Performance at two rearing temperatures of White Leghorn lines selected for increased and decreased survival under heat stress.

Body weight and percent mortality resulting from heat stress at 40.6 C were determined on two lines selected for resistance and susceptibility to heat stress by the Florida Agricultural Experiment Station and reared from 5 to 9 weeks of age in two temperatures, 32.2 C and 21.1 C. The resistant line was significantly smaller in body weight at 5 and 9 weeks of age and had less gain in weight from 5 to 9 weeks of age than the heat susceptible line. Birds reared in the cold environment had significantly larger 9-week body weight and gain from 5 to 9 weeks of age than those reared in the hot environment. At 9 weeks of age the birds were heat stressed at 40.6 C and 50% relative humidity. The heat resistant line had a significantly lower percent mortality than the heat susceptible line. Birds reared at 21.1 C had a significantly greater percent mortality when heat stressed than those reared at 32.2 C. No interaction occurred between the selected lines and the temperatures in which they were reared for percent mortality or any of the body weight traits. Comparison with other experiments involving the same base population from which the Florida lines were selected indicated that the response to selection for resistance was greater than that to selection for susceptibility. It is suggested that natural selection for heat resistance could be responsible for the asymmetry of response. An estimate of the genetic correlations (rG) was derived as the geometric mean of the ratios of correlated to direct responses. The estimate of rG between body weight gain from 5 to 9 weeks of age and percent mortality under heat stress was .16.