Introduction
Genetics of rabbit breeds and populations
Genetics of breeding characters
Genetic improvement: Selection and crossing
Conclusions
Domestic rabbits are the descendants of Oryctolagus cuniculus, a species native to the western Mediterranean basin (Spain and North Africa). Wild rabbits belong to other genera: Sylvilagus, Coprolagus, Nesolagus and Brachylagus. The rabbit was domesticated relatively recently: most breeds are created by humans and are no older than 200 or 300 years, which is why there are few locally adapted land races.
The rabbit has been used as an experimental animal in genetics and reproduction physiology since the beginning of the century, but it was not until 1950 that the first findings on quantitative genetics were published, in Venge's study of maternal influence on rabbit birth weight (Venge, 1950). This work paved the way for research on the genetic improvement of the rabbit for meat production. Scientists at the National Institute for Agricultural Research (INRA) in France initiated research and development in this area in 1961, followed by work in other research laboratories in many countries, such as that of the teams of the University of Zagazig in Egypt, of Gödöllö and Kaposvar in Hungary, of Iztnanagar in India, of Milan and Viterbo in Italy, of Valencia, Saragossa and Barcelona in Spain, the Normal team in the United States and the Chinese teams (particularly in Shanghai) and those working in Nitra in Slovakia and in Cracow in Poland. Robinson's excellent 1958 bibliography in Genetic studies of the rabbit, based on sound genetic and physiological data, is now outdated by this new research.
Work on rabbit genetics has been regularly updated at world rabbit congresses (Rouvier, 1980; Matheron and Poujardieu, 1984; Rochambeau, 1988). However, experience gained under European production conditions cannot be transferred directly to developing countries. To upgrade their rabbits, breeders should use local animals, either native or from imported populations that have been locally adapted, and make use of the genetic variability that is available.
It does seem that priority should be given to research on rural and backyard rabbit production. These would be small, thrifty, autonomous units requiring little investment and using local resources. They would be reasonably productive.
Perhaps the best of the various definitions of breed is Quittet's: "A breed is a collection of individuals within a species which share a certain number of morphological and physiological characters which are passed on to their progeny as long as they breed among themselves."
One way of assessing the genetic uniqueness of different breeds is to study their origins. A breed is the outcome of the combined impact of artificial and natural selection (environmental adaptation). It is difficult to define exactly what is a breed and what is its background. Artificial selection may be based on a number of different criteria, not necessarily all to do with productivity. The breeding conditions may be either artificial or natural, the environment may gradually change and so on.
Rabbit breeds or populations can also be defined in terms of gene frequencies. This is possible with genes identifiable through their visible or major effects on progeny. Coloration and hair structure are classified as visible effects. Thanks to advanced observation techniques the genes governing blood groups, biochemical and protein polymorphism and hereditary anomalies are now also known. (See Zaragoza et al., 1990.)
For quantitative characters, such as litter size or weight at weaning, which are controlled by a great many non-identifiable genes, rabbit populations can also be defined by their performance. These genes are also assumed to have little effect on overall variability and to function independently, according to the standard assumptions of quantitative genetics. Such characters are also influenced by the environment. The environmental characteristics must be carefully described (number of breeders, the direction of selection, the origin of the population and its range) when describing a population.
The genes are carried by chromosomes organized into 22 pairs (2n=44). About 60 markers have been described. These are genes of visible effect such as colour or coat or morphological anomalies, or genes coding for molecules of which the biological impact is being studied. These two approaches are hard to reconcile, for teams often use only one type of marker. Among the markers described, 37 have been placed on eight autosomes and on chromosome X; 23 markers constitute six linkage groups, and the locus of six markers has still not been found. All these markers are spread over a majority of 22 pairs of rabbit chromosomes. The links between the biological markers and the genes for colour or hair have rarely been tested, however.
Experience has shown that the rabbit can support a slow and gradual increase in inbreeding, but research suggests that mating programmes for small populations should minimize its extent and rate of increase among the stock (Rochambeau, 1990).
Breeds created by selectors, particularly amateurs in the United States and Europe, now conform to official standards. The book of the Federation française de cuniculture (FFC) on standards for rabbits describes more than 40 breeds. Each has been bred from animals of local and regional populations, or by crossing existing breeds, or by using mutants for changes in coat colour or structure. Mass selection for size and body morphology has separated these breeds into giant, medium, small and very small. It is interesting to study the origin of the breeds to learn whether they may correspond to original genetic ensembles and to attempt to determine their characteristics.
The characters by which an animal conforms to a breeding standard, such as body size, whether or not it is compact, coat colour and density and ear size, may be related to its resistance to variations in climate. In fact, such factors as coat, skin, body area and weight affect the animals' body temperature.
The currently known genetic determinants of variations in colour and structure are listed below. Coat colour has always been of great interest to breeders.
Coat colour and hair structure
In The genetics of domestic rabbits, published in 1930, Castle described six mutations in coat colour and two mutations in patterns; three mutations in hair structure; one mutation in the yellow colouring of the abdominal fat and two linkage groups. A convenient way to detect the effects of various mutations is to describe the rabbit's "wild" colouring. The coat consists of three types of hair: the longer, rectrix guard hairs, stiff at the base; the more numerous tectrix barbed hairs forming the major part of the coat, which share a hair follicle with the third type - the shorter hairs making up the undercoat.
The coat colour of the wild or "agouti" rabbit consists of grey dorsal fur with a much lighter or white ventral area. The long guard hairs are black but appear deeper black at the tips and bluer at the base. The barbed hairs have zones of colour: black at the tips, with a yellow band in the middle and bluish at the base. The fibres of the underfur are bluish at the base and fringed with yellow at the tips. Colouring is thus basically due to the distribution of black and yellow pigments (eumelanin and phaeomelanin) in the hair, especially in the barbed hairs, and over the whole coat (sides and back in relation to belly fur). Mutations in different loci modify this colouring.
Colouring. There follows a list of the international notation of alleles. Arnold (1984) shows the correspondence with the German system.
· Locus A, agouti: the non-agouti mutation a produces animals without a yellow band in the hair and a lighter belly. Their colouring is uniform. A is dominant over a. A third allele has been described at this locus, at (tan pattern), which is recessive to A and dominant over a.· Locus B, black pigment: a recessive b allele produces a chocolate brown pigment instead of black in agouti hair.
· Locus C: the C gene is required for the development of pigments in the fur, skin and eyes and hence for the expression of colouring. The recessive c gene inhibits the expression of colouring, causing albinism in recessive cc homozygotes. There are several alleles at this locus, quoted below in dominant-to-recessive order:
C: full expression of colouring.cch: chinchilla, suppression of colour in the intermediate band of the coat.
ch: Himalayan. Only the hairs at the body extremities are black. The expression of this gene depends on the ambient temperature.
c: albinism. The albinism locus is epistatic over the colour loci. The cc genotype covers the expression of colour genes situated at other loci.
· Dilution, D, d: the recessive mutant d. allele affects the intensity of the pigmentation, causing a dilution of the pigment granules. The dominant D allele produces normal pigmentation density. The recessive dd homozygote is found in the genotypes of blue (black diluted to blue) or beige (yellow diluted to beige) rabbits.
· Normal extension of black E or yellow e: the e gene mutation causes increased yellow pigment in the hair, tending to replace the black (or brown) pigment. Grey, black or brown breeds have the E gene. Yellow and red breeds are recessive ee homozygotes.
· Vienna White locus: Vienna White rabbits have completely unpigmented fur but coloured eyes (blue). The original gene is called V and its mutated form v. Rabbits of the Vienna White breed are therefore recessive vv homozygotes. Crosses of this breed with albino rabbits produce coloured progeny.
· Mutations producing a mottled coat: these mutations involve the loci for English (En, en) and Dutch (Du, du). The Papillon rabbit is of the En en heterozygous genotype. The En gene is incompletely dominant. The En En homozygotes are whiter than the heterozygotes, while recessive homozygotes are blacker. The colour genotype of the Papillon rabbit (Giant Checker in English, Mariposa in Spanish) cannot be pinpointed. At the other locus the du du genotype produces the white belt characteristic of the Dutch rabbit.
Hair structure mutations. The three main ones are:
· Angora. This is a recessive autosomal mutation expressed as a lengthening of the duration of hair growth at the same speed of growth which produces longer hair. The wild (L dominant) gene has mutated into a recessive I allele to produce the Angora. The mating of two Angora rabbits always produces Angora offspring. Two rabbits with normal hair can sometimes produce a fraction of Angora progeny if they are Ll heterozygotes.· Rex. This is a recessive autosomal mutation that causes almost all of the guard hair to disappear. The coat looks different with shorter hair. The symbol for the Rex gene is r, and for the dominant wild allele R.
· Hairlessness. This is caused by several recessive mutations and is usually lethal.
The genotype of the coat colour and structure in rabbit breeds can be predicted when these loci are known. So far not much gene interaction visibly affecting body colour and breeding characters has been found, but there has been very little research in this area. The Angora and Rex genes are of course exploited to produce angora wool and Rex fur.
Groups of breeds by adult size and origin
There are different kinds of breeds:
· primitive or primary, and geographic, from which all other breeds have come;· breeds obtained through artificial selection from the above, such as Fauve de Bourgogne, New Zealand White and Red and Argenté de Champagne;
· synthetic breeds obtained by planned crosses of several breeds, such as Blanc du Bouscat and Californian;
· Mendelian breeds, obtained by the fixation of a new character of simple genetic determination, appear by mutation, such as Castorrex, Satin and Japanese.
Breeds are conveniently grouped by adult size, which is also related to production characteristics such as precocity, prolificacy, growth rate and age at maturity. A major determinant of adult size is the origin of the breed.
Heavy breeds. Adult weight exceeds 5 kg. Fertility is generally low. The growth potential of the heavy breeds can be exploited, especially in cross-breeding. The Bouscat Giant White, (French) Belier, Flemish Giant and French Giant Papillon are examples. The fur of the (French) Belier varies greatly in colour and can be white, agouti, iron grey or black. Its body build would make it a good meat rabbit. However, it is bred for show and therefore found only in small units, at least in France. The breed is more important in other European countries such as Germany and Denmark.
The Bouscat Giant White is a synthetic albino breed. It is a large rabbit known for its prolificacy and fast growth rate in traditional French rabbitries. The Flemish Giant from Belgium comes in several colours. It is one of the largest rabbits (potential adult weight 7 kg) and is still farm-raised. This breed could furnish a gene pool for improving growth in other breeds; Flemish Giants could be pure-bred for this purpose.
Average breeds. Adult weight varies from 3.5 to 4.5 kg. These are the basic stock of breeds used for intensive rabbit production for meat in western Europe and are the most numerous. Only a few examples are described here.
Silver rabbits are found in several countries (English Silver, German Silver). These varieties differ from the Argenté de Champagne in adult size (English Silver is lighter) and colour. Like Fauve de Bourgogne, Argenté de Champagne is an example of a breed that has developed with selection over many years from a regional population (Champagne). The breed is known for both its fur, once much sought after, and its productivity: high fertility, quick growth, good muscle development and good meat quality. Its adult weight is 4 to 4.5 kg. It is farm-bred in France, usually on straw litter. Research has begun on intensive breeding of Argenté de Champagne.
The Fauve de Bourgogne is also of regional origin. It has spread throughout France and elsewhere in Europe (Italy, Belgium, Switzerland). The Fauve de Bourgogne Rabbit Breeders' Association has established a stud book for this breed, ensuring pure-bred selection.
The New Zealand Red was first exploited in California with a selection system similar to that used in France on the Fauve de Bourgogne, with the difference that the New Zealand breed was raised on wire-mesh floors which were introduced much earlier in the United States than in France.
The Californian is a synthetic American breed. It was presented for the first time in 1928 in California by its breeder, whose objective was a meat animal with very good fur. The adult weight of the Californian is 3.6 to 4 kg.
The New Zealand White originated as a breed in the United States. It is the albino offspring of coloured rabbits. From the outset it was bred selectively in large meat-production units, especially in southern California (San Diego area), for its breeding qualities: prolificacy, maternal performance, fast growth rate and precocious body development which makes it ready for slaughter at 56 days, the objective being a light carcass. The New Zealand White adult weight (4 kg) slightly exceeds that of the Californian. The New Zealand White was used in the first studies on the rabbit at the Fontana Station in California. Since 1960 this breed has spread through Western Europe and other regions with the growing use of mesh floors for rabbit cages.
The Large Chinchilla rabbit raised in Europe is of German origin. Its adult weight averages 4.5 kg. It can be bred for meat and fur.
Lightweight breeds. These breeds have an adult weight of 2.5 to 3 kg. They include the Small Himalayan, the Small Chinchilla, the Dutch and the French Havana.
The Russian or Himalayan rabbit is white with black extremities. It is thought to have originated in China and spread from there to Russia and Poland. It carries the Himalayan Ch gene mutation.
The lightweight breeds usually develop very quickly and make excellent mothers. They eat less than the medium and large breeds and could be crossed or used pure in developing countries to produce a light, meaty carcass of 1 to 1.2 kg.
Small breeds. These breeds weigh about 1 kg at maturity. They are represented chiefly by the Polish rabbit, with its many variations of coat colour. Selection for small size has led to very low fertility and a marked decrease in growth rate. These breeds cannot be used for meat production. They are bred for show, for the laboratory and as pets.
Local populations and strains
Pure-bred animals are usually raised in small groups and their selection for breeding characters is in its infancy. These breeds could therefore constitute interesting potential gene pools for improving local populations.
Most rabbits raised for commercial meat production belong to populations which may resemble one breed or another (a question of appearance only, as they do not meet the criteria for that particular breed in terms of origin and standards) and sometimes resemble no breed at all. These are "common" rabbits, grey, spotted or white, the outcome of various unplanned crosses. They may belong to local populations. Some examples of local populations in developing countries are the Baladi rabbit of the Sudan (baladi means native or local in Arabic), the Maltese rabbit of Tunisia and the Creole rabbit of Guadeloupe. Developing countries planning to develop rabbit production should first identify existing local populations and establish their biological and breeding traits and adaptability before designing selection programmes and improved production systems.
Many countries where rabbit production is recent, dating back only a few decades, have no clearly defined local populations. The populations are highly polymorphic and come from a great many unplanned crosses with imported pure-bred animals. Often these populations are of limited potential and not locally adapted. Even so, they should be studied before deciding to eliminate them.
Finally, there are rabbit strains. The strain is a genetically closed group, small in number, with no outbreeding for several generations. Characteristics of a strain are the number of breeding animals, the year and way the group was constituted, and possibly the mating programme (selection or no selection). These strains can be found in research laboratories which keep them to study their biological and breeding characteristics in order to make the best use of them in selection. The INRA centre in Toulouse conducts selection experiments on strains (Table 28).
Private breeders have fairly recently begun selecting rabbit strains, along the lines of the poultry selection that has been practised since 1930. But some breeders or small groups of breeders, at village level for instance, may also have created strains without realizing it.
Some research laboratories, such as the Jackson Laboratory at Bar Harbor, Maine, United States, keep inbred rabbit strains or lines for use solely as laboratory animals.
Breeders in traditional rabbit-breeding regions use local populations. The genetic patrimony of the population is shaped by the ecology of the region, the characteristic production system and breeders' interventions. Slowly, the population evolves. Barring specific instances, the population is open to bordering populations. This slows the trend towards uniformity and offers new genetic variability for natural and/or artificial selection.
The next stage of evolution is the breed. Here the breeder is more important and defines a standard and looks for animals which conform to it. The ecology of the region and the characteristic production system is less influential than for populations and breeds are usually more homogenous. Selection for confirmation to a standard can lead to excesses. Breeders may be looking only for external characters, neglecting production characters. They may breed close relatives to increase the visual impression of homogeneity. The last evolutionary stage is the strain. There are fewer founders (a few dozen for each sex) and few genes are exchanged with neighbouring populations. A strain is usually artificially selected for a few traits. Strains are often more genetically homogenous than breeds.
Breeding characters
The expression of breeding characters depends on environment and the breeder. A comparison of results from several different environments and geographical locations can reveal general characteristics of the breeds or species. Fecundity, growth rate and tissue development in young rabbits are three sets of basic breeding characters.
Fecundity. Fecundity is defined as the product of fertility (number of kindlings per doe per unit of time) and prolificacy (number of young per kindling).
Prolificacy varies significantly according to several factors which may be inherent in the animal. Litter size increases by 10 to 20 percent from the first to the second litter and then again, but by less, from the second to the third, with no change from the third to the fourth. After the fourth the size may decrease. Inbreeding may reduce prolificacy.
TABLE 28 Characteristics of selected INRA experimental strains
|
Strain and origin |
Selection criteria |
Selection methods |
Population size |
No. of generations |
|
1077 |
Litter size at weaning |
Index |
33 males |
18 |
|
New Zealand White |
|
|
121 females |
|
|
9077 |
Field strain |
|
22 males |
12 |
|
Same origin as 1077 |
|
|
44 females |
|
|
2066 |
Litter size at birth |
Index |
24 males |
18 |
|
Californian and Large Himalayan |
|
|
64 females |
|
Prolificacy also depends on the season and the reproductive rate imposed on the doe. In healthy does receiving normal feed and 12 to 14 hours of light, prolificacy seems to be linked to adult size.
Ovulation potential increases, on average, with size. The first factor affecting prolificacy is the ovulation rate (number of eggs) followed by the viability of blastocysts and embryos before birth.
In 1932, Gregory showed that litter size depends on the number of eggs produced after mating and this number depends on the body size of the breed: 3.97 for Polish does and 12.88 for Flemish Giant. The corresponding litter sizes at birth are 3.24 and 10.17. Small light breeds are generally less prolific than medium and large breeds. Elamin (1978) gives the following average figures from the Sudan for the Baladi, Californian and New Zealand White breeds:
|
|
Baladi |
Californian |
New Zealand White |
|
Total litter size |
4.7 |
7.10 |
7.49 |
|
Live births per litter |
3.5 |
6.67 |
6.94 |
Matheron and Dolet (1986) analyse the results from 682 females in ten rabbitries in Guadeloupe. Their first category is small-size Creole females. These are hard to find and so breeders buy them in France and cross-breed them many times. They then distinguish between New Zealand White and "other" females on which more specific data are lacking. In these complex crosses, breeders have used Argenté de Champagne, Fauve de Bourgogne, Bouscat White, Papillon, etc. in addition to the above two strains. Table 29 shows that New Zealand females are more fertile and more prolific. This is a confirmation of the high adaptability of this breed to local conditions of high temperatures and humidity. Birth-to-weaning mortality is still high, indicating a need for further progress. Creole females are less prolific but more viable than the "other" females. The difference of -0.78 at birth is only -0.12 at weaning. The poor birth-to-weaning viability of young from "other" females is surprising. The literature indicates that these half-breed females often benefit from the effect of heterosis and complementarity, but the performance here shows that this is not always the case. It is also possible that the choice of breeds and crosses was poor.
Paez Campos et al. (1980) give the breeding parameters of New Zealand White, Californian, Chinchilla and Rex breeds raised at the National Rabbit Breeding Centre at Irapuato in Mexico, a tropical zone tempered by the 1 800 m altitude (Table 30).
TABLE 29 Performance of females of three genetic types in Guadeloupe rabbitries
|
Litter size |
Breeds |
Mortality (%) |
||||||
|
Number |
Pregnancy rate (%) |
Total births |
Total live births |
Weaned |
Birth |
Birth-to-weaning |
Total |
|
|
Others |
2159 |
75 |
7.33 |
6.72 |
4.54 |
8 |
32 |
38 |
|
Creole |
78 |
71 |
6.55 |
6.21 |
4.42 |
5 |
29 |
32 |
|
New Zealand White |
291 |
80 |
7.44 |
6.71 |
5.14 |
10 |
23 |
31 |
|
Total |
2528 |
76 |
7.32 |
6.70 |
4.60 |
8 |
31 |
37 |
|
Significance of the breed effect |
|
** |
NS |
NS |
* |
NS |
** |
** |
|
Standard deviation |
|
|
2.78 |
2.86 |
3.00 |
|
|
|
Source: Matheron and Dolet, 1986.
TABLE 30 Average breeding parameters of four breeds raised at the Irapuato National Rabbit Breeding Centre, Mexico
|
Strains |
Litter size |
Live births per litter |
Rabbits weaned per litter |
Age at first mating (days) |
Weight at first mating (kg) |
Number of litters examined |
Number of does |
|
New Zealand |
8.5 |
8.0 |
6.5 |
1.44 |
3.49 |
3723 |
600 |
|
Californian |
8.0 |
7.2 |
5.8 |
140 |
3.50 |
1090 |
200 |
|
Chinchilla |
8.7 |
8.1 |
6.0 |
132 |
3.39 |
562 |
140 |
|
Rex |
6.8 |
6.3 |
5.1 |
153 |
3.02 |
554 |
120 |
TABLE 31 Litter size observations in Cuba for four rabbit breeds
|
|
Total births per litter |
Total live births per litter |
|
Semi-giant White |
9.3 |
8.2 |
|
Californian |
7.8 |
6.6 |
|
New Zealand White |
7.0 |
6.2 |
|
Chinchilla |
7.6 |
6.4 |
Source: Ponce de Léon, 1977.
Ponce de Léon (1977) obtained the results in Table 31 from four breeds researched in Cuba, in a wet tropical climate. The characters of these breeds and this rabbitry are defined in greater detail further on in the chapter. The high rate of stillbirths (11.6 percent) is explained by rearing conditions in the rabbitry.
The development of technical and economic management systems in France and Spain provides series of results describing performance trends in rabbit production units. For the sample regularly followed in France by the Institut technique de l'aviculture, litter size (number of live births) rose from 7.2 in 1974 to 7.8 in 1986, reaching 8.6 in 1992.
Table 32 summarizes other findings comparing breeds reared under rural or southern country conditions. The numerous European and United States comparisons of medium-sized breeds such as New Zealand White and Californian rabbits, for which Rochambeau's (1988) summary might be consulted, have been deliberately omitted. The table stresses the importance of studies in India and Egypt. Regrettably, there are few studies on local populations. The New Zealand White and Californian rabbits are used by many authors, but they are very different strains. Since most authors rarely specify the origin of their animals, it is difficult to compare them and these various populations of white rabbits may well have only the phenotype of colouring in common. This table also shows the importance of specific "giant" populations found in India and Egypt, but without knowing the adult size of these populations it is impossible to know whether they really belong to giant breeds. Other populations such as the Russian Chinchilla or the Sandy also deserve attention.
Biological components of prolificacy. The description of biological traits in local populations and breeds provides useful pointers for better utilization strategies. The procedure is to count the numbers of corpus luteum to estimate the rate of ovulation. The number of implantation sites and the number of living and dead embryos are then counted to determine embryo viability. Litter size at birth completes the estimation of foetal viability. Observing the female tractus after embryo implantation (seven days after kindling and before the 15th day of pregnancy), both the rate of ovulation and embryo viability can be estimated. The simplest method is laparotomy, to observe the ovaries and uterus. As this usually requires slaughter of the doe, the technique of choice today is the laparoscopy. The effect on the doe is considerably reduced by the use of an endoscope which allows a normal productive life after the operation, and several observations on the same female. Tables 33 and 34 show that strains differ. The way the strains are classified varies between ovulation and birth, i.e. strain 2066 is penalized by poor pre-implantation viability (Table 33).
Weight gain and anatomical composition. The growth rates of young rabbits are strongly correlated with adult size and weight where there has been no marked dietary deficiency. Table 35 gives average weights of young rabbits at successive ages, from 28 to 78 days, as well as carcass weights at 78 days, for the Small Himalayan and New Zealand White. The table clearly shows the growth rate of young Small Himalayan rabbits (adult weight 2.5 kg) to be slower than that of the New Zealand White breed (adult weight 4 kg). Moreover, at 78 days the New Zealand White is more mature than the Small Himalayan, when its live weight is 63 percent of adult weight against 59 percent for the Small Himalayan. The variation coefficients, the ratio of the standard phenotype deviation from the mean, are typical of the intrabreed variability of these characters for a given feeding system. Variability is greater in young New Zealand White rabbits than in Small Himalayan. Medium breeds slaughtered at the same age also vary in growth performance and carcass composition. Table 36 gives data for young Fauve de Bourgogne, Argenté de Champagne and Large Himalayan rabbits slaughtered at 84 days. Argenté de Champagne has excellent growth, muscle tissue and fat development for meat production. Fauve de Bourgogne is a close second.
TABLE 33 Litter size components in three experimental INRA strains
|
|
Strain |
||
|
2066 |
1077 |
9077 |
|
|
Ovulation rate |
14.5 |
13.8 |
13.0 |
|
Number of embryos implanted |
11.1 |
12.0 |
11.0 |
|
Number of live embryos at 15 days |
9.8 |
10,4 |
9.7 |
|
Number of live + dead young at birth |
8.0 |
8.2 |
8.4 |
Source: Bolet at al., 1990.
TABLE 34 Litter size components in a sample of 233 V-strain females at the University of Valencia
|
|
Average |
Standard deviation |
|
Ovulation rate |
15.0 |
2.1 |
|
Number of embryos implanted |
12.9 |
2.6 |
|
Number of live embryos at 12 days |
12.6 |
2.6 |
|
Number of live + dead young at birth |
10.0 |
2.8 |
Source: Santagreu, 1992.
Weight gain and the growth rate of the main tissues depend on the breed's biological characteristics and on production factors such as feeding. So the criterion for describing a breed in a particular production environment should probably be maturity in terms of weight, defined as weight at a given age divided by adult weight.
The most interesting breeds from the production point of view are those with the best ratio of weight gain to adult weight, which arrive early at the proper live weight for market. Lightweight breeds could be utilized as pure-breds or, better, crossed with medium-weight breeds for a light carcass with good muscle development and quality meat (sufficient fat) where there is consumer demand.
The genetic improvement of breeding characters relevant to the production environment depends on the specific genetic variability expressed in that environment. This variability is expressed in animals of the same breed or local population as well as in different breeds and populations and in interpopulation crosses. Variability is an expression of genetic differences which selection and crossing try to exploit.
The question here is how genetic variability can be exploited in small-scale production, preferably using local resources. Upgrading the potential of a species depends on its biological characteristics, mastery of its reproduction and calculating the genetic parameters for selection.
Biological characters
Controlled breeding. One breeding operation requiring much care and time on the breeder's part is to get the first and successive litters from the doe. In cage breeding the doe should be serviced in the buck's cage. Once sexually mature the doe can theoretically be presented to the buck at any time except during gestation, but in practice she does not always accept the male. Acceptance of the male and subsequent mating produce litters only 70 percent of the time. This figure varies with physiological conditions, season, breed and environment. Figure 17 summarizes the respective genetic roles of the buck and the doe in litter size at weaning.
TABLE 35 Variability in weights of young rabbits from 28 to 78 days, and carcass weights, for two breeds
|
|
Small Himalayan |
New Zealand White |
||
|
x |
v (%) |
x |
v (%) |
|
|
Age (days) |
Live weight (g) |
|||
|
28 |
428 |
8 |
599 |
26 |
|
31 |
485 |
12 |
761 |
16 |
|
38 |
582 |
8 |
1013 |
14 |
|
45 |
770 |
9 |
1248 |
13 |
|
52 |
933 |
9 |
1568 |
15 |
|
59 |
1105 |
10 |
1860 |
14 |
|
66 |
1245 |
10 |
2066 |
11 |
|
73 |
1387 |
10 |
2300 |
10 |
|
78 |
1476 |
10 |
2503 |
10 |
|
|
Carcass weight (g) |
|||
|
78 |
911 |
9 |
1364 |
7 |
Note: Animals bred at INRA (Toulouse Centre). Rational production; weaning at 28 days. Carcasses with head and paws. x = average; v = variation coefficient.
TABLE 36 Average live weight at 84 days, carcass weight, muscle weight/bone weight ratio, weight of fatty tissue in carcass, for three breeds
|
|
Fauve de Bourgogne |
Argenté de Champagne |
Large Himalayan |
|
Live weight at 84 days (g) |
2143 |
2460 |
2055 |
|
Carcass weight (g) |
1305 |
1588 |
1287 |
|
Muscle weight/bone weight ratio (%) |
4.3 |
4.5 |
4.0 |
|
Weight of fatty tissue in carcass (g) |
86 |
107 |
73 |
Source: Rouvier, 1970.
The breeder is dependent on the sexual urges of the buck and doe for the first essential step, mating. Little is known about the biological basis of rabbit sexuality. The urge drops with high temperatures (28° to 30°C). In the hot season the doe must be presented to the buck early in the morning, from 0600 onwards, when the sexual urge is greatest.
Fertility is affected by ovulation, which depends on the doe and takes place ten hours after mating, and by fecundation of the egg, which depends on the buck and occurs 16 hours after mating. The genes of both the buck and the doe equally affect prenatal growth and the viability of the egg. Crossing can improve the viability of the egg, blastocyst and embryo. The doe has more influence in the uterine environment, notably on embryo nourishment. The buck therefore has an influence on litter size.
FIGURE 17 Respective genetic roles of male and female rabbits in determining litter size at weaning
Doe prolificacy is a breed characteristic, but with substantial individual variations (one to 18 young per litter). Once the doe has kindled, the litter must be safely raised to weaning. The breeder affects litter size at weaning by protecting the young and by the feed provided for the nursing doe. The viability of the baby rabbits, maternal behaviour and milk production are also important. Kindling-weaning viability in the litter depends on the number of live births, which varies from breed to breed, as shown in Table 37.
This viability remains fairly constant for the number of live births in litters of three to nine. Small litters (one or two live births) do not offer a favourable environment for the survival of the young. Live young at weaning peak at 8.60 for litters of 12 or more. This suggests practical rules for fostering to increase the total production of young rabbits weaned. The fostered rabbits may come from small (one or two), or more commonly from large (over ten) litters. However, fostering implies both a sufficient number of does in the rabbitry and the breeder's familiarity with their maternal behaviour. After birth and once the young rabbit has suckled, it can be separated from the mother for 24 hours, allowing for easy travel and transfer to a foster mother.
TABLE 37 Birth-weaning viability of young rabbits by litter size at birth
|
No. Of litters |
No. of live births per litter |
No. weaned per litter |
Birth-weaning viability (%) |
|
171 |
1 |
0.35 |
35 |
|
321 |
2 |
1.37 |
68 |
|
487 |
3 |
2.43 |
81 |
|
634 |
4 |
3.23 |
81 |
|
1035 |
5 |
4.06 |
81 |
|
1784 |
6 |
5.05 |
84 |
|
2741 |
7 |
5.80 |
83 |
|
3837 |
8 |
6.68 |
83 |
|
3753 |
9 |
7.34 |
82 |
|
2857 |
10 |
7.82 |
78 |
|
1343 |
11 |
8.21 |
75 |
|
676 |
12 |
8.57 |
71 |
|
221 |
13 |
8.59 |
66 |
|
63 |
14 |
8.60 |
61 |
|
Average |
8.01 |
6.41 |
80 |
Note: Data from a rational rabbit production unit in the Midi-Pyrénées region of France.
Source: Roustan, Matheron and Duzert, 1980.
The biological characteristics of the female rabbit- ovulation induced by mating, acceptance of the male from the day of kindling, no lactation anoestrus, no marked seasonal anoestrus - are such as to afford a wide range of theoretical reproduction rates. As an example, Table 38 compares three different rates of reproduction at a commercial rabbit breeding centre in Mexico.
Both does and bucks have a very high reproduction potential, as confirmed by the latest research. Potential reproduction per doe per year can be evaluated at 150 young. Achieving this, however, will require many more years of research as well as the mastery of environmental factors. For breeding in developing countries it is best at present to aim at using local populations and longer reproductive periods. The best technique is to start by upgrading traditional production techniques and (where they exist) local populations.
Tissue growth. As demonstrated by Cantier et al. (1969), bone tissue in rabbits develops first, followed by muscle and then fat. In a population of common rabbits of average adult weight (4 kg) the skeleton develops rapidly up to a live weight of 900 g. Growth then continues more slowly up to 4 kg. Muscle tissue gains very quickly in weight up to a live weight of 2.3 to 2.6 kg, when the curve falls abruptly. Adipose tissue develops at a fast rate after 2.1 kg. To allow for the differences in the speed of overall weight gain due to breed or feeding, rabbits should be slaughtered at 50 to 60 percent of the normal adult weight for their breed or population. This is the right stage for the best anatomical composition of the carcass and the most efficient utilization of feed.
TABLE 38 Comparison of three reproduction rates
|
Breeding characteristics |
Rates |
||
|
1 |
2 |
3 |
|
|
Breeding does |
75 |
75 |
75 |
|
Age at weaning (days) |
28 |
35 |
42 |
|
Presentation of doe to buck after kindling (days) |
3 |
10 |
17 |
|
Rate of acceptance of male (%) |
85 |
84 |
87 |
|
Gestation rate (%) |
61 |
84 |
87 |
|
Theoretical number of litters/doe/year |
9.0 |
8.0 |
6.95 |
|
Estimate of litters/mother-cage/year |
7.9 |
7.5 |
6.6 |
|
Number born per litter |
7.6 |
7.6 |
7.7 |
|
Live births per litter |
6.8 |
6.9 |
7.0 |
|
Number weaned per litter |
5.7 |
5.9 |
5,8 |
|
Average weight at weaning (g) |
520 |
760 |
990 |
Source: Irapuato National Rabbit Breeding Centre, Mexico.
Poor feed slows down overall weight gain and lowers conversion efficiency - the amount of feed necessary to produce a 1 kg weight gain. This might not be a drawback in a breeding system using local resources for feeding the growing rabbits, but the fastest growing animals in a population have the best carcass composition (muscle/bone ratio, fat percentage) at slaughter age or weight. Young rabbit meat is naturally lean; there is no excess fat. The best slaughter age and weight must be worked out in terms of market demand, the production system and the type of feed used.
Genes and the environment
Most quantitative breeding characters - fertility, viability, growth, etc. - are poly-genetically determined, but they are also subject to the effects of the environment. Phenotype is the outcome of the impact of genotype and environment on a character. The genotype is the outcome of the effects of genes at several loci. The environment is made up of a number of components: climate, habitat, the animals' microclimate, temperature, humidity, air speed, rabbitry equipment, breeding techniques and feeding practices, and the human factor-the breeder. The genetic determination of character variations is of dual interest to the selector and breeder: first, to exploit the genetic variability of animals of the same breed or population; and second, by crossing, to exploit the genetic variability between breeds and populations.
Individual genotypic values are not directly observable, only performance (phenotypes) can be measured. The conventional model of quantitative genetics assumes phenotypic value to be the sum of genetic value and environmental factors. This model assumes genotype and environment to be independent. According to this model, genetic value is partly the result of additive genetic value and partly of gene interaction on the same locus (dominance) or different loci (epistasis). Using a regression coefficient, the additive genetic value of an individual is estimated for the performance of this individual and its relatives. A selection programme tries to create genetic progress, i.e. to increase the average additive genetic value of the population.
Heritability and genetic correlations. The amount of genetic progress depends primarily on how much of the variance is of additive genetic origin. This coefficient is called heritability and it is calculated as the ratio of additive genetic variance to total variance. Heritability therefore varies from zero to one. Heritability is also the regression coefficient of an individual's additive genetic value over his/her own performance. Heritability varies with the character, the population studied and environment. It particularly varies with gene frequencies and thus changes in a selected population.
Figure 18 shows the heritability of the principal characters of interest to breeders. Heritability is read clockwise from the left. Female fertility is near zero in terms of heritability. Then, moving clockwise, heritability increases. The heritability of litter size is about 0.10. The highest heritability is for weight at a standard age (0.2 to 0.3) and this increases as the animal grows and maternal influence wanes. Postweaning growth rate and feed efficiency in collective cages are between 0.3 and 0.4. Above 0.4 lie characters such as carcass weight, muscle/bone ratio, intake in a group cage and slaughter yield.
FIGURE 18 Heritability and genetic correlation of production characters in rabbits
These estimates are relative: the problem of estimating a variance ratio with the available data is compounded by variations of heritability in place and time. Rochambeau's 1988 review is an instructive illustration of this: heritability for the number of live-born young varies from 0.0 to 0.4 when the upper quarter and lower quarter estimates are removed. The variation for individual weight at 14 weeks is 0.2 and 0.8 under the same conditions.
Genetic variability among breeds and populations. Local breeds or populations could be compared with improved breeds in other countries and under different production systems. Breed differences are primarily exploited through cross-breeding. Interbreed comparisons in rabbitries are therefore very useful. Local breeds and populations can be compared with improved breeds in other countries and breeds produced in different conditions. Interbreed differences are basically exploited through crosses. Not all crosses are advantageous, however; they must be tested. The main advantages of cross-breeding are heterosis and interbreed nicking ability.
Heterosis may be defined as better breeding performances of crossed animals than that obtainable with the average of the two pure parent breeds. Heterosis may apply to the young rabbit (its viability, for example), the crossed doe (fertility, milk production) or the crossed buck (vigour, sexual urge, fertility). Characters subject to dominance, such as reproduction characters, are those most likely to benefit from heterosis.
Heterosis may occur where the populations crossed differ genetically, which is not always revealed by a phenotypic study of the pure breeds or populations. The crossed animals are always more heterotic than the animals of the two parent populations and this implies greater adaptability to variable and difficult environmental conditions. Crossing can therefore be useful in improving rabbit breeding in developing countries, but crossing trials must be planned. Where local populations exist their use is recommended.
Cross-breeding makes possible the optimum use of the nicking ability of the breeds or populations crossed. Nicking ability concerns the two groups of characters from the mother and her young which contribute to the quantity of rabbit meat produced by the doe. In cross-breeding this ability is aimed at bringing together either the overall characters relative to the mother and the offspring, or a favourable combination of additive effects on the components of an overall character.
In the first instance, bucks of a breed with high growth potential are crossed with does of another breed or population that exhibit good prolificacy, maternal performance and tolerance of the production environment. The second instance concerns traits making up an overall character. Thus, ovulation rate and egg and embryo viability are components of litter size at birth (prolificacy). Prolificacy and birth-weaning viability are components of litter size at weaning. Crosses can therefore be sought which combine a high ovulation rate and strong embryo viability in the crossed doe. These characters may well be antagonistic on an intrapopulation basis.
The effects of heterosis and nicking ability are not systematic. Crossing programmes are needed to bring out these effects clearly. Let us consider population A and population B. It is recommended that two pure-bred (A × A) and (B × B) be compared with two reciprocal crosses (A × B) and (B × A), to highlight the effects of the maternal and grandmaternal generations.
As an intuitive illustration of the maternal effect, let us assume that breed A has an adult weight of 6 kg and breed B an adult weight of 3 kg. We cross an A male with a B female and a B male with an A female, and compare the weight of the young at weaning. The young AB rabbits have the same genetic heritage on average as the young BA rabbits, as they share half of the paternal and half of the maternal genes. The young have a different maternal environment, however: A females have a larger uterus and produce more milk so the young weigh more at weaning. Thus, even with the same genetic heritage, BA rabbits are heavier at weaning than AB rabbits because of the favourable maternal effect. A more precise definition is given in Matheron and Mauléon (1979). It is recommended that two successive generations of crosses be studied to bring out the direct effect of heterosis on rabbit characters and on the maternal effects, as expressed in the female characters. The first generation includes crosses (A × A), (B × B), (A × B) and (B × A); the second consists of mating pure AA and BB females and half-breed AB and BA females with, for example, males of a third C strain. If more than two populations are studied, the number of genotypes to compare at the second generation increases with the square of the number of populations.
One example is provided by an INRA experiment at the Toulouse Centre in 1987 to 1989. This three-stage experiment used strains 1077, 9077 and 2066, as shown in Table 28. The first stage involves a factorial mating between males and females of three strains: males of each genotype (1077, 9077 and 2066) are crossed with females of each genotype (1077, 9077 and 2066) to obtain litters with nine genotypes (three pure and six cross-bred). At stage two, females from these nine genotypes were mated with males of three pure genotypes. At the last stage, the same females were mated with males belonging to two strains of terminal crossing of different origin. The first three litters of the female were checked at each stage. The females were then slaughtered during their fourth pregnancy and litter size components studied.
Table 40 compares the performances of pure and cross-bred females. Cross-bred females were superior on the whole, the number rising from ovulation to weaning at a rate of one to 13 percent. There are also differences between the pure strains and cross-bred females. The following analyses attempt to explain these differences for further use. 2066 females have a better ovulation rate but this advantage disappears at the next stage. The performances of 2066 and 1077 are fairly close. The 9077 strain performs less well.
Cross-bred genotypes with 2066 genes also have a higher ovulation rate: an advantage maintained up to weaning, where the genotypes 2066 × 1077 and 1077 × 2066 confirm their superiority. Litter size is considerably increased by the use of cross-bred females.
Table 41 analyses the same findings in terms of genetic effect. For direct genetic effect, 2066 has a negative effect on the number of implantation sites and 9077 has a positive impact on litter size at birth. The maternal effect of 9077 on the number of implantation sites contrasts with the positive effect of 1077 on litter size at weaning. While the effect of direct heterosis is weak, maternal heterosis has a major effect on the number of implantation sites and is maintained until weaning, achieving 16 percent for 1077 and 2066.
The results of crossing experiments, of particular interest in selecting an optimum animal utilization strategy, are specific to the animal population studied and cannot be generalized for all animals in a breed. However, they can describe local populations or strains, thus selecting the best way to use them in cross-breeding or pure-breeding.
Cross-breeding in tropical countries. The biological bases for superior crosses should be sought among the available animal populations bred in various environments. Several large-scale studies of interbreed crosses in tropical countries have been made: there follows one from Cuba, and then a synthesis of experimental work in Egypt.
These studies were made on animals from imported acclimatized breeds, not on local rabbit populations. Meat production was improved by using the best crosses. In 1969 to 1971 the Cuban Instituto de Ciencia Animal crossed four breeds on a rotational basis: Semi-giant White, Californian, New Zealand White and Chinchilla. The characters analysed were litter size at birth and weaning, and litter weight at weaning. The experiment was conducted during the dry season (November to April, mean temperature 22.2°C, humidity 75.2 percent) and the rainy season (mean temperature 26.1°C, humidity 77.7 percent). Some of the experimental animals from these four breeds had recently been imported from Canada, others had been in Cuba for some time. Adult weights are given in Table 42.
TABLE 40 Average female performance in nine genotypes: litter size components measured at different stages
|
Female genotypes* |
Number of corpus luteum |
Number of implantation sites |
Litter size at birth |
Litter size at weaning |
|
9077 × 9077 |
13.0 |
11.0 |
7.8 |
6.9 |
|
2066 × 2066 |
14.5 |
11,1 |
8.5 |
7.2 |
|
1077 × 1077 |
13.8 |
12.0 |
8.6 |
7.5 |
|
Average |
13.8 |
11.4 |
8.6 |
7.5 |
|
2066 × 1077 |
15.2 |
13.4 |
9.9 |
8.7 |
|
1077 × 2066 |
15.3 |
13.1 |
9.9 |
8.8 |
|
1077 × 9077 |
12.4 |
10.9 |
8.5 |
7.4 |
|
9077 × 1077 |
12.7 |
11.0 |
8.8 |
7.8 |
|
9077 × 2066 |
13.5 |
11.9 |
8.7 |
7.9 |
|
2066 × 9077 |
15.0 |
12.5 |
9.4 |
8.3 |
|
Average |
14.0 (+1%) |
12.1 (+6%) |
9.2 (+11%) |
8.1 (+13%) |
* Paternal genotype followed by maternal.
Source: Brun, Bolet and Ouhayoun, 1992.
TABLE 41 Genetic parameters of litter size measured at different stages between ovulation and weaning
|
Parameters |
Genotypes |
Implantation sites |
Litter size at birth |
Litter size at weaning |
|
Direct genetic effects |
9077 |
0.8 |
0.4 |
0.3 |
|
2066 |
-1.2 |
-0.4 |
-0.2 |
|
|
1077 |
0.4 |
0.0 |
-0.1 |
|
|
Maternal effects |
9077 |
-0.9 |
-0.8 |
-0.4 |
|
2066 |
0.5 |
0.5 |
0.0 |
|
|
1077 |
0.4 |
0.3 |
0.4 |
|
|
Direct heterosis |
2066 × 1077 |
3 |
5 |
0 |
|
1077 × 9077 |
-1 |
1 |
0 |
|
|
9077 × 2066 |
-1 |
3 |
6 |
|
|
Maternal heterosis |
2066 × 1077 |
15 |
15 |
16 |
|
1077 × 9077 |
-4 |
7 |
7 |
|
|
9077 × 2066 |
10 |
9 |
15 |
Source: Brun, Bolet and Ouhayoun, 1992.
The animals were raised in hutches identical to those used for rabbit breeding in southern California. These are wire cages with wooden nesting boxes arranged in single decks in two rows, in a roofed building open on all four sides. This habitat protects the rabbits from direct sun but in a wet tropical climate cannot protect against rain and wind, which explains the high mortality rate of the rabbits before weaning.
An extensive system of reproduction was used, with weaning at 45 days, followed by mating. The average figures on litter size show a normal prolificacy for breeds of this adult size (7.45 total births per litter); slightly higher than normal stillbirth rate (over 10 percent); and, above all, a high birth-to-weaning mortality (2.5 rabbits weaned per litter). This was caused by inadequate protection of the nests from wind and rain and inadequate feeding of the lactating does. It is interesting to know the positive contribution of cross-breeding in such difficult production conditions.
A comparison among the pure breeds revealed that the Semi-giant White loses fewer young between kindling and weaning than the others, and the weaning weight is better. For simple crosses the highest averages for number of young weaned and lowest total rabbit mortality figures were recorded by New Zealand White × Semi-giant White. Numerical productivity can also be increased by crossing the female progeny of this cross with Californian males. The most productive cross is Semi-giant White does × Chinchilla bucks.
Afifi and Khalil (1992) summarized the findings of nine Egyptian experiments published between 1971 and 1990. They compared pure and cross-bred animals from local or imported populations. There is a long list of breeds used: Bouscat, Chinchilla, Giza White, Baladi White, Red and Yellow, Grey Flemish Giant, White Flemish Giant, New Zealand White and Californian. The trial designs include many simple crosses but, unfortunately, few cross-bred females. The authors of the summary conclude that local breeds (Giza White, Baladi) are superior for characters expressed before birth and the imported breeds (New Zealand White, Californian, Bouscat) better for postnatal characters. The review includes a great many estimations of the effects of direct heterosis, here summarized in Table 43. In these experimental environments, the direct effects of heterosis proved weak for the characters studied. Apart from an average value of 15 percent for litter weight at birth and 7 percent for litter size at weaning, all other values were below 5 percent. They are close to zero for individual weight at four and 12 weeks and for postweaning viability. The effects of maternal heterosis are stronger, even though the low number of experimental results does preclude a categorical statement.
France, Italy and Spain in southwestern Europe are developing genetic improvement programmes to meet the needs of intensive production in a temperate climate. Animals selected in Western Europe are not necessarily the best for small unit production (five to 60 does) in different production conditions. Local rabbit breeds and stock bred locally using various imported populations should be used for genetic improvement.
Efficient genetic improvement should be a group effort with scientific and technical support from the country's research and development organizations. The improvement programme could focus on a village (or preferably a group of villages), on all the rabbitries in a province, or on the whole country. Genetic improvement is a costly operation: the group needs to be big enough to bear the cost and to mobilize the necessary skills.
Genetic improvement demands technical specialization. There should therefore be breeder-selectors and breeder-users, perhaps with breeder-multipliers between the two. While the pyramidal schemes used in Western Europe are efficient in their special context, they are not universally applicable. It is up to the individual to conceptualize networks tailored to the sociospecifics of the country's breeders (although the networks must be genetically efficient). The selectors should also be excellent breeders, making use of production systems, feed resources, housing and other materials adapted to the environment. Sophisticated selection facilities should be avoided, as the objective is to match the best local systems. Health care and sanitation, in particular, must be exemplary.
TABLE 42 Adult live weight of four breeds in a Cuban cross-breeding experiment, 1969 to 1971
|
Breed |
Weight of females |
Weight of males |
|
(kg) |
(kg) |
|
|
Semi-giant White |
4.05 |
3.95 |
|
Californian |
4.05 |
3.87 |
|
New Zealand |
3.80 |
3.90 |
|
Chinchilla |
3.98 |
4.20 |
A selection unit must be effective on two levels: breeding and production. The extra costs entailed in the technical side of the selection work should be borne by the group of breeders benefiting from the genetic improvement. The cost of research devoted to a genetic improvement programme for the whole country should be shared by a larger group. There are several conceivable types of organization. With French assistance, Mexico experimented with a pyramidal system (1976 to 1982) with a state-supervised national breeding station and regional multiplication stations. Development agencies distributed breeding animals to family-scale rabbitries.
Research and development agencies should focus on: first, the real efficiency of selection methods and creating new genetic material to improve rabbit production in the country; and second, the best strategies for utilizing local and exogenous animal populations, making breed comparison studies, doing cross-breeding experiments and testing strains.
The object of selection is to upgrade performance by enhancing an animal's genetic value where husbandry and feeding techniques permit expression of genetic value. In fact, breeding and feeding techniques must be improved at the same time as the genetic value. Selection and crossbreeding should increase the annual output per doe and speed the growth rate for earlier slaughter and better carcass and meat quality.
The definition of a selection trial design requires both the choice of a method and the review of its theoretical efficiency. The crossing is a supplementary benefit to intrapopulation selection. But genetic progress from cross-breeding is not cumulative from one generation to the next, as is progress from selection, except where selection is used to improve crossing. The following are examined below: selection methods, cross-breeding strategies and organizing genetic improvement.
Selection methods
Characters and criteria for selection. A major objective of selection is to improve annual fecundity per doe. This global character depends on the breeder, the animal and the environment. The breeder establishes the theoretical reproduction rate of the does. For backyard rabbitries, it is assumed that weaning takes place at 42 days, servicing at 24 days after kindling, and the average conception rate is 70 percent. This gives an average of six litters per doe per year.
A culled doe is immediately replaced by a young doe ready for mating. If the stock renewal rate is 100 percent per year, the annual numbers of litters per doe will be roughly 5.5. If an average six young per litter are weaned and 5.5 reach slaughter or reproduction age, the objective is then 30 rabbits per doe annually.
This modest goal is realistic for backyard rabbitries not based exclusively on pelleted feed. If necessary, the weaning age can be extended by delaying presentation of the doe for servicing beyond day 24. The theoretical reproduction rate can be stepped up if the goal is too easily achieved or too modest in terms of the potential of the stock and environment. The doe could be brought for servicing at day 17 after kindling with weaning at 35 or 42 days. This would give an additional litter per doe, raising the annual goal to 35 rabbits per doe. A more intensive breeding objective could produce 40 to 50. For many countries, however, this would not be a realistic goal.
Whatever reproduction rate is adopted, it is important to have fertile does which accept the buck and can produce many large litters with good kindling-to-weaning survival rates. This implies a whole range of characters: acceptance of the buck, gestation, fertility, viability of young, milk production and longevity. These characters and performances can be summed up by the selection criterion: average number of weaned per litter from the first three litters obtained within a predetermined period. There is a close correlation between performance during the first three litters and the doe's total output. In practice, the following principle could be followed:
· after the second litter, calculate the selection index of the doe based on the average young weaned per litter;· divide this index by the number of days between the first kindling and the nth kindling (for index for n litters). This gives an index of numerical productivity;
· compare does with the same numbers of litters against this index.
As weaning age is variable, the number of rabbits weaned can be calculated on litter size at 28 days so the doe's genetic value can be estimated more rapidly.
Chapter 9 describes an even simpler system of choosing breeding stock, which can be done directly in the rabbitry.
The other group of characters for selection has to do with weight gain. One selection criterion is average daily weight gain from weaning to slaughter age, say at day 70. The difference between individual weight at day 70 and individual weight at weaning is divided by the number of days elapsed between these two dates. The idea is to speed up postweaning growth. There is no need to measure the quantity of feed consumed, except for experimental purposes or to compare genetic types for selection for feed utilization.
It is not easy to measure the quantity of feed or dry matter eaten by the animals, and when they are given different feeds and local forage feed conversion efficiency is difficult to calculate. Speeding up postweaning growth indirectly reduces the amount of dry matter needed for every kilogram of live-weight gain.
Slaughter yield, carcass quality (meat/bone ratio, fat) and organoleptic qualities of meat are complex characters to select for because they can only be measured in carefully controlled slaughter conditions. Direct intrapopulation selection for these characters would be unrealistic. Breeders can check sample figures for these characters for the population they are using and if improvement is necessary crosses can be made with bucks from good meat breeds (described earlier in this chapter).
Performance control and technical data management. With stock being used for selection it is necessary to:
· identify each breeding animal individually;
· measure the breeding characters needed for genetic and breeding management of the stock;
· record these characters for later exploitation.
All rabbits are identified at weaning when separated from the dam by a numbered ear tag or a number tattooed in the ear. This might be the date of birth plus a day-of-year identity number. Depending on the size of the group an individual number might have four or five digits (up to 999 or 9 999 births a year) or even six if necessary. Another number indicating genetic type (breed or cross) could be added to the animal's cage card.
Troop management involves three types of record card: doe, buck and litter. The buck and doe cards identify the breeding animal, its number, date of birth and the number of its sire and dam; next, the animal's cage, for easier identification within the production system; then the date and cull rate.
On doe cards (see Figure 45) record:
· servicing dates (day, month and year);· identification number of servicing buck;
· result of pregnancy test by abdominal palpation;
· kindling date and litter: parity of doe, number of live and stillborn young (found living or dead at first examination of nest after kindling) and number added or subtracted from litter 36 hours after kindling;
· weaning dates, number weaned per litter and weaned litter weight;
On buck cards (see Figure 46) record:
· date of servicing;
· number of does serviced;
· outcome of abdominal palpation;
· number of live and stillborn young.
While the buck card repeats some of the doe card data, it is very useful for following the pregnancies and prolificacy of does mated to that buck.
A litter card shows:
· litter birth date, number of dam and sire, weaning date per litter and per individual offspring;
· the young rabbit's number, weaning weight, and the preslaughter weighing date and weight.
A "remarks" column on each card allows the breeder to add observations (e.g. animal's health). These cards are designed for manual or computer processing and are used for daily breeding management, genetic management and, perhaps, experimentation.
There are software programs for desktop computers which collect this data on a daily basis and edit the breeder's workplans (particularly for mating, palpations, kindling and weaning). These can calculate the various balance sheets of the enterprise.
Choosing a selection system. Having chosen selection objectives and criteria, the next thing is to determine the selection system that will maximize genetic progress. This is dependent on three parameters: selection intensity, selection precision and the intergeneration gap.
Selection intensity depends on the percentage of individuals retained. Assume, for example, that 100 rabbits are weighed and ten chosen to breed. The rest are slaughtered and so the percentage is equal to 10 percent.
Selection precision depends on the heritability of the character, the number of measurements and the degree of related-ness between the selection candidate and the rabbit measured. For instance, in selecting for litter size, recording the data for the three first litters, not just the first, makes for greater precision. In selecting for slaughter yield, on the other hand, precision diminishes if the rabbits measured are five fraternal half-siblings of the candidate and not five full siblings.
The generation interval is the age of the parents at the birth of their average progeny and it increases if females are chosen after the third litter instead of after the first. There is a conflict between trying to be more precise and trying to reduce the generation interval.
In the end, genetic progress depends on the additive genetic variance of the character, a parameter assumed here to be constant.
There are four selection methods:
· mass or individual selection: measured on the selection candidate;
· pedigree selection: measured on the candidate's ancestors (parents, grandparents, etc.);
· sibling selection: measuring the candidate's siblings (full and half-siblings, etc.);
· progeny selection: measuring the candidate's progeny (young, etc.).
Table 44 illustrates the advantages and drawbacks of each method with respect to the three parameters of genetic progress.
These four methods are complementary: pedigree selection provides an initial sifting of selection candidates when the geneologies and performances of the sire and dam are known. This choice is not very exact however. Mass selection is the simplest and most efficient method and as such is the method of choice. Sibling selection is more complex, but is useful for greater precision when the character selected for is not easily heritable, such as litter size, or when the candidate has to be slaughtered in order to measure the character. Progeny selection is not much in use for rabbits as it considerably increases the generation interval and is very expensive.
Table 45 summarizes the findings of selection experiments on rabbits. It shows that selection is an effective way of increasing litter size and postweaning growth rate, although there is usually almost no progress in litter size. Successful selection depends on full control of rabbit breeding, the collection and management of geneological data and performance, and the selection cycle.
TABLE 44 Four selection techniques compared for effectiveness
|
|
Mass selection |
Pedigree selection |
Sibling selection |
Progeny selection |
|
Intensity |
Average |
High |
Average |
Low |
|
Precision |
Average |
Low |
High to average |
High |
|
Generation interval |
Average |
Low |
Average |
High |
In practice, a synthesis of various theoretical studies suggests the following recommendations.
To improve litter size, the selection criterion is litter size at birth or weaning, measured on the first three litters. For greater precision without increasing the generation interval, the performances of the candidate's full sisters and half-sisters are taken into account. Renewal with the progeny of the female's second or third litter is the next step. Rearing the rabbits in separate generations, as described below, increases selection efficiency but the rabbitry has to be much bigger. It is pointless to attempt selection without the necessary resources.
For a better postweaning growth rate, the selection criterion should be the speed of growth after weaning. This criterion can be measured on both sexes and heritability is average. Simple mass selection is therefore the technique of choice. In order not to reduce the strain's aptitude for reproduction, breeding animals are chosen in litters with at least one shared character and at least four or five births.
A breeder who renews the herd on the basis of the best does for litter size will choose the young rabbits from the litters of these females which weigh the most at slaughter time. In any case, unhealthy young rabbits are culled prior to selection.
Renewal of pure-bred stock and mating programmes. Here there are different cases to consider: first, a rabbitry practising combined selection based on litter size; second, mass selection based on the same character for a sizeable number of breeding does in the strain (200); and third, smaller groups.
Case 1. Selection of a strain on the basis of litter size at weaning (INRA, Toulouse). Combined selection, separate generations. The theoretical plan calls for raising the stock in separate breeding groups, each group constituting a generation. In each generation 196 does are bred with a batch of 42 males. Twenty-five percent of these does are selected according to the results of the first three litters, the theoretical reproduction rate permitting a generation interval of ten months. Each doe selected produces an average of four replacement female offspring, so the group is made up of families of full sisters and paternal half-sisters.
The mating programme is implemented in accordance with the composition of the breeding groups. Table 46 shows that the females of each of the 14 families are distributed among 14 breeding groups with three males (one and two alternates) and 14 females. One breeding female is chosen at random per family from among the 196 does.
This mating programme means the genetic value of each doe can be figured according to her performance and those of related females (family average). The plan can also be implemented with fewer than 14 families and 14 breeding groups (e.g. 10, or a total of 100 breeding females). The breeding groups system offers the practical advantage of matching a production lay out in which families are represented by mother cages. Here, the 14 doe cages and the three buck cages are arranged side by side in rows in the rabbitry.
TABLE 45 Findings of specific selection experiments on rabbits
|
Authors |
Characters selected |
Genetic progress per generation1 |
Strain size |
No. of generations |
|
Poujardieu et al. (1993, pers. comm.) |
Litter size |
+0.05 |
33 M and 121 F |
18 |
|
Baselga et al. (1993) |
Idem |
+0.10 |
24 M and 120 F |
11 |
|
+0.03 |
24 M and 120 F |
8 |
||
|
Mgheni and Christensen (1985) |
Idem |
+0.35 |
20 M and 40 F |
4 |
|
- 0.432 |
20 M and 40 F |
4 |
||
|
Narayan, Rawat and Saxena (1985) |
Idem |
-0.05 |
22 M and 110F |
6 |
|
Rochambeau et al. (1989) |
Individual slaughter weight |
+ 46 g and + 2.4% |
12 M and 30 F |
8 |
|
Mgheni and Christensen (1985) |
Idem |
+ 75 g and +3.4% |
20 M and 20 F |
4 |
|
108 g and-4.3%2 |
20 M and 20 F |
4 |
||
|
Estany et al. (1992) |
Idem |
+ 27 g and 2.0% |
15 M and 60 F |
12 |
|
+ 23 g and 1.6% |
15 M and 60 F |
8 |
Note: M = males; F = females.
1 Expressed in gross and percentage of average.
2 Selection to reduce value of characters selected.
Rearing the generations separately has a number of advantages: the animals compared are the same age and so it is easier to calculate selection indices and estimate genetic progress. It also makes it easier to create a gap between the generations for health purposes.
There are a number of drawbacks, however. If female fecundity is too low it is impossible to produce a new generation every two months. Optimal use of available cages is also impossible and the occupation rate is low. Many breeders therefore prefer a system of overlapping generations (Case 2), but the system does demand a strict adherence to management rules.
Case 2. Selecting a strain for postweaning growth and female fecundity (IRTA, Barcelona, Spain), mass selection and overlapping generations. The selected population includes six breeding groups composed of 16 does and five bucks. As in Case 1, the males remain in their breeding group and one sire is replaced by one male offspring. The females change group: the daughter of a doe is never in the same group as her dam.
Selection is in two stages: first the does are indexed by postlitter weaning weight of their litter. Only does in the first and second kindling category and does in the bottom 20 percent leave no progeny. Does with a negative index are culled as soon as a replacement female is available. All does are culled after the fifth kindling, as are males over the age of 13 months.
In the second selection stage, future breeders are chosen from the progeny of the does selected earlier. The final selection is for animals with the greatest daily weight gain between weaning and sale. Twenty-five percent of the does and 15 percent of the bucks of the weaned population of each lot are kept as replacements as needed.
TABLE 46 Formation of reproduction groups based on family origin
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|
|
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Family 1 |
|
|
|
|
|
|
|
|
Family 2 |
|
|
|
|
|
|
|
|
Family 3 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Family 14 |
|
|
|
|
|
|
|
|
|
G1 |
G2 |
G3 |
G14 |
Males used |
Replacement males | |
Source: Matheron and Rouvier, 1977.
This set of rules governing the selection and culling of breeding animals is designed to maintain a population in demographic equilibrium and increase selection efficiency.
Case 3. Conservation of a numerically small strain. It may be useful to conserve strains or populations that are few in number. Matheron and Chevalet (1977) proposed an appropriate management method (used to manage control strain 9077 at INRA).
The strain is composed of 11 breeding groups with one male and four females. From one generation to the next, each male leaves one male progeny and each doe one female progeny.
The male of group i is the offspring of the i male of the preceding generation. His dam is chosen at random from females in the breeding group. The four females making up group i in generation n+1 are the daughters of four females who were, respectively, in groups i-1, i-2, i-3 and i-4 in generation n. This method is illustrated in Figure 19.
Once the breeding animals for herd renewal have been chosen, a mating programme -is the next step. Selection may be random, but mating between close relatives such as full brothers-sisters, half-brothers-half-sisters, mother-son or father-daughter must be avoided. A practical way to organize a servicing calendar is to assign breeding animals to cages by breeding groups, taking family origin into account (the family is the original breeding group). A breeding group consists of two or three cages of sires and 10 to 14 cages of dams, or proportionately fewer if the colony is numerically small.
Cross-breeding strategies
Three cross-breeding systems are:
Simple or two-breed crossing. Fem
1.1
2.1
1.1
1.2
