GENETIC-RELATIONSHIP BETWEEN GROWTH TRAITS IN BONSMARA HEIFER AND BULL CALVES ON DIFFERENT NUTRITIONAL REGIMES

A system of feeder-breeder dimorphism (large, fast growing offspring i n feedlots obtained from small low maintenance cows on natural pasture s) would be profitable in South Africa. The possibility of a genetic b asis for feeder-breeder dimorphism was estimated by calculating geneti c correlations between body mass gains achieved by beef cattle bulls i n feedlots and body mass gains of half-sib heifers under natural pastu re conditions (nutritional managerial and sex differences between grou ps). The genetic correlations were 0.79 +/- 0.11, 0.01 +/- 0.19 and 0. 43 +/- 0.15, respectively for 12-month weight, ADG (average daily gain , weaning to 12 months) and the Kleiber ratio (ADG/metabolic 12-month weight). The genetic correlations for ADG and the Kleiber ratio may in dicate that different genes affect the measurements in half-sib bulls and heifers in different environments. The weaning to 12-month traits in bulls under feedlot conditions are probably genetically independent of the weaning Lo 18 months or 12 to 18 months traits in heifers unde r pasture conditions, as the genetic correlations for ADGs and Kleiber ratios of these traits are closer to zero than to unity (between -0.1 3 and +0.27). Selection for growth of bulls in feedlots would thus not affect the growth of heifers on pastures, which indicates that feeder -breeder dimorphism can be genetically induced for different nutrition al environments.