Survival under heat stress of lines selected for fast and slow growth at two temperatures.

Two replicates of four lines of White Leghorn chickens previously selected for fast and slow gain from 5 to 9 weeks of age in a hot (32.2 C) and cold (21.1 C) selection environment were grown from 5 to 9 weeks in the hot and cold temperature environments and then subjected to high temperature stress at 40.6 C. Four different experiments were run. The percent mortality was analyzed when approximately 50% of the birds had died. Birds reared in the cold environment were heavier and consistently had higher mortality than those reared in the hot environment when both were subjected to acute heat stress (40.6 C). The importance of acclimation to mild heat stress on response to acute heat stress was demonstrated by the mortality difference. No significant differences in survival under heat stress were found between lines selected in the hot and the cold selection environments. Lines selected for fast growth had significantly higher mortality rates under high temperature stress than lines selected for slow growth in replicate 1 but not in replicate 2. Estimates of the genetic correlation between weight gain from 5 to 9 weeks of age and percent mortality under heat stress after two generations of selection in replicate 2 was -.004, and after three generations of selection in replicate 1 was +.076.