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.

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The effect of chronic heat exposure on carcass quality of broilers: proportion of lean and fat tissues, fat content, and fatty acid composition, was investigated. One hundred and eight 4-wk-old male chickens were brooded in individual battery cages in two controlled-environment rooms at constant ambient temperature (22 or 32 C) until 7 wk of age. They were equally distributed into three treatments: 22 C, ad libitum feeding (22AL); 32 C, ad libitum feeding (32AL); and 22 C, pair-feeding on the daily feed intake of heat-exposed chickens (22PF). At 7 wk of age, heat-exposed chickens (32AL) had a lower body weight gain than the other birds: -47% compared to 22AL and -31% compared to 22PF. At 32 C, broilers exhibited a lower breast to body weight proportion: 11.9 vs 13.4% for 22AL. Abdominal, subcutaneous, and intermuscular fat deposits were enhanced in hot conditions, respectively, 15, 21, and 22% compared to 22AL and 58, 64, and 33% compared to 22PF. However, lipid contents of abdominal, subcutaneous, intermuscular, and intramuscular tissues were not affected by heat exposure but were significantly reduced in the 22PF birds. In heat-exposed birds, although saturated fatty acid proportions, particularly palmitic acid (C16:0), were increased, unsaturated fatty acids as a percentage of total fatty acids were decreased, especially oleic (C18:1) and linoleic (C18:2) acids in fat tissues. Consequently, under ad libitum feeding conditions, heat exposure significantly decreased the unsaturated to saturated fatty acid ratio in the abdominal and subcutaneous fat tissues, but not in intermuscular and intramuscular fats.

Two experiments were conducted on broiler chickens to compare the effect of a new organic Se source, 2-hydroxy-4-methylselenobutanoic acid (HMSeBA; SO), with two practical Se additives, sodium selenite (SS) and Se yeast (SY). The relative bioavailability of the different Se sources was compared on muscle ( pectoralis major) total Se, selenomethionine (SeMet) and selenocysteine (SeCys) concentrations and apparent digestibility of total Se (AD Se ). In the first experiment, from day (d) 0 to d21, Se sources were tested at different supplied levels and compared with an unsupplemented diet (NC). No significant effects were observed on growth performance during the experimental period. However, the different Se sources and levels improved muscle Se concentration compared with the NC, with a significant source effect in the following order: SS , SY , SO (P,0·05). Seleno-amino acids speciation results for NC, SY and SO at 0·3 mg Se/kg feed indicated that muscle Se was only present as SeMet or SeCys, showing a full conversion of Se by the bird. The second experiment (d0 -d24) compared SS, SY or SO at 0·3 mg Se/kg feed. The AD Se measurements carried out between d20 and d23 were 24, 46 and 49 % for SS, SY and SO, respectively, with significant differences between the organic and mineral Se sources (P,0·05). These results confirmed the higher bioavailability of organic Se sources compared with the mineral source and demonstrated a significantly better efficiency of HMSeBA compared with SY for muscle Se enrichment.

The effect of chronic heat exposure on feed digestibility of broilers was investigated. Eighty 4-wk-old male chickens were brooded in individual battery cages in two controlled-environment rooms at a constant ambient temperature (22 or 32 C) until 6 wk of age. They were equally distributed into three treatments: 22 C, ad libitum feed consumption (22AL); 32 C, ad libitum feed consumption (32AL), and 22 C, pair-feeding on the daily feed intake of heat-exposed chickens (22PF). Broilers were fed either a standard corn-soybean meal diet (control diet) or a practical seasonal diet containing several ingredients including wheat, spring pea, and animal fat (summer diet). Digestibility of energy, dry matter, protein, fat, starch, and nitrogen, and total mineral balances were measured between 38 and 42 d of age. Apparent metabolizable energy content of summer diet was significantly decreased in 32AL compared to 22AL, whereas AME of the control diet did not change. Nitrogen retention was significantly reduced in 32AL birds compared to 22AL and 22PF birds, irrespective of the diet. Taking into account these differences in nitrogen balance, AMEn was reduced under hot exposure: -72 and -155 kcal for control and summer diets respectively, in 32AL compared to 22PF chickens. This reduction could be explained by a significant decrease of nutrient digestibility:protein: -4.2 percentage units irrespective of the diet, fat: -1.7 and -5.2 percentage units for control and summer diets respectively, and starch: -4.2 percentage units for summer diet. It thus appears worthwhile to take into account such reduction in digestibility to formulate practical diets for brooding under hot conditions. High quality oil and protein sources should also be used instead of low quality feedstuffs, like animal sources, in such conditions.