter size of
these does was reduced by 1.4 kit.
(ad libitum feeding means they were given as much food as they wanted, literally it means "at will")
The does inseminated at 17.4 weeks faired better than those at 14.5 weeks because they were more mature and did not have to split their energies between their own growth and the growth of their babies.
Ad libitum fed does inseminated at 14.5 week of age, gained weight in
the first gestation and first lactation period. Competition for
nutrients between body growth and production must have occurred, and
resulted in smaller litters and lower milk production than restrictive
fed does inseminated at 17.5 week of age. It was concluded that young
does should have a body weight around 4 kg at first insemination to
optimize litter size.
The long term applicability of this study is stated as
Rearing strategies only affected body weight development, feed intake
in the first parity. Long term effects over three parities were
absent and culling rate of does was not affected.
The authors suggest that the reason they did not see an effect on the second and third parities could be because they didn't have a large enough sample size, or because there is no affect on the later parities. There is a small increase in culling rate with does bred younger, but it is not enough to be statistically significant with the sample sizes that they used (it was a small enough variation that could have been due to random chance).
This research is available online as
Proceedings - 8th World Rabbit Congress –September 7-10, 2004 – Puebla, Mexico
Invited Paper. BREEDING OF YOUNG FEMALES DOES. ROMMERS J. M. web link
I have not found any research discussing waiting longer than 17.4 weeks.