Metabolic and endocrine changes induced by chronic heat exposure in broiler chickens: growth performance, body composition and energy retention.

The present study was performed in order to investigate the effect of chronic heat exposure (32 degrees, constant) on growth, body composition and energy retention of broiler chickens in relation to age. At 2 and 4 weeks of age, fifty-four male Shaver broiler chickens were allocated to three treatments according to the following design: 22 degrees, ad lib. feeding (22AL); 32 degrees, ad lib. feeding (32AL); and 22 degrees, pair-feeding with the 32 degrees group (22PF). Ambient temperature was kept constant at either 22 or 32 degrees for 2 weeks. Heat exposure decreased feed intake by 14% between 2 and 4 weeks and by 24% between 4 and 6 weeks of age. Even with the same feed intake, chicks gained less weight at 32 degrees than at 22 degrees, 5.5% 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 degrees compared with 2.06 at 22 degrees in 22AL birds for the period 4-6 weeks. Body composition appeared significantly affected by high ambient temperature. Feathering was reduced at 32 degrees 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 lib.-fed (22AL) groups at 22 degrees. 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 degrees (0.56) was twice that for those maintained at 32 degrees (0.28). These modifications should be investigated further in relation to metabolic and endocrinological changes.