Estimates of genetic parameters for growth traits of domestic rabbits in the humid tropics

E C Akanno and S N Ibe*

Department of Animal Science and Technology, Federal University of Technology, P.M.B 1526, Owerri, Imo State, Nigeria
chimaeva@yahoo.com
*Department of Non-Ruminant Animal production, Michael Okpara University of Agriculture,
Umudike, P.M.B 7267, Umuahia, Abia State, Nigeria.

Abstract

Data on 363 progeny from a random bred population of New Zealand White and Dutch breeds of rabbits were used to estimate heritability, genetic and phenotypic correlation of growth traits at different ages in the rabbit. Traits studied were Individual Body Weight (IBW), Ear Length (EL), Body Width (BW), Body Length (BL), Head to Shoulder (HS), Shoulder to Tail (ST) and Length of Leg (LL) at 6, 9 and 12 weeks of age.

Heritability (h²s) estimates were high for IBW (0.43+ 0.35 at 6 weeks), HS (0.45+ 0.35 at 9 weeks), ST (0.56+ 0.40 at 9 weeks), LL (0.43+0.19 at 9 weeks), and HS (0.67+0.45 at 12 weeks). Moderate estimates were obtained for BW (0.30+0.30 at 9 weeks), IBW (0.36+0.35 at 12 weeks) and BL (0.35+0.35 at 12 weeks). Also, heritability (h²s) for EL (0.11+0.21 at 6 weeks), BW (0.14+0.24 at 6 weeks), BL (0.18+0.25 at 9 weeks), IBW (0.11+0.22 at 9 weeks), BW (0.09+0.34 at 12 weeks) and ST (0.06+0.23 at 12 weeks) obtained were low estimates. Genetic and phenotypic correlations were moderate to high and positive between the various traits except for the negative values obtained for genetic correlations between IBW and HS (-0.12), and IBW and BL (-0.01) at 9 and 12 weeks of age, respectively.

Keywords: Body weight, correlations, heritability, linear body measurements, rabbit, tropics.

Introduction

Domestic rabbits are raised in the tropics where they serve as a cheap source of meat for reasons of economy of feeding and high fecundity and prolificacy (Cheeke 1986). Genetic improvement of rabbit is important in order to increase their contribution to the much-needed animal protein in this area. One of the pre-requisites for genetic improvement is knowledge of genetic parameters for important economic traits. Unfortunately, information on these parameters is scanty in available literature for rabbits raised in the tropics.

Reports from Tanzania (Mgheni and Christensen 1985), Ghana (Lukefahr et al 1992), Brazil (Ferraz and Eler 1994), Hawaii (Moura et al 1997) and Nigeria (Odubote and Somade 1992) indicate higher heritability for production traits than is generally reported from temperate environments. Ferraz and Eler (1994) gave heritability estimates for body weight as 0.03 (6 weeks), 0.19 (9 weeks) and 0.26 (11 weeks). For linear body measurements, estimate of heritability as reported by Chineke and Adeyemi (2001) ranged from 0.90+1.46 for length of ear at 56 days to 0.99+0.48 for shoulder to tail at 49 days. Estimates of genetic correlations between rabbit body weights and linear body measurements are scarce in available scientific literature.

The objective of this study was to estimate heritability, genetic and phenotypic correlations between some growth traits of domestic rabbit for application in genetic improvement.

Materials and Methods

Location of study

The experiment was conducted at the Rabbitry of the Teaching and Research Farm, Michael Okpara University of Agriculture, Umudike, Abia State, Nigeria. This location lies in latitude 05^o29'N, longitude 07^o33'E and altitude of 122m above sea level. The zone has a maximum daily temperature of about 33^oC. The annual rainfall is from 2100 to 2500 mm. The mean relative humidity is about 85%, although daily values are usually liable to wide variation.

Experimental animals and their management

Five bucks each of the Dutch (DT) and New Zealand White (NZW) breeds, 30 DT and 14 NZW does were randomly sampled from a population of rabbits maintained at the Unit. All the rabbits were housed in cages and fed ad libitum with pelleted rations containing 16.5% protein and 2.80 Mcal/kg gross energy. Panicum maximum grass and Centrosema pubescens legume were also fed as supplementary ration.

Matings were carried out in the morning and 14 days post-partum according to Table 1. The does were palpated to determine pregnancy. Non-pregnant does were put up for mating again until conception occurred. The litters were examined at birth for defects. Inbreeding was avoided in the herd. A total of 363 progeny resulted from the matings.

Experimental design

The statistical design was the nested (hierarchical) design in which each of a number of sires was mated to a number of dams and each mating resulted in a number of progeny. The model is as in Expression (1). There were two parities.

Y_ijklm = μ + P_i + X_j + S_k + D_kl + E_ijklm … (1)

Where:
Y_ijklm = Observation on the m^th progeny of the I^th doe mated to the k^th sire and belonging to the j^th sex and i^thparity.
μ = Overall mean.
P_i = Fixed effect of parity.
X_j = Fixed effect of sex.
S_k = Random effect of sire.
D_kl = Random effect of dam within sire.
E_ijklm = Random error, independently and identically normally distributed with zero mean and constant variance [iind (0, s²)].

Data collection and analysis

Data were collected on the following traits: Individual Body Weight (IBW) in grams, measured with a weighing scale and linear body measurements (LBMs) namely, Ear Length (EL), Body Width (BW), Body Length (BL), Head to Shoulder (HS), Shoulder to Tail (ST) and Length of Leg (LL). The LBMs were measured in centimeter with a tailor's tape. Measurements were taken at 6, 9 and 12 weeks of age. The data were subjected to analysis of variance (ANOVA) for unequal subclass numbers, using Harvey's (1990) Least Squares and Maximum Likelihood computer programme.

Estimates of variance components were obtained using Method 3 of Henderson (1953). By equating the mean square of each random effect to its expectation, variance components for sire (s²_S), dam (s²_D) and error (s²_E) were obtained. These estimates were used to estimate heritability, genetic and phenotypic correlations.

Heritability estimates were obtained with Expressions 2

       (2) 

Standard errors for heritability estimates were calculated using the formula described by Becker (1984).

Estimates of genetic and phenotypic correlations were obtained using appropriate expressions involving the estimated variance components according to Becker (1984).
 

Results and Discussion

Heritability estimates

The heritability estimates obtained from paternal half-sib (PHS) correlation for the various traits are shown in Table 2. PHS estimates were moderate to high for IBW and BL at 6 weeks of age, for BW, HS, ST and LL at 9 weeks and for IBW, BL and HS at 12 weeks. Low estimates were obtained for EL and BW at 6 weeks, for IBW and BL at 9 weeks, and for BW, ST and LL at 12 weeks of age.

The heritability values obtained for body weight at various ages are similar to 0.42 obtained for body weight at 90 days (Lukefahr et al 1992) and 0.19 and 0.26 for body weight at 9 and 11 weeks of ages, respectively (Ferraz and Eler 1994) under tropical conditions. Also, the estimates obtained in this study for body weight are similar to those obtained in temperate environment (Szendro et al 1988; Castelline and Panella 1988; Lukefahr et al 1996). These results indicate strong contribution of additive genes in the expression of these characters and suggest possible improvement of body weight in rabbits by either pedigree or individual selection method.

For linear body traits, the estimate of heritability for EL at 6 weeks from PHS correlation is in agreement with 0.11 ± 0.24 reported by Chineke and Adeyemi (2001), which means that expression of ear length is largely controlled by the environment. The importance of this trait in heat dissipation has been noted (Lukefahr and Ruiz-Feria 2003). In contrast, PHS estimates for BW and ST at 12 weeks are much lower than the estimates obtained by the same researchers (Chineke and Adeyemi 2001) at 7 weeks of age. These could be due to difference in the age of rabbits and in management.

Moderate to high estimates were obtained for BW at 6 and 9 weeks, for BL at 6, 9 and 12 weeks, for HS at 9 and 12 weeks and ST and LL at 9 weeks. These agree with estimates reported by Chineke and Adeyemi (2001) and indicate the potentials for genetic improvement of rabbits through individual selection. These traits reflect the length of long bones, which have been observed as good predictors of live weight and carcass composition (Tiamiyu et al 2000).

That heritability could not be estimated from PHS for HS and LL at 6 weeks and EL at 9 and 12 weeks (Table 2) is as a result of negative sire variance components in the analysis of variance. Negative variance components are regarded as an indication of negligible contribution of additive genes to variation of the traits concerned. Although theoretically impossible, negative variance components may result with analysis of variance procedure of estimation.

Genetic and phenotypic correlations

Tables 3, 4 and 5 give the genetic and phenotypic correlations between growth traits of rabbits at 6, 9 and 12 weeks, respectively. The genetic correlations are relatively high and positive, ranging between 0.45 and 0.95 for all traits at 6 weeks of age.

Similarly, at 9 weeks of age, the coefficients of genetic correlation between growth traits were high and positive except for IBW and BL (0.32), EL and HS (0.39), BW and HS (0.20), BL and HS (0.45), ST and HS (0.37) and HS and LL (0.39), which were low to moderate with a negative and low estimate obtained for IBW and HS (-0.12).

In week 12, the highest genetic correlation of 0.72 was between BL and EL and the lowest coefficient (-0.01) was between IBW and BL.

On the other hand, phenotypic correlations at 6 weeks were moderate to high and positive, ranging between 0.34 and 0.89, while values obtained at 9 weeks ranged from 0.27 (between IBW and HS) to 0.83 (between ST and LL) and were all positive. In week 12, phenotypic correlations between all traits were positive and relatively high. The genetic and phenotypic correlation coefficients between the different growth traits of rabbits at various ages obtained in this study are consistent with the findings of Chineke (2000), Tiamiyu et al (2000) and Abdullah et al (2003).

The implication of these findings is that, for traits with strong positive genetic correlation, selection for one trait will lead to improvement in the other. This is the phenomenon of correlated response. Nevertheless, the negative correlations observed in some cases indicate that selection for one of the traits could lead to the improvement in the other if a reduction of the second trait is desired.

Genetic correlation coefficients are helpful as guides to selection. For example, selection for a wider and longer body in an index could be effective in a breeding programme aimed at achieving increased body size, as long as linear body traits were very highly and positively correlated with body weight. Secondly, linear body parameters could be used to predict body weight for selection purposes.

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