The objectives of this study were to identify the significant 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) and the unselected control line (S3) in Tswana cattle. Phenotypic analysis for average daily gains (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. Growth traits analysed were birth weight (BWT), weaning weight (WWT), yearling weight (YWT) and eighteen months weight (EWT) while the analysed ADGs were pre-weaning average daily gain (ADG1) and post weaning average daily gain (ADG2). S1 resulted in heavier BWT, WWT, YWT and EWT traits and higher ADGs than in S2. Animals from both S1 and S2 exhibited high weight traits and ADGs than those in S3. The identified significant environmental effects for growth traits and ADGs were sex of the animal, dam age, selection line and contemporary group. The significant environmental factors should always be accounted for whenever genetic evaluation for growth performance of Tswana cattle breed is carried out. Therefore, it can be concluded that mass selection for weaning weight can yield optimal improvement on growth performance of Tswana cattle breed when properly carried out.

Keywords: Animal model, environmental factors, growth performance, growth traits