Two trials were conducted to study the effects of heat stress during rearing (trial 1) and crating (trial 2) on broiler stress parameters and fear, breast meat quality, and nutrient composition. The relationships between stress parameters and meat quality traits were also determined. Trial 1 consisted of 3 temperature treatments from 3 to 7 wk: control (temperature was 22 degrees C); diurnal cyclic temperature (temperature was 28 degrees C from 1000 to 1700 h and 22 degrees C from 1700 to 1000 h); and constant high temperature (34 degrees C; temperature was 34 degrees C). In trial 2, broilers from the control and 34 degrees C groups in trial 1 were used. Broilers in each group were placed in transport cages. The 9 cages from the control group were divided into 3 groups and placed into 3 rooms at 15, 22, or 34 degrees C for 2 h. The 3 cages from the 34 degrees C group were also held in the room at 34 degrees C (34-34 degrees C). Diurnal cyclic temperature had no effect on BW up to 5 wk of age. The effect of 34 degrees C constant temperature on BW of broilers increased with age. Plasma levels of glucose and albumin increased by 34 degrees C, but no dramatic change in levels occurred when those broilers were crated at 34 degrees C. The heterophil:lymphocyte (H:L) was higher for the 34-34 degrees C broilers and the control broilers in the 34 degrees C room than those from the 22 and 15 degrees C room. Breast muscle glycogen level decreased in broilers reared under diurnal cyclic or high temperatures. A lower pH and higher lightness (L*) and redness values and redness:yellowness were found in meat for broilers from both 34 degrees C and 34-34 degrees C groups. Higher H:L was associated with breast muscle pH according to first-order polynomial regression. The H:L had a significant effect on L* values, which were described by a second-order polynomial regression. Blood glucose level was positively correlated with L* and redness values. Duration of tonic immobility was neither influenced by rearing and crating temperatures nor associated with meat quality parameters.

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Eighteen hundred eggs obtained from Ross broiler breeders at 32 and 48 wk age were randomly assigned to two incubation treatments: T1 eggs were incubated at 37.6℃ throughout, while for T2 eggs the incubation temperature was reduced 1℃ for 6 h daily at embryonic ages (EA) 10 to 18. Embryo and organ weights and body composition were measured at EA 14, 19 and day of hatch (DOH). Hatchability and hatching duration, as well as serum triiodothyronine (T 3 ), thyroxin (T 4 ), and triglycerides were measured at DOH. T1 eggs contained less water than T2 eggs at EA 18. Hatchability was lower and the incubation period was 4.2 h longer for T2 than T1 chicks. On DOH for older breeders chick weights and serum T 3 levels were higher for T2 than T1; however, those from younger breeders were similar at both incubation temperatures. These results may show a beneficial effect of T2 treatment in older breeders. Incubation temperature did not affect triglycerides levels. On DOH, higher body lipids content of T2 than T1 chicks may contribute to their resistance to cold post hatch.

Cyclically cold incubation temperatures have been suggested as a means to improve resistance of broiler chickens to ascites; however, the underlying mechanisms are not known. Nine hundred eggs obtained from 48 wk Ross broiler breeders were randomly assigned to 2 incubation treatments: control I eggs were incubated at 37.6°C throughout, whereas for cold I eggs the incubation temperature was reduced by 1°C for 6 h daily from 10 to 18 d of incubation. Thereafter, chickens were reared at standard temperatures or under cold exposure that was associated or not with a postnatal cold acclimation at d 5 posthatch. At hatch, hepatic catalase activity and malondialdehyde content were measured. Serum thyroid hormone and triglyceride concentrations, and muscle expression of several genes involved in the regulation of energy metabolism and oxidative stress were also measured at hatch and 5 and 25 d posthatch. Cold incubation induced modifications in antioxidant pathways with higher catalase activity, but lower expression of avian uncoupling protein 3 at hatch. However, long-term enhancement in the expression of avian uncoupling protein 3 was observed, probably caused by an increase in the expression of the transcription factor peroxisome proliferator activated receptor-γ coactivator-1α. These effects were not systematically associated with an increase in serum triiodothyronine concentrations that were observed only in chickens exposed to both cold incubation and later acclimation at 5 d with cold rearing. Our results suggest that these conditions of cyclically cold incubation resulted in the long-term in changes in antioxidant pathways and energy metabolism, which could enhance the health of chickens reared under cold conditions.

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1. The objective was to evaluate the effects of brooding temperature on intestinal development, oxidative organ damage, and performance of chicks acclimated to high temperature during incubation. The effects of acclimation and brooding temperatures on slaughter weights of broilers under heat stress were also investigated. 2. Eggs were incubated at either 378 degrees C (INC(Cont)) or heat-acclimated at 395 degrees C for 6 h daily from d 10 to d 18 of incubation (INC(H)). Brooding temperatures at floor level were set at 32, 335 and 35 degrees C (Bt(32), Bt(335), Bt(35), respectively) for the first 5 d. The temperature was reduced gradually to 30 degrees C from d 6 to d 10. From 21 to 42 d, broilers from INC(Cont) Bt(32) and INC(H) Bt(32) and Bt(35) were divided into two groups; half from each group was exposed to daily cyclic higher ambient temperatures, while the other half was reared at control temperature. 3. INC(H) chicks had lower jejunum, but greater liver and residual yolk sac weights than INC(Cont) chicks on the day of hatching. Although INC(H) chicks from Bt(335) and Bt(35) had lighter body weights than Bt(32) on d 5, no significant differences were observed in the body weight of broilers among treatments at 10 and 21 d. 4. Similar jejunum protein, alkaline phosphatase, maltase, glutathione, and malondialdehyde contents of chicks from INC(Cont) and INC(H) suggested that heat acclimation during incubation has no effect on jejunum enzyme activity or oxidative status of chicks. 5. Taking into account INC(H) Bt(35) chicks having lower T(3) levels on d 5, lower heterophil/lymphocyte (H/L) ratios and similar weights at 42 d under heat stress compared with control broilers, the results suggested that although higher brooding temperatures had no effect on body weights of INC(H) chicks during the brooding period, those broilers may able to cope better with heat stress.

In the past 20 years, broiler production has increased in hot climate countries, due to a greater potential for further growth. Thus, there is a necessity to improve thermotolerance of broilers produced in hot climates. Incubation period will become increasingly important in enhancing thermotolerance, because higher temperatures during incubation may lead to an elevation of the thermoregulatory set-point after hatch. Two recent experiments, conducted to determine the effect of high incubation temperature (INC HIGH ) from d 10 to 18 on hatching performance and carcass characteristics of broilers, have been compared with broilers from incubated at control temperatures (INC CONT ). INC HIGH resulted in a delay in external pipping and hatching times compared with INC CONT . There was no incubation temperature effect on the weights of bursa of fabricius, spleen and lungs, and moisture content of chicks but lowered heart and liver weights. When broilers exposed to daily cyclic high temperature from 21 to 42 d, slaughter weight of broilers from INC CONT reduced while heavier body weight and breast yield were obtained in broilers from INC HIGH . It was concluded that higher incubation temperature from d 10 to 18 for 6 h/d had no effect on chick weights and minimized the negative effect of heat stress on slaughter weight and breast meat yield.

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