| Livestock Research for Rural Development 31 (9) 2019 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
A study was conducted to evaluate the heterotic and combining ability for growth and reproductive traits in nine different genetic groups of rabbits. The genetic groups belonged to three maternal lines namely: New Zealand white (NZ), Blue Vienna (BV) and Chinchilla (CH) and their crosses. Records from 808 purebred kits and 1,206 crossbred (main and reciprocal) kits were used to estimate heterosis, reciprocal effects, general and specific combining abilities (GCA and SCA) for litter size at birth (LSB), litter size at weaning (LSW), kit weight at birth (KWB), pre-weaning mortality (PWM), kit weight at weaning (KWW), body weight at 8 weeks (BW8), growth rate (GR) and market weight (MW). Data were first corrected for the fixed effects of sex, season of kindling, the does’ age and parity using the generalized linear model procedures. The means obtained were used to estimate the heterosis, reciprocal effects and the combining abilities. Results obtained showed breed differences in general combining ability (GCA) for litter size at birth and at weaning with CH (6.4 and 5.9) and NZ (6.3 and 5.6) breeds being superior to BV (5.9 and 5.1) counterparts. Again, in terms of growth, the CH and the BV breeds produced kits with higher body weight at kindling as compared to kits from NZ breeds. Analysis for specific combining ability (SCA) showed that the BV X CH and their reciprocals were the best combiners for litter size at birth and at weaning. The BV and CH breed combination recorded the highest percent positive heterosis of 10.6 and 15.5 for LSB and LSW respectively. In terms of KWB, BW8 and MW, it was observed that NZ and BV cross recorded a positive heterosis of 6.4 and 12.9 for KW and BW8, respectively. The BV and NZ cross combination recorded negative heterosis (-18.2) for BW8. It is therefore recommended that for improvement in litter size, cross combinations involving BV and CH be advocated whereas for growth performance, any cross combination involving NZ as sire line is recommended.
Key words: maternal, general combining ability, specific combining ability, heterosis
According to Ansah et al (2014) rabbit has gained much prominence in recent times due to its high prolificacy, comparatively low production cost, limited or no cultural and religious restriction on its production, the high nutritive value of its meat and as a tool for economic empowerment for the vulnerable in society. In Ghana, there are quite a number of breeds of rabbit reared by farmers with varied performances in terms of growth and reproductive abilities. According to Apori et al (2014) the notable meat breeds of rabbits raised in Ghana are the New Zealand White, the most popular breed, Chinchilla, California white, Blue Vienna and Flanders.
Due to the variations in the performances of these breeds (Apori et al 2014), breeders are trying different crossbreeding strategies aimed at developing the most suitable breed for the Ghanaian conditions. The primary objective of crossbreeding is to produce comparatively better or superior genotypes with faster or higher growth rate and increased litter size, as well as greater adaptability to variable and difficult environments (Baranwal et al 2012). According to Apori et al (2014) crossbreeding has offered a much better alternative for improving on the growth rate and litter size at birth for the local or existing breeds instead of relying on fast-growing foreign breeds, which might have problems adapting into the warm and humid environments, and also the problem of completely eroding the gene pool of locally adapted breeds with positive traits like high survival rate and good mothering ability. One common challenge in crossbreeding is the choice of breed to serve as sire or dam line, hence the reliance on reciprocal crossing. Reciprocal effect is therefore the deviation between crosses of two parental lines in which their roles were reversed, which according to Viana (2007) is useful in defining the extent of genetic dissimilarities between the combining lines.
Diallel analysis is a common method to analyse the genetic variability among combinations of breeds and their crosses and to estimate the combining abilities, heterosis and inheritance of traits (Baranwal et al 2012). The combining abilities could be general or specific. General combining ability (GCA) reflects additive effects and additive interactions, while specific combining ability (SCA) reflects dominance and epistasis, plus components of additive epistasis (El-Bayomi et al 2012). Studies on combining abilities are useful in understanding the nature of genetic variance. They are used in breed improvement programmes to choose suitable parents for developing either hybrids or new lines. Previous studies on combining abilities involving different breeds of rabbits by Fayeye and Ayorinde (2000), Kabir et al (2012) and Adenaike et al (2013) showed that certain breeds combined well while other combinations resulted in reduced performance. In Ghana, there is no work on breed combinations to identify the best combiners apart from the various performance studies on the individual breeds. This study was therefore undertaken to evaluate the growth and reproductive performance of different breed combinations through diallel crossing in order to identify the best combiner or genotypes that could be recommended for commercial production in Ghana.
Information on previous studies of this type should be mentioned, and what make this different.
The study was carried out at the rabbitry of the Teaching and Research Farm of the University of Cape Coast, Ghana. The study area lies within 5º 44' N and 0º 54' W. The area has average annual rainfall ranges of 1000mm to 2000 mm, with average mean annual minimum and maximum temperatures of 200C and 300C, respectively, and a relative humidity of 65 to 80%.
Rabbits used in this study belonged to three maternal breeds, namely New Zealand white, Blue Vienna and Chinchilla white and their crosses. The lines were generated through a diallel crossing involving New Zealand white, Blue Vienna and Chinchilla to obtain F1, F2, F3 and then F4 generations. The records of the F4 (both pure and crossbred) were used for the current study since they were the ones close to the genetic equilibrium. These lines were kept at the experimental rabbitry of the Department of Animal Science, University of Cape Coast, Ghana since 2017. These lines were being selected for increased litter size, at weaning and weaning weight.
The rabbits were selected as the parents of the next generation in two steps. In the first step, does and bucks were selected based on their weaning weight (6-week body weight). After 32 weeks of age, the reproductive performance traits (litter size at birth and at weaning) of the does were recorded. In the second step, does were selected based on litter size at birth and at weaning and bucks were selected based on the performance of their full and half sisters. Average selection proportion of about 40% for does and 5% for bucks were applied in each generation, where after 400 does and 80 bucks were selected to produce the next generations.
Each doe and its litter were fed together during the first 6 weeks and the kits weaned after 6 weeks of age. The weaned rabbits were however kept in groups of four and five in standard galvanised iron cages measuring 75 x 45 x 35 cm and provided with similar management throughout the study period. The animals were raised under the intensive system. In the morning, the animals were fed diets containing 16% crude protein, 14% crude fibre and 2400 kcal metabolizable energy at a rate of 75 g/d up to 6 weeks of age and 100 g/day from the 7th to the 12th week. The lactating does and kits were fed main diets of 200-250g/day, according to their body weight and litter size, and supplemented with green fodder made up of guinea grass, cabbage leaves, Euphorbia spp and Desmanthus virgatus. They were provided with potable water ad libitum. A standard prophylactic endo- and ecto-parasitic control schedule was followed. The bucks and does were used for crossing from 24 weeks old even though they attained sexual maturity between 21-23 weeks. This was aimed at ensuring the rabbits were both sexually and physiologically matured due to its effect on the rabbits’ performance (Saha et al 2013).
Data on 2,014 rabbits made up of 808 purebred kits kindled in 60 litters and 1,206 crossbred kits from 86 litters from the three breeds were used. Growth parameters considered were kit weight at birth (KWB), kit weight at weaning (KWW), body weight at 8 weeks (BW8), market weight (MW) and growth rate (GR). In terms of reproductive performance, data collected were litter size at birth (LSB) and at weaning (LSW) and pre-weaning mortality (PWM), that is, deaths occurring during the first six weeks. The LSB and LSW were calculated as the number of live kits/bunnies kindled and weaned respectively. This was done by counting the number of kits/bunnies per litter at birth and at weaning. The KWB were obtained within 24 hours after kindling. The kits from a particular doe were carefully picked with gloved hand and weighed on a 500 g capacity electronic top-loading balance. The KWW was the weight of the bunny at 6 weeks (weaning age) and it was taken by weighing individually the kits/bunnies using a 2.0 kg capacity electronic top-loading balance with 100 g gradation/precision. The GR was calculated as the weight gained over the 12-week period in grams. The BW8wk and MW were the weights (kg) of the rabbits at 8th and 12th weeks respectively. This was done using a 5.0kg capacity top-loading balance with 500g gradation or precision.
The differences brought about by differences in sex, season of kindling, the does’ age and parity were corrected for using the Generalized Linear Model procedure of GenStat (12th Edition). Where differences in means were identified, the means were separated using the least significant difference at 5% level. The statistical model used was as follows:
| Yijk = μ +gi + gj + sij + mi + mj + rij + εijk |
Where;
Yijk = variable analysed
μ = overall mean
gi = the effect of the general combining ability of the ith maternal breed of rabbit
gj = the effect of the general combining ability of the jth paternal breed of rabbit
sij = the effect of the specific combining ability of the cross (i x j)
mi and mj = maternal effects of the ith and jth breed
rij = reciprocal (sex-linked) effect of the cross
εijk = random error
The mean values obtained were used to estimate heterosis and reciprocal effects for the growth and reproductive performance according to Keambou et al (2010):
| H'= F1 -MP |
Where:
H'= heterotic effect
F1 = mean performance of the crossbred offspring
MP = mid-parents performance
This is expanded as:
|
Where:
A x B = mean performance of the main cross
B x A = mean performance of the reciprocal cross
A x A = performance of the purebred 1
B x B = performance of the purebred 2
The percent heterosis (%H) was calculated as:
| %H = (Hˈ/means of purebreds) * 100 |
The reciprocal effect was:
| RE = PF1 (AB) – PF1 (BA) |
Where:
RE = reciprocal effect of the trait
PFI (AB) = mean performance of the F1 progenies from the cross between A and B
PF1 (BA) = mean performance of the F1 progenies from the cross between B and A
| %RE = (R/Mean of the crossbred) * 100 |
The General Combining Ability (GCA) and Specific Combining Ability (SCA) were estimated using the following formulae as proposed by Kabil et al (2012).
| GCAi = ½n (Yi + Y.i) – 1/n² Y.. SCAij = ½ (Yij + Yji) - ¼n (Yi. +Y.i + Yj. + Y.j) + 1/n²Y.. |
Where:
n = number of records considered
Y.. = sum of all observations
Yi. +Y.i + Yj. + Y.j + … = sum of the respective breeds of rabbits
Several authors (Afifi and Khalil, 1992; Kabir et al 2012; El-Bayomi et al 2012; Fayeye, 2013) have found differences in performances among different rabbit breeds under warm and humid conditions. This variability in LSB and LSW is an opportunity for considering selection among these breeds to improve on the trait. In contrast, Afifi and Emara (1990) found non-significant breed effects on LSB and LSW for similar breeds in Egypt. Intensive and continuous selection within these breeds might have resulted in improved performance and hence differences in their reproductive performance compared with similar results obtained elsewhere. The Chinchilla breeds in Ghana are noted for their high performance in terms of litter size at birth (Apori et al 2014). There was also significant breed effect on growth performance (Table 1) with the CH and NZ breeds being superior in growth rate and market weights. In Ghana, rabbit are reared mainly for meat; therefore it would be worthwhile considering the two breeds for adoption or in any crossbreeding improvement programmes. According to Kabir et al (2012) traits with additive gene effects are expected to respond to selection for their genetic improvement. The current results agree with that of Kabir et al (2012) who also observed significant breed effects on LSB and at LSW for similar breeds studied.
| Table 1. The general combining ability (GCA) for growth and reproductive performance | ||||
| Traits | Breed | p-values | ||
| CH X CH | NZ X NZ | BV X BV | ||
| LSB/no | 6.40±0.1a | 6.31±0.2a | 5.90±0.1b | <0.01 |
| LSW/no | 5.90±0.1a | 5.60±0.2a | 5.10±0.2b | <0.01 |
| PWM/% | 7.00±0.1a | 5.10±0.3b | 6.91±0.3a | 0.01 |
| KWB/g | 55.2±0.3a | 52.1±0.2b | 55.1±0.3a | <0.01 |
| KWW/kg | 1.10±0.1a | 1.00±0.3b | 1.10±0.3a | <0.01 |
| BW8/kg | 1.50±0.5b | 1.61±0.5a | 1.60±0.5a | 0.02 |
| MW/kg | 2.20±0.1a | 2.20±0.3a | 2.00±0.3a | 0.01 |
| GR/g/day | 25.4±0.1a | 25.4±0.1a | 23.8±0.1b | 0.01 |
| ab means within rows with different superscripts are significantly different (p<0.05). Litter size at birth (LSB); Litter size at weaning (LSW); Body weight at 8 weeks (BW8); Kit weight at birth (KWB); Kit weight at weaning (KWW); Market weight (MW); Growth rate (GR); Pre-weaning mortality (PWM); CH- Chinchilla; NZ- New Zealand white; BV- Blue Vienna. In each cross combination, the sire breed is comes first | ||||
Specific Combining Ability (SCA) refers to the degree to which the average performance of a specific cross departs from the additive (Griffing, 1956) which in turn shows the degree of non-additive genetic effect in a population. According to Bora et al (2010) insignificant effect of SCA for any traits is attributed to the limited variance among the genotypes for those traits.
From Table 2, it could be seen that BV X CH and its reciprocal cross recorded the highest SCA values for LSB and LSW as compared to the other cross combinations. This makes them the best combiners when they are to be used as sire and dam lines to exploit non-additive genetic variance. In selecting for litter size at kindling, the BV and CH and the reciprocal might be the best option. Even though the BV breeds were found to be inferior when it comes to litter size at birth and at weaning, it really combined well with the CH breeds. This might probably be due to the wide genetic differences between the two breeds, and hence the need to exploit non-genetic gene action in terms of reproductive capabilities. The SCA for growth (market weight and growth rate) was however low or poor in the BV X CH cross combinations, confirming the assertion that BV and CH breeds are poor meat producers (Obasi and Ibe 2008). This agrees with results obtained by Brun et al (1992); Krogmeier et al (1994) and Kabir et al (2012) who also recorded low SCA for growth traits for similar breed combinations. According to Nagpure et al (1991) negative or low SCA is an indication that crossing to utilize non-additive gene effects will result in depression in value of such traits. In terms of growth traits like growth rate and market weight, it was observed that the progenies from the crossbred significantly outperformed their purebred counterparts with the breed combinations (both main and reciprocal) CH X NZ and NZ X CH recording higher market weights with higher growth rates than all the cross combinations.
| Table 2. The specific combining ability (SCA) for growth and reproductive performance | |||||||
| Traits | Specific Combining Ability (SCA) | p-values | |||||
| Main cross | Reciprocal cross | ||||||
| NZ X BV | CH X NZ | BV X CH | CH X BV | NZ X CH | BV X NZ | ||
| LSB/no | 6.31±0.1b | 6.21±0.1b | 6.81±0.2a | 6.81±0.1a | 6.31±0.1b | 6.20±0.1b | <0.01 |
| LSW/no | 6.00±0.2b | 5.90±0.1b | 6.40±0.1a | 6.31±0.1a | 5.95±0.1b | 5.94±0.2b | <0.01 |
| PWM/% | 5.00±0.1b | 5.00±0.3b | 7.10±0.1a | 7.31±0.1a | 5.01±0.1b | 4.88±0.3b | 0.01 |
| KWB/g | 56.1±0.1b | 58.0±0.3a | 55.0±0.3b | 54.1±0.3b | 58.0±0.1a | 58.0±0.3a | <0.01 |
| KWW/kg | 1.31±0.4a | 1.31±0.3a | 1.01±0.1b | 1.01±0.4b | 1.30±0.3a | 1.30±0.3a | <0.01 |
| BW8/kg | 1.50±0.3b | 1.71±0.3a | 1.71±0.3a | 1.51±0.3b | 1.81±0.4a | 1.81±0.3a | 0.02 |
| MW/kg | 2.30±0.1a | 2.30±0.3a | 2.01±0.3b | 2.01±0.6b | 2.31±0.6a | 2.01±0.3b | 0.01 |
| GR/g/day | 26.7±0.1a | 26.8±0.1a | 23.2±0.1b | 23.2±0.1b | 26.7±0.1a | 23.2±0.2b | 0.01 |
| abc means within rows with different superscripts are significantly different (p<0.05). Litter size at birth (LSB); Litter size at weaning (LSW); Body weight at 8 weeks (BW8); Kit weight at birth (KWB); Kit weight at weaning (KWW); Market weight (MW); Growth rate (GR) and Pre-weaning mortality (PWM); CH- Chinchilla; NZ- New Zealand white; BV- Blue Vienna. In each cross combination, the sire breed is comes first | |||||||
| Table 3. Estimates of percent heterosis (%H) for growth and reproductive traits | ||||||
| Traits | Percentage heterosis (%H) | |||||
| Main | Reciprocal | |||||
| NZ X BV | CH X NZ | BV X CH | CH X BV | NZ X CH | BV X NZ | |
| LSB/no | 2.50 | -1.60 | 10.6 | 0.00 | -1.60 | 1.60 |
| LSW/no | 8.20 | 0.00 | 15.5 | 1.60 | 0.00 | 1.70 |
| ASM/days | 0.60 | -0.90 | -0.30 | 0.00 | 0.00 | 0.00 |
| PWM/% | -18.3 | -17.4 | 3.60 | -2.80 | 0.00 | 4.10 |
| KWB/g | 6.40 | 8.10 | -1.10 | 1.60 | 0.00 | -3.30 |
| KWW/kg | 23.8 | 23.8 | -9.10 | 0.00 | 0.00 | 0.00 |
| BW8/kg | 3.10 | 12.9 | 3.20 | 12.5 | -5.70 | -18.2 |
| MW/kg | 2.40 | 4.50 | -4.80 | 0.00 | 0.00 | 14.0 |
| GR/g/day) | 1.40 | 5.30 | -5.70 | 0.00 | 0.40 | 14.0 |
| Litter size at birth (LSB); Litter size at weaning (LSW); Age at sexual maturity (ASM); Kit weight at birth (KWB); Kit weight at weaning (KWW); Body weight at 8 weeks (BW8); Market weight (MW); Growth rate (GR) and Pre-weaning mortality (PWM); CH- Chinchilla; NZ- New Zealand white; BV- Blue Vienna. In each cross combination, the sire breed comes first | ||||||
Estimates of heterosis and reciprocal effects for growth and reproductive performance are presented in Table 3. Heterosis gives an idea of the relationship between the performance of the crossbred offspring and their parents. Higher estimates of heterosis for a given trait indicate that crossbreeding has improved the performance of that trait in the mating concerned. The impact of heterosis is influenced by the genetic diversity or distinctiveness and purity of the breeds that are crossed (Abo Khadiga et al 2008; Ragab et al 2016). A positive heterosis is an indication of a progress made as a result of the crossbreeding carried out while a negative heterosis mean otherwise. It was observed that all the breed combinations involving BV recorded positive heterosis for LSB, and indication that the offspring from these crosses were better than their purebred parents. There is therefore the need to consider these crosses in future crossbreeding programmes. It was however observed that crosses involving the CH and NZ and their reciprocals recorded negative heterosis for litter size at birth. This meant that New Zealand breeds were not able to nick or combine well with Chinchilla for litter size at birth.
Abdel-Azeem et al (2007) and Nwakpu et al (2015) also recorded positive heterosis for LSB and LSW for New Zealand crosses with California or Flemish giant breeds reared under similar environments. Kabir et al (2012) also reported of a positive heterotic effect for LSB between the New Zealand and California white cross. Many other results of positive heterosis regarding litter size have been reported (Youssef et al 2008 and Ragab, 2012). The positive heterotic effects experienced between the Blue Vienna and the Chinchilla main crosses and NZ and BV main and reciprocal crosses in the present study could be harnessed in future breeding programmes for increased LSB. According to Brun et al (1992) differences in LSB among breeds of rabbits are attributable to maternal heterosis. The superiority which some breeds of doe exhibit for maternal ability can be attributed to increased milk production, maternal behaviour and care of the litter. Results for heterosis for pre-weaning mortality were negative for most of the cross combinations, except the BV X CH main and BV and NZ crosses. The improvement in pre-weaning survivability for bunnies from these crosses could be attributable to better mothering ability or maternal effects expressed by the two breeds. It was possible that the few litters kindled by the BV breeds were compensated for by better mothering ability to ensure the survival of the fewer kits/bunnies produced. It might sometimes be desirable to experience negative heterosis for some traits such as pre-weaning mortality which is an implication of higher parental or purebred performance as against crossbred performance. The strong negative heterosis is an indication of greater genetic distance between the tested breeds (Keambou et al 2010).
Apart from the BV X CH and BV X NZ cross combinations which recorded negative heterosis for KWB, all the other breed combinations yielded positive heterosis for this growth performance trait (Table 3). This agrees with the findings of Abdel-Hamid (2007), El-Bayomi et al (2012), Kabir et al (2012) and Fayeye (2013) who also recorded positive heterosis for bunny weight at birth among different breed combinations involving New Zealand breeds. The heterotic effects of crossbreeding on growth parameters have however been found to be generally low due to the generally high heritability of growth traits. In this present study, it was observed that all the crosses involving NZ breeds resulted in positive heterotic effects for most of the pre-weaning growth performance apart from body weight at 8 weeks, where the percent reciprocal effects were negative for crosses involving the NZ breeds. There is therefore the need to exploit the maternal effects in obtaining higher or better pre-weaning growth performance. With growth performance, it was observed that only the NZ cross combinations maintained consistency in growth performance right from weaning to slaughter. Abdel-Hamid (2015) also found similar observations for NZ white-Rex crosses and explained that it could be due to non-additive inter-breed genetic effects. On the contrary, Abdel-Azeem et al (2007) reported of negative percent heterosis for post-weaning growth performance.
It was observed that within the purebreds, Chinchilla and New Zealand were superior in litter size at birth and at weaning under the hot and humid environments as compared to the other breeds studied. The progenies from the BV and CH main and reciprocal crosses were found to be superior in terms of litter size at birth and at weaning as compared to the progenies from the other cross combinations. Crossbreeding was favourable in improving reproductive traits in rabbits. For crossbreeding purposes or to exploit hybrid vigour, it is recommended that CH X BV combination and its reciprocal are considered due to the superior SCA values.
The progenies from the crossbred outperformed their purebred counterparts with the CH X NZ and NZ X CH breed combinations being superior for market weights and growth rates, even though more can be done to increase the market weight. In this wise, it is advocated that crossbreeding be done to improve litter size at kindling and at weaning as well as market weight and growth rate. It is therefore recommended that for improved reproductive performance, BV could be used as a sire line and CH as a dam line whereas for the improvement of growth performance, it is recommended that NZ is used as a sire line and any of the breeds could be used as a dam line.
The authors would like to express their profound gratitude to the Directorate of Research, Consultancy and Innovation (DRIC) of the University of Cape Coast (UCC), Ghana for the financial support and the Department of Animal Science of UCC for the technical support.
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Received 19 July 2019; Accepted 16 August 2019; Published 1 September 2019