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°C); diurnal cyclic temperature (temperature was 28°C from 1000 to 1700 h and 22°C from 1700 to 1000 h); and constant high temperature (34°C; temperature was 34°C). In trial 2, broilers from the control and 34°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°C for 2 h. The 3 cages from the 34°C group were also held in the room at 34°C (34–34°C). Diurnal cyclic temperature had no effect on BW up to 5 wk of age. The effect of 34°C constant temperature on BW of broilers increased with age. Plasma levels of glucose and albumin increased by 34°C, but no dramatic change in levels occurred when those broilers were crated at 34°C. The heterophil:lymphocyte (H:L) was higher for the 34–34°C broilers and the control broilers in the 34°C room than those from the 22 and 15°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°C and 34–34°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.

During incubation, the content of the egg is converted into a chick. This process is controlled by incubation conditions, which must meet the requirements of the chick embryo to obtain the best chick quality and maximum hatchability. Incubation temperature and light are the two main factors influencing embryo development and post-hatch performance. Because chicken embryos are poikilothermic, embryo metabolic development relies on the incubation temperature, which influences the use of egg nutrients and embryo development. Incubation temperature ranging between 37 and 38°C (typically 37.5–37.8°C) optimizes hatchability. However, the temperature inside the egg called “embryo temperature” is not equal to the incubator air temperature. Moreover, embryo temperature is not constant, depending on the balance between embryonic heat production and heat transfer between the eggshell and its environment. Recently, many studies have been conducted on eggshell and/or incubation temperature to meet the needs of the embryo and to understand the embryonic requirements. Numerous studies have also demonstrated that cyclic increases in incubation temperature during the critical period of incubation could induce adaptive responses and increase the thermotolerance of chickens without affecting hatchability. Although the commercial incubation procedure does not have a constant lighting component, light during incubation can modify embryo development, physiology, and post-hatch behavior indicated by lowering stress responses and fearful behavior and improving spatial abilities and cognitive functions of chicken. Light-induced changes may be attributed to hemispheric lateralization and the entrainment of circadian rhythms in the embryo before the hatching. There is also evidence that light affects embryonic melatonin rhythms associated with body temperature regulation. The authors’ preliminary findings suggest that combining light and cyclic higher eggshell temperatures during incubation increases pineal aralkylamine N-acetyltransferase, which is a rate-limiting enzyme for melatonin hormone production. Therefore, combining light and thermal manipulation during the incubation could be a new approach to improve the resistance of broilers to heat stress. This review aims to provide an overview of studies investigating temperature and light manipulations to improve embryonic development, post-hatch growth, and adaptive stress response in chickens.

Eggs obtained from broiler breeders at 32 (young), 42 (middle aged), and 65 wk (old) were used to measure the effects of heat acclimation during incubation on morphological, physiological, and metabolic traits at internal pipping (IP) and at hatch. All eggs were from the same stock, and hatching performance was also evaluated. Eggs from each breeder age were incubated at control (CONT) or 38.5 degrees C for 6 h daily from d 10 to 18 of incubation (HA). On d 10 after heat exposure and on d 14, absolute and proportional weights were significantly lower for HA than CONT embryos. By the time of hatching, HA chicks were heavier than CONT chicks, which suggested accelerated growth. This effect was consistent across ages. Liver and heart weights were lower for HA than CONT chicks. At IP, pH was similar for HA and CONT embryos, whereas pO(2) and Na(+) were significantly higher and pCO(2), HCO(3)-, and K(+) significantly lower for HA than CONT embryos. Blood pH was higher in embryos from older than for younger and mid-aged parents at IP. At hatch there was no effect of heat acclimation for blood HCO(3)-, Na(+), and K(+) levels, whereas plasma triglyceride and T(3) levels were higher and plasma uric acid, glucose, and lipid peroxidation levels were lower for HA than CONT chicks. Embryonic mortality was similar among parental ages for CONT. In contrast for HA, embryonic mortality from older parents was higher than for younger and middle-aged parents. A delay in external pipping and hatching time with high incubation temperature was consistent across the breeder ages. It was concluded that lower blood pCO(2), HCO(3)-, K(+), and higher pO(2) at IP stage, plus increased plasma triglyceride concentrations at hatch, indicate adaptive responses of embryos.

The aim of the present study was to investigate the effects of parental age and heat acclimation during incubation on BW, cloacal temperature, and bl

The aim of the research was to evaluate embryo composition and changes in egg yolk fatty acid composition during embryonic development as a function of incubation temperature and age of breeders. Eggs obtained from a common breeder stock at 3 ages: 32 (younger), 42 (mid age), and 65 (older) wk were divided into 2 groups and placed into 2 incubators: the control and the second where eggs were heat-acclimated (HA) at 38.5 degrees C for 6 h daily from d 10 to 18 of incubation. Body composition of embryos and chicks were measured on d 14, 18, and at hatch, respectively. Fatty acid profiles of yolk and residual egg yolk sac of chicks were analyzed before incubation and at hatch, respectively. Moisture content of embryos was highest on d 14 and then decreased regardless of parental age and incubation temperature. Moisture content of chicks at hatch from 42- and 65-wk parents were lower than those of chicks from 32-wk parents, whereas the trend in chick fat content was opposite. Incubation temperature had no effect on composition of chicks. Consistently lower cis-4,7,10,13,16,19-eicosapentaenoic (docosahexaenoic acid, DHA; 22:6n-3) and cis-11,14,17-eicosatrienoic (20:3n-3) fatty acids in the residual yolk sac of chicks than in egg yolks before incubation may have resulted from preferential uptake from the yolk. The DHA content in the residual yolk sac was considerably higher in chicks from older parents incubated at HA, whereas, in contrast, levels of 18:3n-3 were lower. Also, chicks from younger parents in the HA treatment had lower transported 18:3n-3 and higher levels of transported DHA. It may be concluded that this process observed during the high incubation temperature may be related to a protective strategy and thus contributes to postnatal heat adaptation.