Rabbit Growth Rate

From the time the egg is fertilized until the resulting rabbit becomes really mature and even later it increases in weight and changes in conformation and the proportion of the various parts of its body. The increase in weight is growth. The change in conformation is development. Both are important to the rabbit breeder.

During the first half of pregnancy there is relatively little growth of the embryo, but during the second half the unborn rabbit grows very rapidly. It is for this reason that the nutrition of the doe during the second half of pregnancy is so important. Although the unborn rabbit has priority over the needs of the doe in the way of nutrition, a doe which receives insufficient food will produce progeny much underweight. There is a number of factors which influence birth weight that is growth before birth, but the most influential are controlled by the doe. Were this not so, across between very large bucks to a small doe would produce progeny of such size as to cause difficulty at birth, and this only very rarely occurs.

After birth the typical growth rate of rabbits, if plotted on a graph of weight against age, shows a drawn-out S shaped curve. The live weight gain gradually increases to a peak and then levels off. Thus it is possible to calculate the age at which the most economic gain in terms of feeding stuff is obtained.

A number of different factors affects growth rate. There is a basic pattern of growth determined by the inheritance of the animal. The larger breeds obviously grow faster than do the smaller breeds. But superimposed upon the basic inherited growth rate are limitations due to the environment. The most important of these is the nutrition of the animal. Unless the food supply is sufficient, then the rabbit cannot grow at its maximum rate. This can be very clearly seen in different sized litters. A doe suckling one young rabbit will not yield as much milk as she would if she were suckling several. But nevertheless the single rabbit will have much more than if it were in a litter of say five. After weaning the food supply also has a considerable influence on the rate of growth. It is not only the amount of food but also its composition which is important. A deficiency of one constituent will affect growth rate, sometimes very severely.

Rabbits may be permanently stunted if their nutrition is poor. The earlier this occurs, and the longer the duration of poor feeding, the more severe is the stunting. Temporary stunting may also occur, but although the final growth weight may be achieved, the conformation of the animal so stunted may be changed.

Other factors affecting growth rate are disease, temperature, the season of year, and hutch size. This latter point is often overlooked. Overcrowding will almost invariably lead to a lowered growth rate.

The different parts of the body, and indeed the different tissues of which it is composed, change in proportion as the animal develops. For example, at birth, the head of the rabbit, compared with the rest of the body, is relatively very large, weighing as much as one-fifth of the entire weight. When the animal is fully adult, the head will weigh as little as one-twelfth of the entire weight.

These changes in body proportions arc brought about by the different parts growing at different rates. Some parts mature early and others late. In the same way the different organs of the body develop at different times. The brain, lungs, heart and digestive system develop at an early stage, whilst the reproductive systems and the mammary glands, which are not required until a much later stage, develop later. As well as the different regions and organs of the body, the different tissues develop at varying rates of growth. The order of growth is brain tissue, bone tissue, muscle tissue and then fat. Thus if an animal is underdeveloped, its muscles and fat deposits will be much slighter than those in an animal well developed, although there will be less difference between the bones of the two, and even less between the brains.

An excellent example of the above laws can be seen in a rabbit of three or four months of age reared badly. Its head will be large in proportion to the rest of its body, as will also be the intestines. The amount of bone will be in greater pro-portion to the muscle tissue than it would be if the animal had been well reared, and there will be very little fat. The development of the sexual organs will be retarded.

To produce young stock with a high dressing out percentage, that is the proportion of carcase to live weight, methods to increase the early development of bone, muscle and fat must be adopted, and must be brought into operation at an early age. Conversely, it is possible to increase the fineness of bone by slightly retarding the growth of the animal only when it is relatively young.

As the rabbit grows and develops, so the dressing out percentage increases. At birth this percentage may be between3o and 35%. At weaning it will have increased to 40 to 45%, whilst as an adult the animal may dress out at 5o to 55%.These percentages are about average for ordinary stock, but may be increased or decreased by the rapidity of growth. Thus young rabbits at weaning, which have been fed on a very high level of nutrition, may dress out at over 55%. Knowledge of these facts is most important to the breeder who wishes to obtain the heaviest carcase at the cheapest cost. By rearing his stock slowly, the best advantage, purely its terms of feed, is lost.

The appearance of yellow fat in the rabbit is controlled by a hereditary mechanism. In those rabbits which carry the factor for yellow fat, the yellow pigment, derived from the pigment in green foods, is not eliminated. Thus if the animal becomes fat and is then reduced in condition, the yellow pigment is concentrated. The depth of yellow pigmentation increases as the rabbit grows older.

Although the quality, i.e. hardness of fat, depends to some extent on the foods given to the rabbit, fat deposited its a slowly growing animal will usually be of a softer quality, and therefore less desirable, than fat laid down during periods of rapid growth.