Correlated responses in productive and adaptive traits and temperament following selection for growth and heat resistance in tropical beef cattle

Selection experiments were established in zebu-derived beef cattle grazed at pasture in the tropics to determine direct and correlated responses to selection. Two lines (AX and UPWT) that differed marginally in their level of adaptation to environmental stressors were selected for high growth, measured by estimated breeding value (EBV) for weight at 600 days. A third line (TEMP) was selected for resistance to heat, measured by low EBV for repeated records of rectal temperature under conditions of heat stress. A control line, CONT (AXBX), was also established, with minimum selection differential for 600-day weight. A derived control line, CONT (AX), based on differences between AX and AXBX lines in the foundation generation was used with CONT (AXBX) and genetic trends to measure responses. Selection for high growth increased weights, period weight gains and direct and maternal genetic components of weights between birth and 18 months of age, but did not always increase mature cow weights. Calf birth and mature cow weights were restricted due to environmental effects. Resistance to heat stress improved in UPWT by selection for high growth. Resistance to ticks improved in the more tick-susceptible AX line, but did not change in the more resistant UPWT line. Resistance to worms consistently decreased (worm counts increased) in UPWT, but did not change in AX. Worm numbers over the experimental period were low and may not have biologically impacted on growth rates. There were no general changes in male or female fertility traits, carcass and meat quality attributes or feed efficiency due to selection for high growth. Selection for low EBV for rectal temperature reduced rectal temperatures in TEMP animals. Small correlated improvements in resistance to ticks and worms occurred in TEMP animals. Changes also occurred in pattern of fat distribution in the carcass. TEMP carcasses had higher marbling than CONT (AXBX) carcasses. There were no significant changes to other attributes through selection for low EBV for rectal temperature. Selection within this line may reflect selection for factors other than resistance to heat stress per se. (C) 2003 Elsevier B.V. All rights reserved.