Biological characters

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• 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 16 summarizes the respective genetic roles of the buck and the doe in litter size at weaning.

FIGURE 16.-Respective genetic roles of male and female rabbits in determining litter size at weaning (Matheron and Mauleon, 1979)

TABLE 34.-BIRTH-WEANING VIABILITY OF YOUNG BY LETTER SIZE AT BIRTH¹

Source: Rouston et al, 1980.

¹ Data from a rational rabbit production unit in the Midi-Pyrénées region of France.

The breeder is dependent on the sexual urges of the buck and doe for the first essential step, the mating. Little is known about the biological basis of rabbit sexuality. The urge drops with high temperatures (2830°C). In the hot season the doe must be presented to the buck early in the morning, from 06.00 onwards, when the sexual urge is greater.

Fertility is affected by ovulation, which depends on the doe and takes place 10 hours after mating, and by fecondation 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 pre-natal 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.

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 34.

This viability remains fairly constant for the number of live births in litters of 3-9. Small litters ( 1-2 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. Up to 3 young can be fostered by 'good" mothers. The fostered rabbits may come from small (12), or more commonly from large (over 10) litters. However, fostering implies both a sufficient number of does in the rabbitry. and the breeder's familiarity with their maternal behaviour.

The biological characteristics of the female rabbit -ovulation brought on 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 35 compares 3 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 thing is to start by upgrading traditional production techniques.

• 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 liveweight of 900 g. Growth then continues more slowly up to 4 kg. Muscle tissue gains very quickly in weight up to a liveweight of 2.3-2.6 kg, when the curve falls abruptly. Adipose tissue develops at a fast rate after 2.2 kg. To allow for the differences in the speed of overall weight gain due to breed or feeding, rabbits should be slaughtered at 50-60 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 35.-COMPARISON OF 3 REPRODUCTION RATES

A 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 polygenetically 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, the 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.

• Genetic variability in animals of the same population

Individual genotypic values are not directly observable, only phenotypes can be measured. From the beginning quantitative genetics has used the simple model P = m + G + E, where:

m is the general average of the character in a population
P is the phenotype of the character
G is the genotype of the character expressed in such a way that mean value for individuals of the population is nil
E is the effect of the environment expressed in such a way that mean average value for individuals of the population is nil.

This model assumes genotype and environment to be independent. There are three ways in which genes affect genotype. First, each gene acts individually and the additive effect is called A. Second, two genes at the same locus may interact, producing a dominant effect called D. Dominance occurs when the heterozygotous genotype has a different, generally higher value than the average of two dominant and recessive homozygotous genotypes. Third, when several loci are involved, the genes at one locus may interact (alone or in pairs) with genes at another locus. Epistasis (I) then occurs. The total genotypic value G thus breaks down as: G = A + D + I.

A population of animals is selected on the basis of the estimated additive genetic value of the breeding animals expressed through phenotype regression.

The amount of genetic progress that can be expected from intrapopulation selection depends especially on the coefficient of additive genetic value on the phenotypic value heritability (h²), or inheritance of character. This regression coefficient is equal to the ratio additive genetic variance (s²_G)/phenotypic variance (s²p). This fundamental parameter of genetics and selection, between 0 and 1, is governed by character, population and environment. Data on the heritability of characters are much more complete for other domestic mammals and chickens than for rabbits, which have received proportionately less attention from researchers. Table 36 gives some indicative figures for rabbit characters of economic importance.

In Table 37 Rouvier gives the figures on heritability for young rabbits in a crossed population. They were fed balanced pelleted feed ad lib. and reared in a protected environment. Theoretically, these conditions favour the expression of genetic variability.

TABLE36.-FIGURES ON HERITABILITY FOR SOME ECONOMIC RABBIT CHARACTERS

Source:Rouvier, 1975,1980,1981.

TABLE 37.-HERITABILITY VALUES (h²) AND STANDARD DEVIATIONS (s_h2) OF 5012 CROSSBRED RABBITS RAISED IN 8-RABBIT COLONY CAGES

Source: Rouvier, 1981.

There are virtually no estimates on the heritability of rabbit breeding characters for populations in warm and tropical climates except the work of Rollins et a/. (1963) in California. The authors examined the genetic variability of rabbit mortality rates between days 15 and 56 (weaning) in New Zealand White rabbits at the Fontana Experimental Station. In this age group overall mortality was 12 percent, mainly from enteritis and pneumonia. The heritable paternal component of this mortality was h² = 0.12 ± 0.02. The heritable maternal component was h² = 0.45 ± 0.05 for the mortality from enteritis and h² = 0.58 ± 0.05 for the mortality from pneumonia. This indicates a significant additive genetic variability for mortality or viability of young rabbits for the two main causes of death detected at this centre-enteritis and pneumonia. The maternal component of heritability proved far superior to the paternal. This reflects the mother's impact on the viability of the young.

In working out a selection programme, as well as the heritability of characters their phenotypic and genetic correlations need to be known. Concerning phenotypes, there is a positive correlation between total litter weight and litter size at birth and weaning. There is, however, a negative correlation between the individual weights of young rabbits and litter size at birth. Thus, the correlation between individual weight and litter size at 28 days is r =-0.6.

The effect on growth of litter size at birth was first demonstrated by Venge ( 1950). It shows in the weight of the rabbits at successive ages up to 12 weeks. There is no linear ratio, however, between weaning to slaughter growth rate and litter size.

The rg genetic correlations between growth rate and individual weight at 28 days (weaning) and at 70 days are respectively rg = 0.35 and rg = 0.93. The genetic correlation between the growth rate and carcass weight (slaughter at 11 weeks) is rg = 0.87. Selection can thus affect average daily growth rate from weaning to slaughter. A response to selection for this character would include correlated responses on weaning and slaughter weights.

There is a genetic correlation between average post-weaning daily growth rate and the intake of pelleted feed (rg = 0.7 to 0.8). Intake is also heritable, which indicates that appetite is heritable. Selection for post-weaning growth amounts to selecting those rabbits which have best adapted to weaning stress, and have the best appetites and greatest viability.

In conclusion it should be recalled that expression of genotype and additive genetic value can depend on the environment. This raises the question of choosing the selection environment according to the production environment and how breeding stock is used (pure breeds or crosses). The interaction between genotype and environment is still not clear for rabbits because genotypes have only been compared in a single type of environment. As environmental differences are likely to be much greater in developing countries, this interaction should be given careful attention in research.

• Genetic variability among breeds and populations

The comparison of breeds raised in the same environment may reveal breeding peculiarities traceable to differences in the average genotype values of animals belonging to different breeds. 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 crossbreeding are Heterosis and interbreed nicking ability.

Heterosis may be defined as better average breeding performances of crossed animals than 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 2 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.

TABLE 38.-GAINS IN PRODUCTIVITY IN E LITTER CHARACTERS DUE TO SIMPLE (2-BREE D) AND 3-BREED 2-FACTOR CROSSBREEDING

Table 38 shows increases in prolificacy, number of young weaned, kindling-weaning viability and litter weight at weaning, obtained by 2-and 3breed crosses. Averages are for the 4 breeds used in experimental breeding at INRA, Toulouse.

There can be a 12 percent gain in prolificacy due to heterosis in the simple crossing of 2 breeds (better viability of crossed eggs and embryos). Another 6 percent is added by crossing 3 breeds (improved fertility of crossed does). The gain in number of young weaned per litter is even greater: 15 percent for simple crosses, plus 10 percent for 3-breed crosses. This means a 25 percent overall increase from the pure breed to double-stage crossing with 3 breeds using a crossed doe. Heterosis affecting kindling-weaning viability thus joins heterosis affecting prolificacy by increasing litter size at weaning. The result is an effect of heterosis on the total weight of the weaned litter (considering the correration with litter size). So the simple-crossed young rabbits are more vigorous and the crossed does produce more milk.

Crossbreeding makes possible the optimum use of the nicking ability of the breeds or populations crossed. Nicking ability concerns the 2 groups of characters from the mother and her young which contribute to the quantity of rabbit meat produced by the doe. In crossbreeding 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. It is recommended that two reciprocal crosses of each breed or population combination be compared to discover the effects of the maternal and grand-maternal generations. It is also recommended that 2 successive generations of crosses be studied to find the direct heterotic effects on the characters of the young rabbits, the impact on the does, and the effects of the preceding maternal generations.

Where n breeds or populations are available for study, a complete crossing programme will include: in the first generation, n²mating of bucks and does of the same n breeds. In the second generation, the female progeny of these n² genetic types will be mated with the male offspring of an (n + 1 )^th population.

Where males are to be compared the n2 male genetic types obtained from the first crosses should be used with a single female population. The main interest in such a study would be the effects of heterosis on the characters of sexual urge and fertility in the buck.

The result of experiments at INRA, Toulouse, in 1970-72 provide an example. Table 39 shows the paternal and maternal effects on the litters of crosses between Burgundy Fawn (BF), Champagne d'Argent (CA), Bouscat Giant White (BGW), Californian (CL), New Zealand White (NZW) and Small Himalayan (SH) males, mated in accordance with a factorial scheme, with does of the 3 breeds CL, NZW, and SH.

The best buck breeds for the characters researched were Burgundy Fawn and Champagne d'Argent. This offers an answer to the question of what breed of male to cross with several female populations (here, the CL, NZW and SH breeds). In choosing a doe crossbred for productivity and the effects of the doe's parental stock, the breeds of the doe's sire and dam should be analysed in terms of their impact as well as the heterotic effect on maternal impact. Table 40 gives the findings for the CL, NZW and SH breeds.

TABLE 39.-MATERNAL AND PATERNAL EFFECTS ON LITTER CHARACTERS, BY BREED

(estimates expressed in deviations from the mean)

Source: Rouvier. 1980

FB = Fauve de Bourgogne. CA = Champagne d'Argent; BOW = Bouscat Giant White; CL = Californian; NZW = New Zealand White; SH = Small Himalayan.

The does of the 9 genetic types resulting from the factorial crossing of the CL, NZW and SH breeds were all mated with Burgundy Fawn bucks. The progeny of does mated with this breed or with Champagne d'Argent have the highest weights at weaning. Of the 3 breeds compared in crossings to improve litter size and weight at weaning, the Californian should be used as the sire (+0.5 rabbits weaned) and the New Zealand White as the dam (+0.5 rabbits weaned, +303 g total litter weight at weaning) for a productive crossed doe.

The effects of heterosis must also be studied when choosing a crossed doe. For each pair of breeds considered the heterotic effect is measured by the difference between the average of the 2 genetic types of the reciprocal crossed does and the average of the purebred does of the 2 breeds, expressed as a percentage of the general average of the character (Table 41).

Table 40.-Estimated effects of the breeds

Source: Rouvier, 1980.
¹ See Table 39 for names of breeds.

Table 41.-Heterosis¹ on maternal genetic effects in crossing 3 breeds²

* = Significant.
** = Very significant

¹ Heterosis on maternal impact calculated from data on interaction obtained by analysis of variance, differences between averages of reciprocal crossed does and purebred does, as percentage of the latter.
² See Table 39 for names of breeds.
³ No heterosis.

The crossing of a Californian sire and a New Zealand White dam does not produce heterosis. The attraction is the favourable combination of the additive effects of the sire and the dam. Reciprocal crossing, however, is not advantageous. Concerning the litter characters of the crossed doe CL x NZW, there is a favourable combination of direct genetic effects on the ovulation rate transmitted by the sire and the effects of the maternal line of the dam. It has been confirmed that the ovulation rate in the Californian strain of the sire exceeds the New Zealand White strain by 2 eggs.

Table: 42.-Specific combining ability of 3 strain (Averages of characters; estimates of effects of each genetic type, measured in deviations from the mean)

See Table 39 for names of breeds.

Crossed does with Small Himalayan dams show heterosis: a doe with a Californian sire and a Small Himalayan dam has a bigger, heavier litter at weaning. Table 42 shows the estimated effects of each of the 9 genetic types, expressed in deviations from the mean. These estimates show specific performances for the strain combinations, tracing the additive genetic and heterotic effects on the characters of the litters.

The findings of crossing trials are very useful in planning breeding programmes, but they only apply to the animal populations studied and cannot be applied to all the animals of a particular breed. But they could typify local populations or strains of the breed and thus be used in selecting for pure- and crossbreeding.

• Crossbreeding in tropical countries

The biological bases for superior crosses should be sought among the available animal populations bred in various environments. The only known large-scale studies of interbreed crosses in tropical countries are from Cuba (wet tropics) and Mexico (dry tropics, tempered by altitude).

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 1969-71 the Cuban Instituto de Ciencia Animal crossed 4 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-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 4 breeds had recently been imported from Canada, others had been in Cuba for some time. Adult weights were:

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 2 rows, in a roofed building open on all 4 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 note the advantages of crossbreeding 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 NZW x 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 x Chinchilla bucks.

These findings show how important it is to experiment for the best results with purebreds and crossbreds where several breeds are available for trials.

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