3. Reproduction

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Anatomy and physiology
Feeding behaviour
Nutritional needs
Types of feed

 

Anatomy of the genitals

In the male, the oval-shaped testes within the scrotum remain in communication with the abdominal cavity, where they were at birth. The rabbit is actually able to withdraw its testes when frightened or fighting with other males. The testicles descend at about 2 months. The short, backslanting penis points forward when erect. Figure 6 shows the relative position of the various organs.

FIGURE 6.-Genital apparatus of male rabbit

In the female, ovaries are oval-shaped and do not exceed 1-1.5 cm. Beneath the ovaries is the oviduct, made up of the duct, the ampulla and the isthmus. Though outwardly the uterine horns are joined at the back into a single organ, there are actually two independent uteri of about 7 cm, opening separately through two cervical ducts into the 6-10 cm vagina. The urethra opens midway along the vagina at the vaginal vestibule. The glands of Bartholin and the preputial glands can be identified. The whole is supported by the broad ligament attached at 4 main points under the vertebral column.

Figure 7 shows the relative position of the various organs.

FIGURE 7.-Genital apparatus of female rabbit

Male reproduction physiology

Gonad development and puberty

The gonads begin to differentiate on the 16th day after fertilization. After birth the testes develop less quickly than the rest of the body. From the age of 5 weeks they begin to grow very rapidly. Accessory glands undergo a similar development, but at a more even rate and are less precocious.

Spermatogenesis begins between days 40 and 50. The testicular tubes become active at about 84 days. The first spermatozoa are present in the ejaculate at about 110 days.

Sexual maturity, defined as the moment when daily sperm production ceases to increase, is reached at 32 weeks by New Zealand White rabbits in temperate climates. However, a young buck in these same conditions can be used for reproduction from the age of 20 weeks. Indeed the first manifestations of sexual behaviour appear at days 60-70 when the rabbit makes its first attempts at riding. Coitus may occur for the first time at about 100 days, but the viability of the sperm cells is very weak or nil in the first ejaculates. So first mating should be timed for age 135-140 days.

All these figures are to be considered approximate. The onset of puberty varies from breed to breed, but conditions in the rabbitry also play an essential role, particularly feeding, which is even more important than climate.

The volume of semen ejaculated is about 0.3-0.6 ml. Concentration is evaluated at 150-500 x 106 spermatozoa per ml, but both volume and concentration are liable to vary. False mountings, 1 or 2 minutes before copulation, increase the concentration of the ejaculate. In 2 successive servicings the first acts as a preparation for the second, which is less voluminous but more concentrated. During subsequent matings the volume of the ejaculate decreases, while concentration increases between the first and the second ejaculate and then diminishes. The total number of spermatozoa per ejaculate follows the same trend.

Maximum spermatozoa production is obtained by using the buck regularly once a day. If the buck is used regularly twice a day, each ejaculate has only one half the concentration of spermatozoa. On the other hand, if bucks service several times a day, 1 day a week, the 3 or 4 ejaculates may be concentrated enough to effect fertilization. Further ejaculates contain very little spermatozoa and cannot effect fertilization often enough to be worthwhile.

Female reproduction physiology

As in the male foetus, sexual differentiation takes place on the 16th day after fertilization. Ovogonial division begins on the 21st day of foetal life and continues until birth.

The first follicles appear on the 13th day after birth, and the first antrum follicles at about 65-70 days. Does are able to mate first at 10-12 weeks, but as a rule this will not produce ovulation. The onset of puberty varies greatly with:

Does generally reach puberty when they have grown to 70-75 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.

In most domestic mammals ovulation takes place at regular intervals when the female is in heat or oestrus. The interval between two periods of oestrus represents the length of the oestrus cycle (4 days for rats, 17 for ewes, 21 for sows and cows).

The female rabbit, however, does not have an oestrus cycle with regular periods of heat during which ovulation will occur spontaneously. Does are considered to be in oestrus more or less permanently. Ovulation occurs only after mating. A female rabbit is therefore considered to be in heat when she accepts service and in dioestrus when she refuses.

FIGURE 8.-Sexual behaviour and duration of oestrus in 5 pubescent nulliparous does (Moret, 1980)

There are many observations which denote the alternating periods of oestrus during which the doe accepts mating, and dioestrus in which she refuses (Figure 8). But the present state of knowledge does not make it possible to predict either the respective lengths of oestrus and dioestrus or the environmental or hormonal factors determining them.

It has been noted, however, that 90 percent of the time when a doe has a red vulva she will accept mating and ovulate, whereas when the vulva is not red the doe will accept service and become fertilized only 10 percent of the time. A red vulva is therefore a strong indication, though not a proof, of oestrus. A doe in heat assumes a characteristic pose, called lordosis, with the back arched downwards and hindquarters raised. A doe in dioestrus tends to crouch in a corner of the cage or exhibit aggression towards the buck.

The sexual behaviour of a female rabbit is thus very special. She has no cycle and can stay in heat for several days running. On the ovary, follicles not having evolved to the ovulation stage through lack of stimulation undergo regression and are replaced by new follicles, which remain for a few days in the pre-ovulating state and may then in turn regress.

In most mammals the progesterone secreted during gestation inhibits oestrus and the pregnant female refuses to mate, but a pregnant doe may accept mating throughout the gestation period. Indeed, in the second half of pregnancy this is the most common behaviour (Figure 9).

A breeder cannot therefore use the sexual behaviour of does as an indication of pregnancy. Mating occurring during gestation has no dire consequences for the embryos. Unlike the phenomenon observed in the female hare, superfoetation (2 simultaneous pregnancies at 2 different stages of development) never occurs in rabbits.

FIGURE 9.-Mating acceptance trends in gestating does (Moret, 1980)

Ovulation is normally induced by the stimuli associated with coitus and occurs 10-12 hours after mating, as outlined in Figure 10.

Given this sort of pattern, ovulation can be induced artificially by various techniques. Mechanical stimulation of the vagina can cause ovulation, but the outcome is quite random. Injections of LHRH or LH can produce results, though repeated injections of the LH hormone lead to immunization and loss of effect beyond the 5th or 6th injection. Injections of LHRH repeated at 35 days for 2 years, however, have involved no loss of effect: 65 to 80 percent of the does became pregnant from this injection followed by artificial insemination.

FIGURE 10.-Onset of ovulation following coitus

At the moment the ovary follicles are ruptured the oviduct pavilion or infundibulum covers the ovary. When liberated the ovocytes are sucked in by the pavilion. The ovocytes are in fact fertilizable from the moment they are liberated, but they are not actually fertilized until about an hour and a half after release. The sperm is deposited by the male in the upper part of the vagina. The spermatozoa make their way upwards rapidly. They can reach the fertilization area (in the distal ampulla, near the isthmus) 30 minutes after coitus. During their journey the spermatozoa undergo a maturing process which enables them to fertilize the ovocytes. Of the 150200 million spermatozoa ejaculated, only 2 million (1 percent) will reach the uterus. The rest are defeated by obstacles at the cervix and uterotubal junction.

The egg reaches the uterus 72 hours after ovulation. On its way through the oviduct the egg divides. The uterine wall differentiates, but the uterine dentellus appears only 5-8 days after coitus. It is the synchronization of these phenomena that makes possible the implantation of the egg. Implantation proper takes place 7 days after mating, at the blastocyst stage. Distribution of the blastocysts is roughly equidistant in each horn, but the blastocysts never move from one uterine horn to the other. From the 3rd to the 15th day after mating the progesterone rate continues to increase, then remains stationary and finally drops rapidly before parturition.

The maternal placenta develops along with the foetus, reaching its maximum weight towards the 16th day of pregnancy. The foetal placenta is visible about the 10th day, and becomes larger until birth (Figure 1 1).

Embryo losses, measured by comparing the numbers of corpus luteum and living embryos, are usually very extensive. Generally speaking only 60-70 percent of the eggs become live rabbits. Most embryo mortalities occur in the 15 days before birth. Mortality is partly due to the viability of the embryos and partly to their situation in the uterine horns. External factors also play a part: the season, and the physiological condition of the doe (especially her age).

Liberated ova which are not fertilized may occasion a pseudopregnancy lasting 15-17 days. At first the corpus luteum and uterus develop as in an ordinary pregnancy, but they do not reach the size or the level of progesterone production of the corpus luteum in pregnancy. Towards the 12th day they regress and disappear under the action of a luteolytic factor secreted by the uterus, undoubtedly prostaglandin. The end of pseudopregnancy is marked by the maternal behaviour of the doe and nestmaking. While such pseudopregnancy is much used in research laboratories on the physiology of reproduction, it is very uncommon in rabbits. Indeed, when a doe is serviced under unfavourable conditions she does not ovulate, and it is exceptional for ovulation to occur without fertilization (as in mating with a sterile but sexually active buck).

FIGURE 11.-Changing weights of foetus and embryonic membranes during gestation (Hammond and Marshall, 1925)

The mechanism of parturition is not very well known. It seems that the secretion of corticosteroids by the suprarenals of the young plays a part, as in other animal species, in giving the signal for parturition. At the end of gestation the doe makes a nest for the litter with her own fur and materials she has available such as straw and shavings. This behaviour is linked with an increase in the oestrogen/progesterone ratio and with the secretion of prolactin. The doe does not always make a nest, or she may kindle outside the nesting box.

Kindling lasts from 15 to 30 minutes, according to the size of the litter. Litter size varies as much as from 1 to 20 young. Most litters range between 3 and 12. In rabbit production units the average is 7-9, but there are great variations.

After parturition the uterus retracts very quickly, losing more than half its weight in less than 48 hours.

Milk synthesis depends on prolactin, a lactogenic hormone. During pregnancy prolactin is inhibited by the oestrogens and by progesterone. At parturition there is a rapid drop in the progesterone level. As oxytocin is freed the action of the prolactin is stimulated and permits the milk to mount in a predeveloped gland.

Milk is let down as follows: the doe comes into the nest box to nurse her litter. The stimulus of nursing provokes the secretion of oxytocin. inframammary pressure mounts, the milk is let down and the young suckle. The amount of oxytocin secreted is proportional to the number of young feeding. But the doe sets the number of feeds: just once in 24 hours. Suckling alone will not provoke the secretion of oxytocin; the mother must want to nurse.

Doe's milk is much more concentrated than cow's milk except for the lactose component (see Table 25). After the 3rd week of lactation the milk becomes markedly richer in proteins and especially fats (up to 2022 percent). The already low lactose content tapers off to almost zero after the 30th day of lactation.

Daily milk production increases from 30-50 g in the first 2 days to 200250 g towards the end of the 3rd week of lactation. It then drops rapidly. The decrease is even swifter if the doe has been fertilized immediately after kindling (Figure 12). The lactation curve varies from doe to doe, especially with regard to duration.

TABLE 25.-AVERAGE COMPOSITION OF COW'S AND RABBIT'S MILK

Components Rabbit's (4th to 21st day) Cow's milk
  Percent
Dry matter 26.1 - 26.4 13
Proteins 13.2 - 13.7 3.5
Fats 9.2 - 9.7 4
Minerals 2 4 - 2.5 0.7
Lactose 0.86- 0.87 5

Source:Lebas, 1971a

Measuring the young rabbits' weight at 21 days gives a fairly good estimate of total lactation, as milk production between days 0 and 21 is closely correlated with total milk production (r + = 0.92). The doe's milk output increases with litter size but the baby rabbits get less milk each than they would in a smaller litter.

FIGURE 12.-Pattern of milk production in lactating and gestating, lactating does (Lebas,1972)

Reproduction and environment

Males exposed to artificial lighting for only 8 out of the 24 hours produce significantly more sperm than those exposed to light for 16 hours.

Does, however, are far more opposed to mating with only 8 hours of light than they are with 16. For both males and females 12 hours of light a day produce average results. The practice in rational European rabbit production units is to light breeding areas artificially for 15-16 hours a day. The and females are together in the same room. The advantage of the does' behaviour and fertilization more than compensates for the drawback of the males' reaction to the light.

FIGURE 13.-Changing liveweights of young does aged from 37 to 112 days reared in different temperatures (Matheron and Martial, 1981 )

The impact of temperature on spermatogenesis has been studied by various authors, but usually for short periods ranging from just a few hours to a few weeks at most. In a prolonged 5-week trial, Oloufa et al. (1951) noted actual falls in the volume and concentration of ejaculates at a high temperature (33C). A high temperature also affects sperm motility even after such short periods of exposure as 8 hours at 36C, or medium periods such as 14 days at 30C. Furthermore, and this seems to be the worst effect, temperatures in excess of 30C reduce the bucks' sexual urge.

However, these findings should not obscure the fact that rabbits do reproduce effectively in hot tropical or equatorial climates. Breeders should take the precaution of protecting their rabbits against extreme heat; they should avoid direct sunshine and protect the cages with an insulating roof, not just a corrugated metal sheet (which in fact transmits too much heat).

It should be noted that humidity does not seem to have been recorded in the various laboratory tests on the effects of temperature on spermatogenesis.

High temperatures also seem to affect female rabbits negatively. The lower prolificacy attributed to does reared in hot climates (30-31) would appear to be due not so much to the temperature itself as to a reduction in body weight caused by a lower feed intake in the heat (Figure 13). It would seem, however, that embryo mortality increases when the temperature exceeds 30-33C, though here again decreased feed intake needs to be considered as a possible cause.

In Europe the season is usually analysed in terms of the combined effects of light and temperature. In tropical climates the temperature effect seems to be dominant, but an effect due to variations in the length of daylight cannot be excluded. The reproduction cycles of the European wild rabbit are strongly influenced by the season. Does breed from the end of winter until early summer (Figure 14). The reproduction period can he longer or shorter, at either end, according to both temperature and availability of feed.

FIGURE 14.-Seasonal variation in percentage of gestating and/or lactating wild does in the United Kingdom (Stephen, 1952)

Exposing domestic does to light for 16 out of 24 hours in Europe considerably attenuates this seasonal variation; indeed it nearly suppresses it. Even so, reproduction problems sometimes appear at the end of summer with no direct relation to the temperature. In tropical climates a drop in the rate of reproduction is noted during the same period, the wet season, when temperatures are high and so is humidity.

Rates of reproduction

The physiological features of the male and especially the female are such that the breeder has great latitude in choosing a reproduction method. But for successful rabbit production the choice of method must be preceded by careful study and planning. The goal is to increase doe productivity and reduce inputs.

Productivity, defined as the number of young per doe per unit of time, depends on: the interval between successive kindlings; litter size at birth; and the survival rate of the young.

These criteria can be improved by slow, methodical selection and careful management of the rabbitry environment. In practice the crucial factor in increasing productivity is shortening the kindling-tomating interval. This means nonproductive periods must be pared as far as possible. Before such a strategy is adopted the breeder should consider:

AGE AT FIRST MATING

Before discussing the rate of reproduction, the first factor to consider is the age at first mating. If the unproductive period before the first litter can be shortened rabbit productivity will naturally be increased. Studies conducted in France on does receiving a balanced concentrated feed showed that female rabbits first serviced at 51/2 months had a lower annual productivity than females serviced 3 weeks earlier. The first group had virtually reached their adult weight and were too fat. The best plan is to have does serviced as soon as they reach 80, or at the most, 85 percent of the mature weight for their breed. Females can be serviced even earlier if their feed is extremely well balanced.

The three basic reproduction rates

The second method of stepping up production, after earlier servicing, is to accelerate the rate of reproduction. This amounts to shortening the theoretical interval between 2 successive litters. In fact, the true rate of reproduction is always slower than the theoretical rate because not all does immediately accept the buck and not all are fertilized when retired. There are 3 basic rates of reproduction: extensive, semi-intensive and intensive.

The breeder fully utilizes the does' maternal instincts by allowing them to nurse their young for 5-6 weeks, rebreeding them soon after weaning. Does are therefore serviced once every 21/2 months.

Later weaning is in no way advantageous except for fryer production -very young animals which can be sold at 8 weeks and have not undergone weaning shock. In the United States and the United Kingdom fryers with a liveweight of 1.7-1.8 kg are produced this way, using breeds such as the New Zealand White. The mother can be serviced before weaning, about 5-6 weeks after kindling, which allows 21/2 months between litters.

When feed is not quite adequate in quality or quantity, it is preferable to wean rabbits at about 40 days. At the same time the breeder should slightly lengthen the resting period between weaning and rebreeding so the doe can build up her reserves again. In any case, weaning later than 6 weeks offers no particular nutritional advantage. The milk produced by the doe after this period provides at most 3 to 5 percent of the young rabbits' daily feed intake.

The breeder has does serviced 10-20 days after kindling and the young are weaned at 4-5 weeks. For 10-20 days the doe is newly pregnant while still nursing. The most important phase of embryo development takes place during the slump in milk production, which may even have ceased, so there is no real competition between the demands of gestation and lactation. As these does never have a resting period they need sufficient and well balanced concentrate feed.

The breeder has the does reserviced just after kindling, taking advantage of the fact that they are then in heat. Weaning should take place at 4 weeks at the latest' usually 26-28 days. There are 3 main techniques:

To arrange ad lib. mating, breeders have worked out 2 types of rabbit housing. The first is the corridor-collar type: the does live in individual cages. They have a broad collar around their necks to prevent them from leaving the cage through the calibrated opening leading into a communicating corridor. The buck, however, has free access (at least temporarily) to the does' cages and can mate whenever the doe is ready.

The second is the group system: a buck and perhaps 10 does live together in the same cage. They can mate at the optimum times. However, special arrangements must be made to curb the natural tendency of females to kill the offspring of other does when they themselves are lactating or ready to kindle.

Choosing the reproduction rate

Considering the greater nutritional needs of the pregnant doe, especially one which is also lactating, semi-intensive and intensive reproduction systems are only suitable where does get the right quantity and quality of feed. If these conditions are not met, the does will usually accept the male but will abort.

Abortion extends the interval between litters to make it as long as the extensive breeding interval. Figure 15 shows the main periods in the reproduction cycle and how under intensive reproduction the doe has no opportunity to build up reserves.

FIGURE 15.-Distribution of gestation, lactation and resting periods in does used at different rates of reproduction

Numerous comparisons of intensive and extensive reproduction have been made, principally in France. Ten years ago the litters of does mated post-partum numbered one less than those of does mated 10 or more days after kindling. This is virtually no longer true, mainly because of improved feeding and the selection of strains and lines suitable for intensive reproduction. Where it is feasible it is the most productive method, though also the most demanding.

In fact breeders in France, Italy and Spain adopt a variable rate of reproduction depending on the condition of the does. For instance, a good healthy doe which produces a litter of fewer than 7 or 8 is immediately remated. If she has given birth to 10 or so young the breeder waits about 12 days before having her serviced. In autumn, when it is harder to get the rabbits to mate, breeders systematically take the does for servicing after parturition. This is to take advantage of the strong post-partum oestrus during which 95-99 percent accept servicing.

In intensive reproduction in European rabbitries 1 doe can produce 5060 weaned young annually. At the same technological level 45-55 rabbits can be produced in a semi-intensive reproduction system. In a wet tropical climate, with balanced feeding, it is possible for a doe to produce 40-50 weaned young a year by a combination of intensive and semi-intensive rates.

Using the extensive rate the best breeders obtain 30-35 weaned young per doe per year. In a tropical climate, depending on the region and especially on feeding, 15-30 weaned young can be produced under extensive reproduction.


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