Response to selection at two temperatures for fast and slow growth from five to nine weeks of age in poultry.

Cornell Control White Leghorn chicks were grown in a common environment to five weeks of age and selected for fast and slow gain in body weight from five to nine weeks of age at two temperatures, 21.1 degrees (cold) and 32.2 degrees (hot), during which time a constant 50% relative humidity was maintained. All lines were tested each generation in both temperature environments. Selection continued for four generations, with a second replicate started six weeks after the first replicate in each generation. In the hot environment, a 20% reduction (104 g) in five-to-nine-week weight gain was found. The responses to selection for fast and slow growth were symmetrical except in the first generation, when an outbreak of bronchitis confounded selection for body weight with selection for disease resistance and allowed little gain in the slow lines. No genotype-by-environment interactions were found, indicating that selection in either direction in either selection temperature produced equal responses in either test temperature. This suggests that any interactions observed between the growth of strains in tropical vs. temperature climates must be due to some difference between these environments other than the temperature differences studied.