Growth and Carcass Traits of Burgundy Fawn, Flemish Giant and New Zealand White Rabbits and their Crosses

The results of rabbit meat production could be improved by using appropriate breeding methods, namely, selection or crossing. New Zealand White rabbits are counted among typical broiler breeds intended for meat production. Crossing females of medium-size breeds with males of large-size breeds, such as Flemish Giant, aims to enhance the reproduction indices as well as the growth and carcass traits of the offspring (Lavanya et al., 2017). The best effect of such crossing is observed when the animals are slaughtered between the 85^th and 120^th day of life; in this case dressing percentage may even exceed 56% (Zając, 2003). The birth weights of rabbits, reported by various authors, differ according to litter size, birth date as well as the feeding and health condition of females (Ajayi et al., 2018). Such traits as body weight and its daily gain are important indicators of the value of animal productivity traits, constituting a basis for assessing the suitability of rabbit breeds for meat production. High daily gains provide evidence about good feed conversion and high fattening ability (Lukefahr et al., 1996). The present study was designed to determine the growth and carcass traits of purebred and crossbred rabbits of three breeds, in particular the possibility of using Burgundy Fawn and Flemish Giant rabbit breed in commercial farming.

The experiments were performed in standardized experimental conditions, which means that the animals were housed in a heated hall (avg. temp. 15^oC) equipped with water-supply system (nipple drinkers), lighting (14L:10D) and exhaust ventilation. The data for the study consisted of the results concerning the growth of 371 rabbits and slaughter traits of 132 rabbits, the offspring of pure- and crossbred rabbits of three breeds: Burgundy Fawn (BF), Flemish Giant (FG) and New Zealand White (NZW). The experiment was performed according to the breeding program: mating purebreed FG, NZW, BF rabbits and crossing NZW females with FG males (NZW × FG), BF females with FG males (BF × FG) and NZW females with BF males (NZW × BF).
    
Young animals were weaned on the 35^th day of their life and housed in metal cages arranged in batteries. The rabbits were fed ad libitum with pelleted feed containing 16% protein, 18.9% crude fibre and 2.1% fat, constituting 10.2 MJ metabolic energy. The animals were weighed at 7-day intervals from birth to the 84th day of life.
    
Rabbits were slaughtered on the 84^th day of their life, after 24 hours of fasting; after which the carcasses were eviscerated and were subjected to 24 hours of cooling in the temperature of 4^oC. The process of slaughter was conducted with the methods described by Blasco et al., (1993). Hot carcass weight, chilled carcass weight, head weight and giblets weight (liver, lung and heart, kidneys) were recorded. Chilled carcass was cut in two points: between the last thoracic and the first lumbar vertebra and between the last lumbar and the first sacral vertebra, then fore part weight, loin weight and hind part weight were recorded.
The dressing out percentages were calculated according the following formulas:
    
Hot dressing out percentage (I) was calculated as (hot carcass weight / slaughter weight) × 100; hot dressing out percentage (II), as (hot carcass weight with giblets / slaughter weight) × 100 and hot dressing out percentage (III), as (hot carcass weight with giblets and head / slaughter weight) × 100. Chilled dressing out percentage (I) was calculated as (chilled carcass weight / slaughter weight) × 10; chilled dressing out percentage (II), as (chilled carcass weight with giblets / slaughter weight) × 100 and chilled dressing out percentage (III), as (chilled carcass weight with giblets and head / slaughter weight) × 100.
 
Statistical analysis
 
In order to examine the differences between the means for genetic groups, the General Linear Model (GLM) procedure of the SAS software with Tukey’s HSD test (p<0.05) had been used (SAS, 2014). The linear model included genetic group, sex and slaughter year as fixed effects; interaction of fixed effects; litter size at birth as a linear covariate - for growth and average daily gains, then slaughter weight as a linear covariate - for carcass traits.

Growth performance
 
The interaction genotype × sex and the differences between males and females were found to be non-significant.
    
Kids of the FG breed had the heaviest weight at birth than those from the other groups and the differences were significant (Table 1). In the 1^st week of life, the weight of the FG rabbits remained significantly bigger than that of animals from the other groups, while the BF rabbits weighed the least. The other genetic groups of rabbits did not differ significantly in weight. In the 2^nd and 3^rd week of life, the body weight of purebred FG rabbits was significantly heavier than in the other genetic groups. In the 3^rd week, significantly smaller body weights than that of rabbits from the other groups were recorded in the BF × FG and NZW × FG crosses, except for purebred BF and NZW rabbits. In the 4^th week of life, the body weight of the NZW × FG crosses was significantly lower than that of rabbits from the other groups, except for the BF × FG crosses and NZW rabbits. Animals with the FG, BF and NZW × BF genotypes were relatively heavier. At the time of weaning, purebred FG rabbits and the NZW × BF crosses were significantly heavier than rabbits with other genotypes, except for the BF × FG and the NZW × FG crosses. The body weights of purebred NZW and BF rabbits were similar. In the 6^th week of life, the BF × FG crosses and the FG rabbits had the biggest body weight, significantly bigger than rabbits with other genotypes, except for the NZW × BF crosses. In the 7^th week of life, the NZW rabbits had the lowest body weight and that was significantly lower than in the rabbits with the FG and BF × FG genotypes. From the 8^th to 10^th week of life, no significant differences in weight occurred between purebred FG rabbits and the BF × FG crosses, while rabbits with other genotypes weighed less and had similar body weights, whereas significantly smaller body weights were recorded in the NZW rabbits. In the 11^th and 12^th week of life, the body weights of purebred FGs were significantly heavier than those of the rabbits with other genotypes. There were no significant differences in body weight among other genetic groups, except for purebred NZW rabbits. In the 12^th week, the smallest body weight was recorded in purebred NZW rabbits. For the NZW rabbits, other authors reported birth weights from 45.51 g (Jimoh and Ewuola, 2017) to 76.06 g and the weight of 338.14 g in the 3^rd week of life (Topczewska et al., 2013). Body weight at weaning varied depending on climate and feeding. Animals fed with complete pelleted feed reached a heavier body weight, between 759 and 789 g (Kowalska and Bielański, 2011), with the result being similar to that obtained in our research. From the 6^th week of life onwards, when rabbits are fed exclusively with pellets, their growth rate and daily weight gain increase. Chandra et al., (2014) recorded smaller body weight in the 6^th week of life for the NZW rabbits, namely, 535.29 g. In the study conducted by Kowalska and Bielañski (2011), rabbits on the 90^th day of life weighed 2619.5 g, which is more than the animals in our study. The weight of newborn BF kits in our research was similar to that reported by Dalle Zotte and Paci (2013) - 64.1 g and by Bolet et al., (2004) - 71 g. Body weight at weaning noted by Bolet et al., (2004), at 763 g, was lower than the result recorded for our BF rabbits. According to Dalle Zotte and Paci (2013) 6-week-old BF rabbits weighed 1040 g, for animals aged 11 weeks, Dalle Zotte (2005) reported the weight of 2737 g, which is more compared to our experimental animals. For FG rabbits reared in a hot climate, Chandra et al., (2014) observed smaller body weight at weaning (558.54 g) compared to those recorded in our research. Prayaga and Eady (2003) reported a similar weaning weight, about 900 g, while Strychalski et al., (2014) noted similar result for body weight on the 42^nd day of life, that is, 1133 g. According to Prayaga and Eady (2003), the body weight of the FG and their crosses in the 10^th week of life was 1900 g, less than reported by Strychalski et al., (2014) at about 1828 g and less than observed in our research. Bolet (2002) obtained rabbits weighing heavier on the 80^th day of life (3126 g) than our FG rabbits at the same age (2741 g). Compared to our results, crosses of the NZW with the FG rabbits in the study by Zając (2003) showed a lower body weight at birth, 60 g and in the 5^th week of life, 1017 g. The body weight of NZW × FG crosses, reported by Prayaga and Eady (2003) to be 900 g in the 6^th week of life, was close to that in our research. As found by Zając (2003), the body weight of crossbred NZW × FG rabbits between the 12^th and 13^th week was 3059 g. The available literature does not contain any other data on body weight for the BF females crossed with the FG males.
 

 
Slaughter traits
 
The interaction genotype × sex and the differences between males and females were found to be significant for the following traits: slaughter weight, hot carcass weight, chilled carcass weight, loin weight and hind part weight.
    
Rabbits of the FG breed had the heaviest slaughter weight compared to those from the other groups and the differences were significant (Table 2). Significantly smaller slaughter weights were noted for the BF×FG crossbred as well as the NZW and BF purebred rabbits. Significant differences were observed for hot and chilled carcass weight, with the highest results obtained for the FG rabbits and the lowest, for the NZW rabbits. Significant differences were also observed for the weight of fore and hind parts of carcass, whereas the parts from carcasses of purebred FG rabbits were significantly heavier than in the other genetic groups. The weight of loin was similar for all the genetic groups, except for purebred NZW rabbits, with the lowest loin weight. The head weight of purebred FG rabbits was significantly heavier than in the other genetic groups. The effect of genotype was observed for giblet weights; also, significant differences were observed in liver and lungs with heart and kidneys weight. Rabbits of the NZW × FG group were characterized with the lowest liver weight, but high weight of lungs with heart. Significantly higher giblet weights for purebred FG rabbits were observed, with similar results of liver weight for animals from the BF, NZW × BF and BF × FG genetic groups.
 

 
Significant differences in dressing out percentage of rabbits from different genotypes were observed (Table 3). The highest hot and chilled dressing out percentage was obtained for purebred BF rabbits and for the BF × FG crosses, whereas similar results of hot I and chilled I dressing out percentage (without giblets and head) were noted for the NZW × FG rabbits and the recorded differences were significant. Purebred FG rabbits were characterized with significantly lower dressing out percentage, despite the high slaughter weight, with similar result to the NZW rabbits, with the lowest slaughter weight. Compared to our results, Dalle Zotte and Paci (2014) obtained lower results of hot and chilled dressing out percentage, calculated with head and giblets weight, 60.3% and 58.3% respectively, despite the higher slaughter weight amounting to 2805 g and higher hot and chilled carcass weight (1690 g and 1633 g) of BF rabbits on the 112^th day of life. For purebred NZW rabbits, Łapiñski et al., (2018) obtained similar results for hot dressing out percentage, calculated with head and without the giblets- 60.00%, whereas slaughter weight and hot carcass weight on the 91st day of life were higher, 2653 g and 1591 g, respectively. Kowalska and Bielañski (2011) noted similar giblets weight but higher head weight; slaughter weight on the 90th day of life was 2619.5 g, hot and chilled carcass weights were 1232 g and 1213.5 g, while hot dressing out percentage was 53.8%, with the results being similar to our findings. Khanna et al., (2014) obtained similar parts of carcasses of NZW rabbits, with results between 26.31% for fore part, 31.57% for loin and 33.93% for hind part, while in our research loin part weighted almost 200 grams less than fore and hind parts. The slaughter weight of purebred FG rabbits characterized by Strychalski et al., (2014) was lower than body weight of animals from our research - 2545.6 g on the 90th day of life, despite that fact that the chilled dressing out percentage (I) was 48.65%, that is 1.09% higher than those calculated in our research. Bolet (2002) in his work calculated higher hot dressing out percentage (III) of the FG rabbits, at 61.2% for animals with similar slaughter weight, namely 3193 g on the 80^th day of life. Similar results of dressing out percentage were calculated for NZW × FG crosses by Migdał et al., (2018), the dressing out percentage of rabbits was estimated between 48.7 and 51.5% (hot I) and between 47.8 and 49.9% (chilled I), depending on the genotype of animals. Sternstein et al., (2014) noted similar results for hot and chilled carcass weight, fore and hind part weight as well as chilled dressing out percentage of the NZW × FG crosses, namely, between 47.8 and 49.9% (chilled I) for animals on the 84^th day of life. Prayaga and Eady (2003) compared dressing out percentages for the NZW and the FG breeds and their crosses and they obtained similar results for all the experimental groups, above 50% and averaged on 53.2%.

It was found that crossing Burgundy Fawn or New Zealand White females with Flemish Giant males makes it possible to achieve a high final body weight and dressing out percentage; therefore, such crossing should be recommended for the production of slaughter rabbits.

This Research was financed by the Ministry of Science and Higher Education of the Republic of Poland from subwention for University of Agriculture in Krakow for 2019 year.