The objectives of this study were to identify the environmental factors that influenced growth traits and to evaluate phenotypic response of growth traits to selection at weaning (S1) and 18 months of age (S2) in Tswana cattle. Phenotypic analysis for average daily gains (ADGs) and growth traits were conducted using 7226 records of animals, which were born between 1995 and 2013 from 1662 dams and 188 sires in 54 contemporary groups, using mixed models. Growth traits analysed were birth weight (BWT), weaning weight (WWT), yearling weight (YWT) and 18-month weight (EWT). The analysed ADGs were pre-weaning average daily gain (ADG1) and post-weaning average daily gain (ADG2). The identified significant environmental effects for growth traits and ADGs were sex of the animal, dam age, selection line and contemporary group. S1 resulted in increased BWT, WWT, YWT, EWT and ADGs compared with S2. Animals from both S1 and S2 exhibited higher weights and ADGs than those in the unselected control line (S3). The significant environmental factors should be accounted for whenever genetic evaluation for growth performance of Tswana cattle breed is carried out. Mass selection for weaning weight can yield optimal improvement on growth performance of Tswana cattle when properly carried out.
Key words: Environmental factors, growth performance, growth traits, cattle, selection.
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