Rabbit Fertility

The ability of animals to produce living young is obviouslyof the greatest importance. Although rabbits are often thesubject of jokes about great fertility, the rabbit breeder maysometimes be faced with difficulties in getting his stock toproduce regularly. On occasions permanent sterility, fromone of a number of causes may occur, but this is generally not of such importance to the rabbit breeder, for the animalmay be eliminated without serious loss. Even more occa-sionally a strain may become infertile, when of course thereis little which can be done except to introduce new blood,or recommence with another strain. It is generally tem-porary sterility with which the breeder must contend.Fertility and good bodily condition are closely related.One of the first bodily functions to suffer from poor manage-ment, and in particular, poor feeding, is the reproductivesystem. Excessive fatness and very poor condition both leadto sterility, but it must be remembered when the reasons forsterility are being sought, that sterility itself will often leadto excessive fatness.

Apart from nutrition, probably the most important factor(excluding disease) is under or over use. In the buckperiods of rest may result in a temporary sterility, whichhowever, usually ceases after the first services. Long rests,or more frequently very much delayed first litters in doesmay result in difficulty in getting the does pregnant. Itoften occurs that a doe being shown for a year or possiblytwo, is then unsuccessfully mated. The reason is that theovaries do not function normally and the solution in bothcases is to ensure that the animals are bred at the proper times.A doe will be extremely fertile immediately after kindling.She will be of above average fertility immediately followingthe termination of pseudo-pregnancy, and she will kindlemore easily immediately after weaning. A difficult doeshould therefore be bred at one of these times.
As to the causes of other types of sterility, there are abnor-malities of the reproductive system, but these are rare. In-breeding may cause one of two types of sterility. In onetype sterility is permanent, and the majority of the animalsproduced are completely infertile. In the other type, someearly breeding is possible, but then a permanent sterilityoccurs. An in-breeding degeneracy to sterility shouldtherefore be closely watched.

A buck whose testicles have failed to descend into thescrotal sacs is known as a cryptorchicl or rig, and is invariablysterile. In some cases one testicle only descends and then the animal, a half rig, may or may not be sterile, althoughusually he is capable of producing sperm.

Temporary sterility may be produced in bucks by hightemperature. In Britain temperatures are rarely highenough for this to occur, but it is a fairly common cause oftemporary sterility in other countries with a hot climate.There are infrequent cases of sterility between two animals.That is a doe may never hold to a particular buck, but isfertile to other bucks. The buck itself is fertile to otherdoes. A type of sterility in which the embryos do not surviveuntil birth is inherited.

There is some evidence that sterility in a strain may beassociated with some forms of vices such as cannibalism ordesertion of young, and also that good maternal characteristicsusually go together. Good mothers are generally speakinghighly fertile.
It is probable that hormone treatment may result in theclearing up of some cases of sterility, but generally speakingthis approach to the problem would be expensive, and thebetter way to tackle the matter is to breed for good fertilityin the same way as for other desirable characteristics.Apart from the question of permanent or temporarysterility there is the problem of infertile matings. In somestuds, where the standard of management is high, the numberof matings per hundred litters may be as low as Ito, whilst insome studs the average may be about 300. On averagethroughout the year probably half the matings are infertile.Infertile matings are a great inconvenience to the rabbitbreeder, no matter what his ultimate aim. In the case ofthe commercial breeder there is loss financially due to decreasedoutput with the same overheads. For the fancier there maybe a missed chance of exhibiting at a particular show. Forthe laboratory there may be some disarrangement of experi-mental work.
The highest percentage of fertile matings occurs duringthe early slimmer. This is probably the result of improvednutrition, which gives an indication of the methods of main-taining good fertility. Good feeding, good management,and regular use are the most important.