Domestic animals have been modified by selecting individuals exhibiting desirable traits and culling the others. To investigate the alterations introduced by domestication and selective breeding in heat stress response, 2 experiments were conducted using Red Jungle Fowl (RJF), village fowl (VF), and commercial broilers (CB). In experiment 1, RJF, VF, and CB of a common chronological age (30 d old) were exposed to 36 ± 1°C for 3 h. In experiment 2, RJF, VF, and CB of common BW (930 ± 15 g) were subjected to similar procedures as in experiment 1. Heat treatment significantly increased body temperature, heterophil:lymphocyte ratio, and plasma corticosterone concentration in CB but not in VF and RJF. In both experiments and irrespective of stage of heat treatment, RJF showed lower heterophil:lymphocyte ratio, higher plasma corticosterone concentration, and higher heat shock protein 70 expression than VF and CB. It can be concluded that selective breeding for phenotypic traits in the domestication process has resulted in alterations in the physiology of CB and concomitantly the ability to withstand high ambient temperature compared with RJF and VF. In other words, domestication and selective breeding are leading to individuals that are more susceptible to stress rather than resistant. It is also apparent that genetic differences in body size and age per se may not determine breed or strain variations in response to heat stress.

Two hundred thirty-five 1-d-old broiler chickens showing short or long tonic immobility responses were classified as low fear (LF) or high fear (HF) responders, respectively. On d 41, they were subjected to either crating or heat challenge (34 +/- 1 degrees C) for 3 h and its effect on plasma corticosterone concentration, heterophil/lymphocyte ratios, and heat shock protein (HSP) 70 expression in brain tissue were determined. Crating and heat exposure elevated heterophil/lymphocyte ratios in both LF and HF birds. Circulating corticosterone, however, was greater in HF than LF birds after crating and heat challenge. Although differences between fear responder group for HSP 70 were negligible before heat challenge, after 3 h of heat exposure, the response was greater for the HF than the LF group. Both LF and HF showed similar increases in HSP 70 after crating.

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This study was conducted to determine whether early age feed restriction improves heat tolerance in female broiler chickens. Chicks were brooded for 3 wk and then maintained at 24+/-1 C. On Day 0, chicks were assigned to one of four feeding regimens; each regimen was applied to four cages of chicks. The feeding regimens were 1) ad libitum feeding (ALF); 2) 40% feed restriction at 4, 5, and 6 d of age (F40); 3) 60% feed restriction at 4, 5, and 6 d of age (F60); and (4) 80% feed restriction at 4, 5, and 6 d of age (F80). From 35 to 41 d of age, all birds were exposed to 38+/-1 C for 2 h/d. Serum concentrations of glucose were elevated by the heat challenge, but were not affected by the feeding regimen. The heat treatment resulted in hypocholesteremia among ALF and F80 chicks, whereas the concentrations increased and remained constant in the F60 and F40 birds, respectively. Subjecting chicks to F60 improved growth and survivability and reduced heterophil to lymphocyte ratios (H/L) in response to the heat treatment as compared with the ALF and F80 regimens. The survivability rate and H/L of F40 chicks were similar to those attained by chicks on other regimens. Newcastle disease antibody titer of ALF birds declined with duration of heat treatment. It is concluded that the F60 regimen is beneficial for alleviating, at least in part, the detrimental effects of heat stress in female broiler chickens.

We examined the effects of early age feed restriction and heat conditioning on heterophil/lymphocyte ratios (HLR), heat shock protein (hsp) 70 expression and body temperature of heat-stressed male broiler chickens. On day (d) 1,chicks were subjected to (1) 60% feed restriction on d 4, 5, and 6 (FR); (2) exposure to 36±1°C for 1 h from d 1 to 21 (HT); (3) both FR and HT (FRHT); or (4) control. On d 35, all the birds were exposed to 39±1°C for 6 h. Subjecting chicks to FR, HT and FRHT reduced HLR response to the heat challenge. The FR and FRHT birds had improved hsp 70 response and the latter were more hyperthermic than controls during the heat exposure.