Abstract
The objective of this study is to estimate variance-covariance components and genetic parameters for growth traits in Tswana cattle. Genetic analyses for average daily gain (ADGs) and growth traits were conducted using 7223 records of animals which were born between 1996 and 2013 from 1662 dams and 188 sires in 54 contemporary groups. Both univariate and bivariate animal models were used. Heritability estimates for growth traits ranged from 0.12±0.03 for BWT to 0.45±0.03 for EWT while those obtained for ADGs were 0.24±0.03 and 0.31±0.04 for ADG1 and ADG2, respectively. Permanent maternal environmental effects were significant for WWT and ADG1. Substantial maternal genetic effects were observed in BWT, WWT and ADG1. Genetic correlations among growth traits and ADGs ranged from 0.19±0.07 between BWT and ADG1 to 0.99±0.02 between WWT and ADG1. Phenotypic correlations among growth traits and ADGs ranged from 0.19±0.01 between BWT and ADG1 to 0.94±0.01 between WWT and ADG1. The two selected lines significantly improved annual genetic gain for all the growth traits and ADG1. Genetic gain for EWT was optimally enhanced in S2 than in S1. The control line exhibited significant annual genetic gain in WWT which was not anticipated and perhaps attributed to asymmetry of selection response. Substantial genetic variations were observed in all growth traits and ADGs suggesting that growth improvement can be attained through selection for growth rate. High genetic correlations between growth traits and ADGs indicated that selection for one of these traits may result in indirect correlated response on the other traits. Key words: Bivariate analysis, genetic correlations, heritability, indigenous cattle, maternal effects, genetic variation.
Highlights
Genetic parameter estimates are necessary for the implementation of genetic evaluations, subsequent assessment and quantification of genetic progress that can be achieved through on-going long term selection in economically based multi-trait breeding objectives (Enns and Nicoll, 2008)
Model 2 was suitable for yearling weight (YWT) and EWT while Model 1 was suitable for ADG2
Direct genetic variances and corresponding heritability estimates obtained in the current study for weaning weight (WWT), YWT, EWT, ADG1 and ADG2 were high, demonstrating the breed’s potential for genetic improvement if planned selection has to be carried out
Summary
Genetic parameter estimates are necessary for the implementation of genetic evaluations, subsequent assessment and quantification of genetic progress that can be achieved through on-going long term selection in economically based multi-trait breeding objectives (Enns and Nicoll, 2008). Many studies have been conducted in recent years across the world with the aim of estimating both phenotypic and genetic parameters and to evaluate their importance in designing breeding programmes for different beef cattle breeds in different production environments (Chevraux and Bailey, 1977; Davis, 1987; Mercadante et al, 2003; Koch et al, 2004; Yilmaz et al, 2004; Enns and Nicoll, 2008; Boligon et al, 2010; Cervantes et al, 2010; Boligon et al, 2013; Shumbusho et al, 2013) This has led to validation of practically achieved progress with theoretically predicted direct and correlated responses for setting up of more efficient breeding programs. Due to the differences in the importance of direct and maternal genetic effects between the breeds, several studies have been conducted to estimate genetic and phenotypic parameters among weight traits of different breeds (Knights et al, 1984; Smith et al, 1989; Lee et al, 2000; Forni and Albuquerque, 2005)
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