Abstract
Problem statement: Acute heat stress at marketing age especially in b roiler chickens raised in open houses with reduced means of heat exchange leads to economic losses. The objective of this study was to determine beneficial effects of early age thermal conditioning in reducing adverse effect s of acute heat stress and decrease losses. Approach: Ninety one day-old broiler chicks were randomly assigned to one of three treatments (n = 30): (1) c ontrol (normally raised), (2) early age thermal conditioning (exposed to temperature of 40±1°C for 24 h at 5th day of age, then raised as control chicks) and (3) chronic stress (exposed to 33±2°C f rom day one till 6 weeks of age). At 42nd day of age, all chicks were subjected to acute heat stress of 39±2°C for 2 h. Blood samples were collected from all groups before and after exposure to acute heat stress. Results: Blood pH increased in both controls and thermally-conditioned chicks after exp osure to acute heat stress coinciding with significant decrease in blood carbon dioxide pressu re (pCo 2) in controls only. Blood potassium level decreased in controls, while in thermally-condition ed or chronically-stressed no significant changes were observed. Blood sodium level showed a trend to ward decreased levels in controls while a trend toward increased levels was observed in both therma lly-conditioned and chronically-stressed birds. Importantly, significant reductions were observed i n total erythrocyte count and hemoglobin level in chronically-stressed birds as compared to other gro ups before and after acute stress exposure. Hetrophil/lymphocyte ratio increased in both contro ls and thermally-conditioned chicks after acute heat exposure, but not in chronically-stressed bird s. Conclusion: When exposed to acute heat stress at marketing age, chicks subjected to early age therma l conditioning responded very similar to birds adapted to chronic heat stress indicating a protect ive role of early age thermal conditioning.
Highlights
Poultry production in tropical countries is affected by many challenges especially during the hot humid summer season
There was a significant reduction in its level after exposure to acute heat stress
With reference to changes in blood pH and blood gases, it was clear that chronically-stressed chicks revealed no significant changes following acute heat stress at marketing age
Summary
Poultry production in tropical countries is affected by many challenges especially during the hot humid summer season. Economic losses result from decreased productivity and increased mortality due to acute heat stress. Chick's age plays an important role in its resistance to acute heat stress. Heat loss from the surface of chicken to the environment increases from day one till four weeks of age and decrease during the last two weeks of the growth period of broiler chicken (Cangar et al, 2008). Heavier broilers (approaching marketing age) are sensitive to high-temperature challenges. This sensitivity can be explained by the fact that broilers have greater difficulty keeping thermal homeostasis due to the large body mass and high rate of metabolism associated with rapid growth (Borges et al, 2003). Low ability of heat loss of broiler chicks during the last two-weeks increases the impact and risk of exposure to elevated environmental temperature
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