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Selection in broiler chickens has increased muscle mass without similar development of the cardiovascular and respiratory systems, result-ing in limited ability to sustain high ambient tempera-tures. The aim of this study was to determine the long-lasting effects of heat manipulation of the embryo on the physiology, body temperature (Tb), growth rate and meat processing quality of broiler chickens reared in floor pens. Broiler chicken eggs were incubated in control conditions (37.8°C, 56% relative humidity; RH) or exposed to thermal manipulation (TM; 12 h/d, 39.5°C, 65% RH) from d 7 to 16 of embryogenesis. This study was planned in a pedigree design to identi-fy possible heritable characters for further selection of broiler chickens to improve thermotolerance. Thermal manipulation did not affect hatchability but resulted in lower Tb at hatching and until d 28 post-hatch, with associated changes in plasma thyroid hormone con-centrations. At d 34, chickens were exposed to a mod-erate heat challenge (5 h, 32°C). Greater 02 satura-tion and reduced CO-, partial pressure were observed (P < 0.05) in the venous blood of TM than in that of control chickens, suggesting long-term respiratory adaptation. At slaughter age, TM chickens were 1.4% lighter and exhibited 8% less relative abdominal fat pad than controls. Breast muscle yield was enhanced by TM, especially in females, but without significant change in breast meat characteristics (pH, color, drip loss). Plasma glucose/insulin balance was affected (P < 0.05) by thermal treatments. The heat challenge increased the heterophil/lymphocyte ratio in controls (P < 0.05) but not in TM birds, possibly reflecting a lower stress status in TM chickens. Interestingly, broiler chickens had moderate heritability estimates for the plasma triiodothyronine/thyroxine concentra-tion ratio at d 28 and comb temperature during the heat challenge on d 34 (h² > 0.17). In conclusion, TM of the embryo modified the physiology of broilers in the long term as a possible adaptation for heat tolerance, without affecting breast meat quality. This study high-lights the value of 2 new heritable characters involved in thermoregulation for further broiler selection. [ABSTRACT FROM AUTHOR]

In the context of climate change where farmers have to face the consequences of increasing frequency of extreme thermal events in Northern countries, and of chronic heat in Southern countries, the adaptive capacities of fastgrowing chickens are strongly challenged and become a critical issue. One strategy for improving broiler robustness while potentially limiting energy use is the conception of new incubation strategies exposing eggs directly in incubators to environmental fluctuations. This could prepare the developing bird to sustain later thermal fluctuations on farm. However, the consequences of the combination of both heat and cold embryo stimulations on animal physiology, health status and performance remain to be determined in thermally fluctuating or standard rearing conditions. A multidisciplinary approach was applied to evaluate the effects on selected pathophysiological markers of standard conditions or temperature variations during incubation combined with standard rearing conditions or cool chick start followed by late heat exposure. Incubation condition I1 including elevated incubation temperatures during mid-incubation and cold stimulations at the end of embryogenesis did not affect hatchability as compared to standard incubation conditions I0. Hatched-chick BW was not different between incubation groups. Postnatal chicken BW was affected by both incubation conditions and postnatal conditions, with lower BW in I1 group than in I0, and lower BW in the postnatal variable conditions than in standard conditions. Feed conversion ratio (FCR) was lower in the I1 treated chickens reared in standard conditions compared to both incubation groups reared in variable postnatal conditions (P = 0.006). Chickens exposed to variable conditions were less affected by diarrhea than those in standard conditions at 11 d of age. Mortality was greater in I1 chicks than in I0 before 11 d of age (P = 0.049), whereas at 35 d, there was no effect of incubation or postnatal conditions on mortality rates. At 41 d, overall mortality and morbidity rates were higher in standard conditions than in variable conditions (P = 0.046). Meat quality was affected by both incubation and postnatal conditions: incubation I1 induced lower values of meat ultimate pH (pHu) measured 24 h after slaughter, but increased drip loss and meat shear force compared to I0 incubation. Postnatal thermal variations decreased the occurrence of the meat defects white striping and wooden breast, breast yield, drip and cooking loss and yellow color of the meat, but increased abdominal fat percentage, thigh yield, pHu and curing-cooking yield of breast meat. Both factors interacted to affect the lightness of breast meat and its intramuscular lipid content that were lower in I1 than in I0 chickens when reared in standard conditions. Only postnatal conditions significantly affected blood gas and ion concentrations. Exposure to postnatal variable conditions increased plasma uric acid concentrations as well as blood total antioxidant status, but decreased the 'reduced to oxidized' glutathione ratio, suggesting a disrupted redox equilibrium in these conditions as compared to control postnatal conditions. In conclusion, in broiler chickens postnatal variable thermal conditions and, to a lesser extent, variable incubation conditions, affected health, performance and meat quality criteria. The adaptive capacities and metabolism of fastgrowing chickens were strongly challenged by postnatal temperature fluctuations, which were not mitigated by incubation conditions including heat and cold stimulations. However, postnatal varying environment limiting growth reduced mortality and meat quality defects as compared to standard conditions at six weeks of age. [ABSTRACT FROM AUTHOR]

Cold stimulations during egg incubation were reported to limit the occurrence of ascites in broilers subjected to cold temperature after 14 d of age. However, data are lacking on the impacts of such strategy in case of cold temperature conditions at start. This study aimed to evaluate the effects of incubation and posthatch cold challenge on performance, breast muscle integrity, and meat processing quality in broiler chickens. Ross 308 eggs were incubated under control temperature (I0, 37.6°C) or subjected to 15°C during 30 min on day 18 and 19 of incubation (I1). Chicks from each group were reared in floor pens either at standard rearing temperature (T0), from 32°C at 0 d to 21°C at 21 d of age, or exposed to colder rearing temperature (T1), from 29°C at 0 to 21°C at 21 d of age. All birds were then kept at 21°C until slaughter (day 40), when body weights (BW), feed conversion ratio (FCR), breast muscle yield, meat processing quality, and the occurrences of meat defects, hock burns, and pododermatitis were recorded. No significant impact of incubation conditions on hatchability was observed. At day 40, BW was more under T1 than under T0 conditions, with T0 females (but not males) presenting more BW after I1 than after I0 conditions. In the whole period, T1 chickens presented lower FCR than T0 chickens and higher breast meat yields at day 40. The occurrence of white striping was more in I1T1 males than in all other groups, except for the I0T1 males. Hock burns were more frequent in I1T1 males than in all females and I0T0 males, whereas the occurrence of pododermatitis was lower in T0 males than in other groups. Despite some positive effects of I1 incubation on growth after starting under low ambient temperature, this study reveals the limits of such strategy concerning chicken health and welfare, demonstrating that early thermal environment is a major component of the quality and sustainability of chicken meat production. [ABSTRACT FROM AUTHOR]