ANTAGONISM BETWEEN GROWTH AND PRODUCTIVE ADAPTABILITY IN MICE

Six lines of mice, which had been previously selected for high and low protein retention, growth and fat content to a differentiation up to 70% in body weight, were tested for growth, body composition and repro ductive performance in two litters under standard and heat-stress clim atic conditions. The heavier, lean lines showed clear superiority in g rowth and body composition in both environments, in spite of depressio ns in performance under heat stress. Traits of fertility and criteria of productivity and efficiency of reproduction were highly influenced by environment with marked differences between the lines. Medium body size was associated with best reproductive fitness under heat stress, the heaviest lines fell below performance of the highest ones under wa rm stall conditions. The results presented can serve as a model-case f or general considerations about livestock breeding in hot environments .