CONSEQUENCES OF SELECTION FOR GROWTH AND HEAT-RESISTANCE ON GROWTH, FEED CONVERSION EFFICIENCY, COMMERCIAL CARCASS TRAITS AND MEAT QUALITY OF ZEBU CROSSBRED CATTLE

Thirty-five steers and 37 heifers were grown at pasture and finished in a feedlot to meet Australian domestic market specifications, i.e. 160-200 kg carcass weight with 5-9 mm subcutaneous fat at the P8 rump site. The animals were from lines that had been selected for either high growth rate to 600 days (UPWT) or for low rectal temperature under conditions of high ambient temperatures (TEMP) and from a control line (CONT). They were slaughtered in six groups at fortnightly intervals when liveweight of individuals was estimated to satisfy market requirements. Animals from the UPWT line were heavier at all ages than animals from the TEMP and CONT lines (P < 0.05) and had higher liveweight gains at pasture (P < 0.01). There was no difference between the lines in liveweight gains in the feedlot. At the same carcass weight, UPWT animals had leaner carcasses (P < 0.01) but similar levels of marbling to CONT line animals. TEMP line animals were also leaner than CONT line animals (P < 0.01), but had more marbling (P < 0.01). There was no significant difference between lines in feed conversion efficiency. Steers were heavier (P < 0.001) than heifers at all ages, grew faster (P < 0.001) except during the immediate post-castration period, had higher feed intakes (P < 0.05) but better feed conversion efficiencies (P < 0.05), and at the same carcass weight were leaner (P < 0.01) and had less marbling (P < 0.05). Meat colour, cooking loss, ultimate pH and meat tenderness did not differ significantly between lines or sexes. These results are discussed with reference to the use of selection for growth or heat resistance to improve growth and carcass traits of beef herds in tropical areas.