The present study was performed in order to investigate the effect of chronic heat exposure (32",constant) on growth, body composition and energy retention of broiler chickens in relation to age. At 2and 4 weeks of age, fifty-four male Shaver broiler chickens were allocated to three treatments accordingto the following design: 22", ad lib. feeding (22AL); 32", ad lib. feeding (32AL); and 22", pair-feeding with the 32" group (22PF). Ambient temperature was kept constant at either 22 or 32" for 2 weeks. Heatexposure decreased feed intake by 14% YO between 2 and 4 weeks and by 24% YO between 4 and 6 weeks of age. Even with the same feed intake, chicks gained less weight at 32" than at 22", 55% less in young chickens and 22% less in older ones. Hot environmental conditions thus resulted in decreased feed efficiency; the feed:gain ratio was 2·85 at 32" compared with 2·06 at 22" in 22AL birds for the period4—6 weeks. Body composition appeared significantly affected by high ambient temperature. Feathering was reduced at 32" in absolute weight but not as a proportion of body weight. Heat-exposed birds showed a decrease in body protein content, protein gain and protein retention. Group 32AL birds were fatter than the pair-fed (22PF) or ad fib.-fed (22AL) groups at 22". The percentage of energy retained as fat was 79 in heat-exposed chickens compared with 64 in the control groups. The energy retained as protein:energy retained as fat for groups maintained at 22" (0·56) was twice that forthose maintained at 32" (0·28). These modifications should be investigated further in relation to metabolic and endocrinological changes.
