Phenotypic and genetic performance of Tswana cattle selected for early growth traits in Botswana
Keoletile, Mogomotsi Innocent
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The objective of this study was to evaluate the phenotypic and genetic performance of Tswana cattle mass selected for early growth traits. The approach will help optimize genetic selection in Tswana cattle through identification of important genetic and none genetic factors influencing growth, reproductive and calf survival traits and ultimately aid in redesigning active breeding program for this breed. Non-genetic effects on growth, average daily gains (ADGs), mature cow weight (MCW), reproductive and calf survival to weaning traits were identified so that they can be adjusted during the genetic analyses of these traits. Phenotypic and genetic analyses for 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 contemporaries using both univariate and bivariate animal models. Analyses of environmental and genetic effects for calf survival traits were done using 7223 records of animals which were born between 1996 and 2013 from 1659 dams and 188 sires in 54 contemporaries. Analyses of environmental and genetic effects for age at first calving were done using 818 records of animals born between 1998 and 2013 from 611 dams and 136 sires in 49 contemporaries, while calving interval analyses were done using 1804 records of cows born between 1999 and 2013 from 496 dams and 121 sires in 45 contemporaries. Analyses of environmental and genetic effects for mature cow weight trait were done using 19301 records of cows born between 1996 and 2010 from 610 dams and 13 7 sires in 54 contemporaries. Growth traits and average daily gains were analysed using mixed animal models that include and exclude maternal genetic effects fitted using the Restricted Maximum Likelihood (REML) procedures in Animal and Sire Restricted Maximum Likelihood (ASREML) program. The best model for each trait analysis was selected based on a log likelihood ratio test (LRT). Growth traits analysed were birth weight (BWT), weaning weight (WWT), yearling weight (YWT), eighteen months weight (EWT), pre-weaning average daily gain (ADGl) and post weaning average daily gain (ADG2). Reproductive traits analysed were age at first calving (AFC) and calving interval (CI). Age at first calving was analysed using univariate animal model while calving interval was analysed using repeatability model. Mature cow weight trait was also analysed using repeatability model. Calf survival to weaning was analysed as binomial trait using generalised mixed linear logistic model with logit link function in the ASREML program. The identified significant environmental effects for growth traits and ADGs were sex of the animal, dam age, selection line and contemporary group while for reproductive traits the significant effects were selection line, calving year and season. Calf survival to weaning was significantly influenced by calf sex, selection line, calf-birth weight and dam age while the significant environmental effects for mature cow weight were selection line, cow age and contemporary group. Heritability estimates for growth traits ranged from 0.12±0.03 for BWT to 0.45±0.03 for EWT while the estimates obtained for ADGs were 0.24±0.03 and 0.31±0.04 for ADG 1 and ADG2, respectively. The estimated heritability values for reproductive traits were 0.07±0.02 for CI and 0.10±0.07 for AFC. The respective heritability estimates for calf survival to weaning and mature cow weight traits were 0.07±0.05 and 0.26±0.03 . Permanent maternal environmental effects were significant for WWT and ADG 1. Substantial maternal genetic effects were observed in BWT, WWT and ADG 1. Genetic correlations among growth traits and ADGs ranged from 0.19±0.07 between BWT and ADGl to 0.99±0.02 between WWT and ADGl. Phenotypic correlations among growth traits and ADGs ranged from 0.19±0.01 between BWT and ADGl to 0.94±0.01 between WWT and ADG 1. Genetic correlations between growth traits and MCW ranged from 0.15±0.17 between BWT and MCW to 0.84±0.19 between YWT and MCW. Phenotypic correlations between growth traits and MCW ranged from 0.15±0.04 between BWT and MCW to 0.31±0.03 between EWT and MCW. Substantial genetic variations were observed in all growth traits and ADGs suggesting that genetic 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. Low genetic variability obtained in reproductive traits and calf survival to weaning trait indicates that improvement of these traits through genetic selection may be slow. The existence of significant genetic variability and moderate repeatability in mature cow weight trait coupled with high genetic correlation between this trait and early growth traits suggest that caution should be exercised when selecting for growth traits to avoid undesirable resultant change in mature cow weight. Improved performance in Tswana cattle breed can be attained through selection based on breeding values estimated from multi-trait analysis. Economic values should be established for growth and reproductive traits of this breed and selection indices consisting of these traits should be considered in future breeding efforts.