Estimates of genetic and phenotypic parameters for preweaning growth traits of N'Dama (Bos taurus) calves in the humid tropics of Nigeria

A R Abdullah* and O Olutogun

Department of Animal Science, University of Ibadan, Ibadan
^*Department of Agriculture, Babcock University, Ilishan-Remo, Ogun State, PMB 21244, Ikeja, Lagos, Nigeria
arabdullah2002@yahoo.co.uk

Abstract

Birth weight (BWT), 205-day adjusted weaning weight (205DWT) and average daily gain till weaning (ADG) data of 1277 N'Dama calves were analyzed by nested analysis of variance procedure of SAS to derive half-sib estimate of variance and covariance components which were used to estimate heritability and both genetic and phenotypic correlations between the pairs of traits. The repeatability estimates were also computed for the traits using records of 594 calves.

The heritability estimates increased with age of calves from 0.10 ± 0.052 for BWT to 0.39 ± 0.09 for 205DWT. The genetic correlation ranged from 0.47 between ADG and BWT, 0.53 between BWT and 205DWT to 0.99 between BWT and ADG. Phenotypic correlations between the traits followed the same trend ranging from 0.15 between ADG and BWT, 0.58 between BWT and 205DWT to 0.99 between 205DWT and ADG. The repeatability estimate was 0.10 ± 0.004 for each of the three traits. The low estimates of all the genetic parameters in this study confirmed the effect of poor management and nutrition in limiting the expression of the true genetic worth of the indigenous stock.

Keywords: Genetic and phenotypic parameters, humid tropics, N'Dama, Nigeria, preweaning growth traits

Introduction

The potential for genetic change in economically important traits of livestock such as beef cattle depends to a large extent on the magnitude of the genetic variances and heritability of each traits considered in selection. The estimates provide information about the genetic nature of traits and are needed to predict direct and correlated responses to selection and determine the method of selection (Mohiuddin 1993). Genetic and phenotypic correlations between traits are equally essential for determining the optimum weighting and expected response to selection in a multi-trait selection programme (Nicholas 1991; Falconer 1994).

Estimates of heritability and genetic correlations are functions of variance and covariance components and are, in general, specific for a particular population in which they were estimated and may vary over time due to selection and management decisions (Aaron et al 1987; Koots et al 1994). Thus, reliable estimates of variance and covariance components are invaluable in the development of efficient long-term improvement schemes.

Predicting breeding value of an individual animal is very important in animal breeding. It enables the breeder to determine the transmitting ability of each animal for the desired economic traits. The use of lifetime averages as an aid to selection for traits on which repeated observations of phenotype can be obtained involves using estimates of repeatability to calculate adjusted averages appropriate for comparing individuals with different numbers of records. Repeatability is the proportion of total phenotypic variance for a trait attributable to permanent difference among individuals. It quantifies the proportion of an individual's superiority or inferiority (based on a single measurement of a trait) that is expected to be expressed in future measurement. Hence repeatability is used in estimating lifetime producing abilities of individual animal in predicting the change in performance of a population due to culling. Early estimation of the future Most Probable Producing Ability (MPPA) of each dam with respect to preweaning growth traits of her calves is of immense advantage because early growth trait of calves has been shown to be directly related to later performance in the feedlot (Christian et al 1965) and subsequent growth traits (Petty and Cartwright 1966).

Preweaning growth rate has been recognized as one of the most important traits that determine economic efficiency of beef cattle production (Robertson et al 1986) and also as an important selection criterion because of the ease with which it can be measured (Gregory 1965). Calf performance may be considered as a trait of the cow in beef herds analogous to milk production of diary cows in successive lactations (Turner and Shrode 1986). A cow's genetic influence on the weaning weights of her progeny is attributable to components from the additive genotype of the cow for growth potential and also to her genotype for milk production and maternal ability (Hohenboken and Brinks 1969). Consequently, the estimates of repeatability of repeated performance of the dam enables early selection for that trait.

The parameter estimates reported in literature for preweaning traits in N'Dama cattle are quite variable being estimated from small datasets (Carew et al 1986; Abanikannda and Olutogun 1999). Kaplon et al (1991) explains that utilization of such parameter estimates to set up a breeding programme for a specific population can be very misleading.

The objectives of this study therefore was to estimate variance components for heritability and repeatability of birth weight (BWT), 205-d adjusted weaning weight (205DWT) and average daily gain until weaning (ADG) as well as the genetic, environmental and phenotypic correlations between these preweaning growth traits in N'Dama calves maintained at Fashola stock farm in the humid tropics of Nigeria.

Materials and methods

Data source

The data used for this study were preweaning performance records of N'Dama calves obtained from Fashola Stock farm located in a low-to-medium tse-tse fly challenge belt on the southwestern fringes of the derived Guinea savanna zone of Nigeria (7^o 54 ¹ N ,3^o 43¹ E and 229 m above sea level. Detailed description of the breed, breeding and management of the herd has been discussed previously (Abdullah 2004)

Data preparation and analysis

The data used for the estimation of heritability, genetic, environmental and phenotypic correlations consisted of 1277 records of birth and weaning weights of purebred N'Dama calves from 1032 dams and 199 sires while those used for the estimation of repeatability were birth and weaning weight records of 594 calves from 172 dams and 112 sires. The dams selected for repeatability estimation were those with number of calves ranging from 3 to 7. The calves included in both datasets were those with weaning ages between 160 and 250 days and the adjusted 205-day weaning weight and average daily gain till weaning was completed for each calf using the procedure of BIF (1990) as adapted by Abdullah (2004).

Statistical analysis

The variance components used for the estimation of heritability and repeatability of each preweaning trait were obtained by Variance Components Procedure (PROC VARCOMP) of SAS (1999) using Restricted Maximum Likelihood (REML) method.

The fitted random model for paternal half-sib heritability estimation is as follows (Becker 1975; Khan and Singh 2002 ).

Y_ij = μ + α_i + e_ij

Where:

Y_ij= the record of BWT, 205DWT and ADG of individual calf of each sire;
μ = overall mean; α_i= the random effect of i^th sire and
e_ij= the uncontrolled environmental and genetic deviations attributable to individual calves within each sire group.
All effects are random, normal and independent with expectations equal to zero.

The variance components used for the repeatability estimates for each trait were derived based on a model that consider only the dam variance as follows (Becker 1975; Khan and Singh 2002).

Y_ij = μ + α_i + e_ij

Where:

Y_ij= the record of BWT, 205DWT and ADG of individual calf of each dam;
μ = overall mean;
α_i = the random effect of i^th dam and
e_ij= the uncontrolled environmental and genetic deviations attributable to individual calves within each dam group.
All effects are random, normal and independent with expectations equal to zero.

The standard errors for heritability and repeatability were computed using the formula given by Swiger et al (1964) for unequal number of calves per sire and dam respectively.

The covariance components due to sire and progeny within sire for all pairs of traits were obtained by Nested Procedure (PROC NESTED) of SAS (1999) while the genetic, environmental and phenotypic correlations between all pairs of traits based on sire components of variance and covariance using single parent design were computed accordingly (Becker 1975; Khan and Singh 2002).

Results and Discussion

Heritability Estimates

The variance components and heritability estimates ± standard errors of BWT, 205DWT and ADG are shown in Table 1. The estimates of additive and residual components of variance when expressed as a percentage of the total variance was about 30 percent and 70 percent respectively and were almost identical for both 205DWT and ADG. This trend was also observed by Kennedy and Henderson (1975a). The heritability values were low but in agreement with values reported by other researchers. The heritability value of 0.10±0.05 for BWT in this study was close to 0.149 reported by Peters et al (1998) for the same breed in similar environment but lower than 0.37 reported by Johnson et al (1992). The heritability values of 0.39 ± 0.09 and 0.38 ± 0.09 recorded for 205DWT and ADG were close to 0.35 ± 0.03 and 0.34 ± 0.03 reported by Tizikara (1988) for 200-day adjusted weaning weight and ADG respectively for the same breed on the same farm. Leigh et al (1984) also using a sub-set of the data from the same farm reported a value of 0.30 ± 0.14 for ADG. The heritability obtained for these pre-weaning traits in this study is however lower than 0.66 ± 0.25 estimated by Olutogun and Dettmers (1977) for 205DWT for a ranch-managed indigenous beef cattle population in the southern Guinea Savannah of Nigeria and 0.84 ± 0.50 given by Wheat et al (1972) for six-month's weight in White Fulani at Shika Research Station in the sub-humid zone of Nigeria. The lower estimates of heritability value for the preweaning traits obtained in this study could be due to trypanosomiasis challenge on the animal. The heritability of 205DWT recorded in this study is equally higher than the average value of 0.20 reported by Mohiuddin (1993) across breeds of cattle from different environments and production systems.

The similarity in the percentage values of both the additive and residual estimates as well as the almost equal heritability estimates recorded for the two preweaning growth traits support the contention that both traits are under the control of the same set of genes (Kennedy and Henderson (1975b).

Genetic and phenotypic correlations

The estimates of genetic and phenotypic correlations between all the preweaning growth traits are presented in Table 2. The genetic correlations between all pairs of preweaning growth traits were consistently high and positive. They ranged between 0.47 for BWT and ADG and 0.99 for 205DWT and ADG. Phenotypic correlations between BWT and all other traits followed the same trend. The genetic correlation of 0.47 between BWT and ADG and 0.53 between BWT and 205DWT estimated in this study is similar to 0.46 between BWT and ADG and 0.68 between BWT and 205DWT reported in a review by Barlow (1978). The phenotypic correlation of 0.58 between BWT and 205DWT obtained in this study is higher than the range of 0.37 - 0.41 reported in the review of Barlow (1978) and this implies that BWT is a good indicator of subsequent development of the calf. The high and positive genetic correlations between all the growth traits in this study implies that they are all being controlled by similar genes and thus selection for any one of these traits would lead to positive changes in the other. This agrees with the report of Kennedy and Henderson (1975b) and supports the contention that both traits are essentially the same measure of growth and are thus under the influence of similar genes. Thus the two traits can be regarded as the same trait in a selection programme.

Repeatability estimates

The estimates of repeatability for all the preweaning growth traits are shown in Table 3. The repeatability estimate of 0.15±0.004 obtained in this study for BWT is within the range of -0.03 and 0.41 reported for various breeds of cattle in a review by Mohiuddin (1993). However, it is below the preferred range of 0.20 and 0.30 suggested for the trait (Legates and Warwick 1990). The estimates of repeatability for 205DWT and ADG are below the range of 0.21 and 0.52 reported by Mohiuddin (1993) for different cattle breeds.

The low estimates of repeatability for the three preweaning traits suggests the relative importance of permanent effect of the unfavourable climatic conditions of the humid tropical climate coupled with the trypanosomiasis challenge on the animal and the substandard level of management prevalent on the farm earlier reported by Abdullah and Olutogun (2002).

Conclusions

• The results of this study revealed that all the preweaning traits in N'Dama cattle are of low heritability, and repeatability and are all positively correlated.

• The low estimates of all the genetic parameters in this study confirmed the effect of poor management and nutrition in limiting the expression of the true genetic worth of the indigenous stock.

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