Anatomy of the genitals
Reproduction and environment
Rates of reproduction
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 two months. The short, back-slanting penis points forward when erect. Figure 7 shows the relative position of the various organs.
In the female, ovaries are oval-shaped and do not exceed 1 to 1.5 cm. Beneath the ovaries is the oviduct, made up of the duct, the ampulla and the isthmus. Although 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 to 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 four main points under the vertebral column.
Figure 8 shows the relative position of the various organs.
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 five 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 to 70 when the rabbit makes it 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 to 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.
Sperm production. The volume of semen ejaculated is about 0.3 to 0.6 ml. Concentration is evaluated at 150 to 500 × 106 spermatozoa per ml, but both volume and concentration are liable to vary. False mountings, one or two minutes before copulation, increase the concentration of the ejaculate. In two 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.
FIGURE 7 Genital apparatus of male rabbit
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, one day a week, the three or four ejaculates may be concentrated enough to effect fertilization. Further ejaculates contain very few spermatozoa and cannot effect fertilization often enough to be worth while. Daily spermatozoa production is roughly 150 to 300 million, independent of the rate of ejaculation. The maximum epididymis reserve is only one to two billion spermatozoa, only partially mobilizable for repeated ejaculations.
Gonad development, puberty and sexual maturity. 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 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 developed 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.
FIGURE 8 Genital apparatus of female rabbit
The oestrus cycle. 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 (four 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.
There are many observations which denote the alternating periods of oestrus during which the doe accepts mating and dioestrus in which she refuses (Figure 9). 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 10).
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 (two simultaneous pregnancies at two different stages of development) never occurs in rabbits.
Ovulation. Ovulation is normally induced by the stimuli associated with coitus and occurs ten to 12 hours after mating, as outlined in Figure 11.
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 luteinizing hormones (LH) or LH releasing hormones (LHRH) can produce results, although repeated injections of the LH hormone lead to immunization and loss of effect beyond the fifth or sixth injection. Injections of LHRH repeated at 35 days for two years, however, have involved no loss of effect: 65 to 80 percent of the does became pregnant from this injection followed by artificial insemination.
Fertilization and gestation. 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 150 to 200 million spermatozoa ejaculated, only two million (1 percent) will reach the uterus. The rest are defeated by obstacles at the cervix and uterotubal junction.
FIGURE 9 Sexual behaviour and duration of oestrus in five pubescent nulliparous does
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 five to eight days after coitus. It is the synchronization of these phenomena that makes possible the implantation of the egg. Implantation proper takes place seven 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 third 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 tenth day and becomes larger until birth (Figure 12).
Embryo losses, measured by comparing the numbers of corpus luteum and living embryos, are usually very extensive. Generally speaking only 60 to 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). For a simultaneously lactating and post-partum pregnant doe (fertile mating 24 hours after giving birth), late embryonic mortality is increased with respect to that observed in a simply pregnant doe under the same circumstances.
FIGURE 10 Mating acceptance trends in gestating does
Pseudopregnancy. Liberated ova which are not fertilized may occasion a pseudopregnancy lasting 15 to 18 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 nest-making, linked to the swift drop in blood progesterone. While such pseudopregnancy is much used in research laboratories on the physiology of reproduction, it is very uncommon in natural-mating rabbitries. 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). Unfertilized ovulation can occur in 20 to 30 percent of artificially inseminated does injected with GnRH. In this case, an injection of prostaglandin PGF2a on the 10th or 11th day will halt the pseudopregnancy and the doe can be fertilized just 14 days after an earlier infertile insemination. Without prostaglandin treatment, the doe cannot be fertilized again until another week has gone by.
Kindling. The mechanism of parturition is not very well known. It seems that the secretion of corticosteroids by the supra-renals of the young plays a part, as in other animal species, in giving the signal for parturition. PGF2a prostaglandins may also be instrumental in starting the process. 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.
FIGURE 11 Onset of emulation following coitus
Kindling lasts from 15 to 30 minutes, according to the size of the litter. Litter size varies as much as from one to 20 young. Most litters range between three and 12. In rabbit production units the average is seven to nine, but there are great variations.
After parturition the uterus retracts very quickly, losing more than half its weight in less than 48 hours.
FIGURE 12 Changing weights of foetus and embryonic membranes during gestation
Artifical insemination (AI) is a growing practice in European rabbitries, particularly in Italy and France. Currently, a little under 1000 production units are involved, but the practice is growing primarily because of the opportunities for work organization involved: AI can impregnate a great many does on the same day without the need to maintain an excessive number of bucks. This paper, while not fully covering the topic of AI, will simply list the main advantages and drawbacks of the method.
Semen collection and control. A doe in heat is put into the buck's cage. The operator holds the artificial vagina with its collection tube between the rabbit's paws. The artificial vagina is kept at a temperature of about 40° to 42°C prior to use, so that it will be at 39°C, the normal vaginal temperature of a doe, at the moment of use. Ejaculation usually takes place immediately following the presentation of the doe.
A basic control of the biological quality of the semen is made for selection of the best ejaculates: no urine, sufficient motility and concentration, etc. The semen is then diluted five to ten times, perhaps in physiological salt solution, within 30 minutes after semen collection, or, always the preferable choice, with a special diluent if it is to be applied within 12 hours. It is possible to freeze the semen, but the poor performance of frozen semen relegates this technique to research laboratory use where there is some interest in maintaining the semen of a specific buck for a long period.
The fact that a high percentage of the ejaculates has to be eliminated on the grounds of poor biological quality means that only a few males need to be retained for every 100 productive females, compared with natural mating.
It is clearly preferable to raise males on wire netting or grating than straw litter which considerably increases bateriological contamination in the semen collected.
Insemination. The semen can be packaged in 0.5 ml pellets or presented in 20,50 or 100 0.5 ml flacons for insertion with a glass cannula. Two techniques co-exist: an insemination gun covered with a single-use sheath, and the glass (or throwaway plastic) cannula. Both techniques have their partisans and their detractors and for both the diluted semen must be delicately inserted deep into the rabbit vagina.
As ovulation is not spontaneous in rabbits, intramuscular injection of an artificial analogue of GnRH (gonadoreline 20 m g, busereline 0.8 m g) is used to provoke ovulation at the moment of insemination. AI in rabbits involves a dual intervention: insemination and the injection of an ovulation-producing hormone.
Successful artificial insemination. Assuming that every operation involved in AI is strictly adhered to, practical success in this reproduction method is equivalent to that in natural mating for the same reproductive rate (percentage of gestation, litter size, etc.).
To ensure adherence, insemination centres are now springing up in Italy and France where male rabbits are maintained and their semen collected, controlled and packed by expert staff possessing the necessary techniques and resources. Like the bucks, these resources give full value for money as such centres can work every day of the week. The semen packaged ready for use is then shipped to specially equipped rabbit production units from the insemination centre. Once apprenticed, rabbit breeders can practise insemination themselves, which requires one or two operators, depending on the insemination technique chosen.
A number of breeders owning more than 30 to 40 breeding does do carry out all operations in their own establishment with good technical results. There have, however, been too many failures to suggest that a breeder should begin by practising every operation, from the preparation of artificial vaginas to insemination in the rabbit's genital tract, including the essential quality controls and disinfestation.
From the purely technical standpoint, does found not to be pregnant when palpated have ovulated after artificial insemination, thus developing a pseudopregnancy that made them temporarily infertile. It is therefore futile to reinseminate an empty doe less than 21 days after the preceding insemination, when the pseudo-pregnancy is over. In natural mating, however, a doe can successfully be represented to the male once it is realized she is not pregnant (10 to 12 days after mating). In this case the absence of pregnancy is almost always linked to an absence of ovulation, whereas after artificial insemination the absence of pregnancy is linked to early embryo mortality or the fact that the rabbit has not been fertilized. Treating pseudopregnant rabbits with prostaglandin may reduce the length of the infertile period and the rabbit can successfully be reinseminated after an unsuccessful AI, but not enough is yet known about the specific modalities involved.
Overall, the highest fertilization rates with AI are obtained with receptive does, i.e. those which would have accepted natural mating. This is particularly true for lactating does and is why all (light, hormonal, etc.) treatments that increase doe receptivity also improve the performance of artificial insemination.
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.
Aspects of milk production. Doe's milk is much more concentrated than cow's milk except for the lactose component (see Table 27). After the third week of lactation the milk becomes markedly richer in proteins and especially fats (up to 20 to 22 percent). The already low lactose content tapers off to almost zero after the 30th day of lactation.
Daily milk production increases from 30 to 50 g in the first two days to 200 to 250 g towards the end of the third week of lactation. It then drops rapidly. The decrease is even swifter if the doe has been fertilized immediately after kindling (Figure 13). The lactation curve varies from doe to doe, especially with regard to duration. Measuring the young rabbits' weight at 21 days gives a fairly good estimate of total lactation, as milk production between days zero and 21 is closely correlated with total milk production (r + = 0.92).
An important point is that the doe's milk output increases with litter size but the baby rabbits get less milk each than they would in a smaller litter. Depending on genetic type, milk production will not increase above eight to 12 baby rabbits.
In males exposed to artificial lighting for only eight out of 24 hours significantly more spermatozoa are present in the gonads than in those exposed to light for 16 hours, although a slightly larger amount is usually collected in ejaculates from the latter.
Does, however, are far more opposed to mating with only eight 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 to 16 hours a day. The males and females are together in the same room.
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 five-week trial, Oloufa, Bogart and McKenzie (1951) noted actual falls in the volume and concentration of ejaculates at a high temperature (33°C). A high temperature also affects sperm motility even after such short periods of exposure as eight hours at 36°C, or medium periods such as 14 days at 30°C. Furthermore, and this seems to be the worst effect, temperatures in excess of 30°C reduce the bucks' sexual urge.
TABLE 27 Average composition of cow's and rabbit's milk
Rabbit's milk (days 4 to 21)
Source: Lebas, 1971a.
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 insulated 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° to 31°C) would appear to be the result not so much of the temperature itself as a reduction in body weight caused by a lower feed intake in the heat (Figure 14). It would seem, however, that embryo mortality increases when the temperature exceeds 30° to 33°C, although 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 day light 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 15). The reproduction period can be longer or shorter, at either end, according to both temperature and availability of feed.
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.
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.
FIGURE 13 Pattern of milk production in does
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-to-mating interval. This means non-productive periods must be reduced to the minimum. Before such a strategy is adopted the breeder should consider:
· whether or not it will be exhausting for the does, perhaps leading to premature culling (this depends mainly on feeding conditions);
· whether or not it might cause a spontaneous reduction in doe fertility and prolificacy;
· whether it will lead to more work for the breeder.
The breeder's desire to improve working conditions and reduce labour costs must also be considered. The final objective criteria for selection must be the production of good rabbits for sale or for consumption per unit of time or per production unit labour cost per hour.
Age at first mating
Before discussing the rate of reproduction, the first factor to consider is the age at first mating. Shortening the unproductive period before the first litter would automatically increase productivity. Studies conducted in France on does receiving a balanced concentrated feed showed that female rabbits first serviced at five and a half months had lower annual productivity than females serviced three 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 (see earlier section on female rabbit physiology).
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 two 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 rebred. There are three basic rates of reproduction: extensive, semi-intensive and intensive, but all intervening stages are or have been used; the distinction is retained here for illustrative purposes.
FIGURE 14 Changing live weights of young does aged from 37 to 112 days reared in different temperatures
Extensive reproduction rate. The breeder fully utilizes the does' maternal instincts by allowing them to nurse their young for five to six weeks, rebreeding them soon after weaning. Does are therefore serviced once every two and a half months.
Later weaning is in no way advantageous except for fryer production - very young animals which can be sold at eight weeks and have not undergone weaning shock. In the United States and the United Kingdom fryers with a live weight of 1.7 to 1.8 kg are produced this way, using breeds such as the New Zealand White. The mother can be serviced before weaning, about five or six weeks after kindling, which allows two and a half months between litters.
FIGURE 15 Seasonal variation in percentage of gestating and/or lactating wild does in the United Kingdom
Where the quality or quantity of the feed is not up to standard, 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 six 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.
Semi-intensive rate. The breeder has does serviced 10 to 20 days after kindling and the young are weaned at four to five weeks. There is no real contrast between pregnancy and lactation for does. For 10 to 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 (milk production 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.
In rational European rabbit production units, a semi-intensive reproduction rate has basically been the rule since the late 1980s: rebreeding 10 to 11 days after kindling; weaning at about 34 to 38 days. At this rate, the work can be programmed by the days of the week, as the plan involves an interval of 42 days (exactly six weeks) between matings: 30 to 31 days of pregnancy + 10 to 11 days following kindling.
Intensive rate. The breeder has the does reserviced just after kindling, taking advantage of the fact that they are then on heat. Weaning should take place at four weeks at the latest, usually at 26 to 28 days. There are three main techniques:
· servicing the same day or the day after kindling: the true postpartum rate;
· servicing scheduled for a specific day, generally three or four days after kindling. This corresponds to a constant interval of 35 days (five weeks) between litters; the results of this 35-day rate are economically disappointing because the rate of female acceptance of servicing three or four days after kindling is very low in most rabbit production units, although not all;
· ad lib mating. A buck left together with postpartum does will serve them several times during the 48 hours following kindling. This is the natural rhythm of wild rabbits.
To arrange ad lib mating, breeders have worked out two 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 ten 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, especially, 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 later abort.
Abortion extends the interval between litters to match the extensive breeding interval. Figure 16 shows the main periods in the reproduction cycle and how under intensive reproduction the doe has no opportunity to build up reserves.
Numerous comparisons of intensive, semi-intensive and extensive reproduction have been made, principally in France. Twenty years ago the litters of does mated postpartum numbered one less than those of does remated ten 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. The systematic use of the most intensive reproduction method, however, makes it difficult to keep female brood stock in good condition, particularly primiparous females. This means a more rapid turnover of stock and the risk of an unfavourable subclinical disease situation, making does more sensitive to any agent of disease or environmental perturbation. After extensive experimentation from 1970 to 1985, European breeders have in fact almost all abandoned the systematic use of postpartum servicing.
In many cases, breeders 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 seven or eight is immediately remated. If she has given birth to ten 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 postpartum oestrus during which 95 to 99 percent accept servicing. Even so, breeders avoid postpartum remating of primiparous does. As has already been mentioned, breeders are increasingly adopting a semi-intensive 42-day rate organized on a weekly basis, as will be discussed in the chapter on rabbit management.
With careful use of a semi-intensive rate, a good breed and balanced feed, European breeders are obtaining 55 to 65 weaned young annually per doe. In the tropics under identical production conditions of rate, breed and feeding, the number of young produced per doe is about 30 to 40.
Using the extensive rate the best breeders obtain 30 to 35 weaned young per doe per year. In a tropical climate, depending on the region and especially on feeding, 15 to 30 weaned young can be produced under extensive reproduction.
FIGURE 16 Distribution (as percentage of productive life) of gestation, lactation and resting periods in does used at different rates of reproduction (EXTENSIVE)
FIGURE 16 Distribution (as percentage of productive life) of gestation, lactation and resting periods in does used at different rates of reproduction (SEMI-INTENSIVE)
FIGURE 16 Distribution (as percentage of productive life) of gestation, lactation and resting periods in does used at different rates of reproduction (INTENSIVE)