Rabbit Growth Rates

From the time the egg is fertilised until the resulting rabbitbecomes really mature, and even later it increases in weightand changes in conformation and the proportion of the variousparts of its body. The increase in weight is growth. Thechange in conformation is development. Both are importantto the rabbit breeder.

During the first half of pregnancy there is relatively little growth of the embryo, but during the second half the unbornrabbit grows very rapidly. It is for this reason that thenutrition of the doe during the second half of pregnancy is soimportant. Although the unborn rabbit has priority over theneeds of the doe in the way of nutrition, a doe which receivesinsufficient food will produce progeny much underweight.There is a number of factors which influence birth weightthat is growth before birth, but the mostinfluential are controlled by the doe. Were this not so, across between a very large buck to a small doe would produceprogeny of such size as to cause difficulty at birth, and thisonly very rarely occurs.

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

A number of different factors affects growth rate. There isa basic pattern of growth determined by the inheritance of theanimal. The larger breeds obviously grow faster than do thesmaller breeds. But superimposed upon the basic inheritedgrowth rate are limitations due to the environment. Themost important of these is the nutrition of the animal. Unlessthe food supply is sufficient, then the rabbit cannot grow at itsmaximum rate. This can be very clearly seen in differentsized litters. A doe suckling one young rabbit will not yieldas much milk as she would if she were suckling several, butnevertheless the single rabbit will have much more than if itwere in a litter of say five. After weaning the food supplyalso has a considerable influence on the rate of growth. It isnot only the amount of food but also its composition which isimportant. A deficiency of one constituent will affect growthrate, sometimes very severely.
Rabbits may be permanently stunted if their nutrition ispoor. The earlier this occurs, and the longer the durationof poor feeding, the more severe is the stunting. Temporarystunting may also occur, but although the final growth weightmay be achieved, the conformation of the animal so stuntedmay be changed.

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

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

These changes in body proportions arc brought about bythe different parts growing at different rates. Some partsmature 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 andthen fat. Thus if an animal is underdeveloped, its muscles andfat deposits will be much slighter than those in an animal welldeveloped, although there will be less difference between thebones of the two, and even less between the brains.
An excellent example of the above laws can be seen in arabbit of three or four months of age reared badly. Its headwill be large in proportion to the rest of its body, as will also bethe 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. Thedevelopment 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 toincrease the early development of bone, muscle and fat must beadopted, and must be brought into operation at an early age.Conversely, it is possible to increase the fineness of bone byslightly retarding the growth of the animal only when it isrelatively young.

As the rabbit grows and develops, so the dressing out per-centage 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, butmay be increased or decreased by the rapidity of growth.Thus young rabbits at weaning, which have been fed on a veryhigh level of nutrition, may dress out at over 55%. A knowledge of these facts is most important to the breeder whowishes to obtain the heaviest carcase at the cheapest cost.By rearing his stock slowly, the best advantage, purely its termsof feed, is lost.

The appearance of yellow fat in the rabbit is controlled by ahereditary mechanism. In those rabbits which carry thefactor for yellow fat, the yellow pigment, derived from thepigment in green foods, is not eliminated. Thus if the animalbecomes fat and is then reduced in condition, the yellowpigment is concentrated. As the rabbit gets older the depthof yellow increases.

Although the quality, i.e. hardness of fat, depends to someextent on the foods given to the rabbit, fat deposited its a slowlygrowing animal will usually be of a softer quality, and thereforeless desirable, than fat laid down during periods of rapid growth.