Early age thermal conditioning immediately reduces body temperature of broiler chicks in a tropical environment

Embryonic thermal manipulation: a potential strategy to mitigate heat stress in broiler chickens for sustainable poultry production

Due to high environmental temperatures and climate change, heat stress is a severe concern for poultry health and production, increasing the propensity for food insecurity. With climate change causing higher temperatures and erratic weather patterns in recent years, poultry are increasingly vulnerable to this environmental stressor. To mitigate heat stress, nutritional, genetic, and managerial strategies have been implemented with some success. However, these strategies did not adequately and sustainably reduce the heat stress. Therefore, it is crucial to take proactive measures to mitigate the effects of heat stress on poultry, ensuring optimal production and promoting poultry well-being. Embryonic thermal manipulation (TM) involves manipulating the embryonic environment's temperature to enhance broilers' thermotolerance and growth performance. One of the most significant benefits of this approach is its cost-effectiveness and saving time associated with traditional management practices. Given its numerous advantages, embryonic TM  is a promising strategy for enhancing broiler production and profitability in the poultry industry. TM increases the standard incubation temperature in the mid or late embryonic stage to induce epigenetic thermal adaption and embryonic metabolism. Therefore, this review aims to summarize the available literature and scientific evidence of the beneficial effect of pre-hatch thermal manipulation on broiler health and performance.

Strategies to combat heat stress in poultry production-A review

The effects of heat stress (HS) caused by high temperatures continue to be a global concern in poultry production. Poultry birds are homoeothermic, however, modern-day chickens are highly susceptible to HS due to their inefficiency in dissipating heat from their body due to the lack of sweat glands. During HS, the heat load is higher than the chickens' ability to regulate it. This can disturb normal physiological functioning, affect metabolism and cause behavioural changes, respiratory alkalosis and immune dysregulation in birds. These adverse effects cause gut dysbiosis and, therefore, reduce nutrient absorption and energy metabolism. This consequently reduces production performances and causes economic losses. Several strategies have been explored to combat the effects of HS. These include environmentally controlled houses, provision of clean cold water, low stocking density, supplementation of appropriate feed additives, dual and restricted feeding regimes, early heat conditioning and genetic selection of poultry lines to produce heat-resistant birds. Despite all these efforts, HS still remains a challenge in the poultry sector. Therefore, there is a need to explore effective strategies to address this long-lasting problem. The most recent strategy to ameliorate HS in poultry is early perinatal programming using the in ovo technology. Such an approach seems particularly justified in broilers because chick embryo development (21 days) equals half of the chickens' posthatch lifespan (42 days). As such, this strategy is expected to be more efficient and cost-effective to mitigate the effects of HS on poultry and improve the performance and health of birds. Therefore, this review discusses the impact of HS on poultry, the advantages and limitations of the different strategies. Finally recommend a promising strategy that could be efficient in ameliorating the adverse effects of HS in poultry.

Naked neck gene and intermittent thermal manipulations during embryogenesis improve posthatch performance and thermotolerance in slow-growing chickens under tropical climates

Many studies have shown that thermal manipulations during the incubation (TMI) and naked neck gene (Na) positively affect heat-stressed broilers' thermotolerance, hatching process, and posthatch performance. Their combination could increase the beneficial effect on broilers reared under natural tropical climatic conditions. The aim of this study was to investigate the effects of the Na gene and TMI on hatching and posthatch performance of slow-growing broilers under tropical climates. The study included 1,200 hatching eggs from 2 different crosses: 1) females and males, both with a normal or fully feathered neck (na na group), and 2) females (with a normal neck) and males (bare neck) (Na na group), incubated in similar conditions until d 7. Thereafter, they were assigned to 3 subgroups for each cross: the control group (C) was incubated at standard incubation conditions (37.8°C, 60% RH). The TMI-1 group was subjected to TMI-1 (T = 38.5°C, RH = 65%, E10-18, 6 h/d) and TMI-2 group to TMI-2 (T = 39.5°C, RH = 65%, E7-16, 12 h/d). Between 450 and 504 h of incubation, eggs were checked for hatching events. During the posthatch phase, chicks from each incubation subgroups (Na na-C, Na na-TMI-1, Na na-TMI-2, na na-C, na na-TMI-1, na na-TMI-2) were raised for 12 wk at a tropical natural ambient temperature. Hatchability, hatching time, chick's temperature, final body weight (FBW), and feed conversion ratio (FCR) were determined. The results revealed that the Na gene reduced (P ˂ 0.05) hatchability. The control group had the highest mortality rate compared to TMI-1 and TMI-2 groups. There was an interaction between genotype and TMI on incubation duration, hatching weight, chick quality, FBW, and FCR (P ˂ 0.05). In conclusion, the Na gene influenced the effects of thermal manipulation. TMI-1 combined with Na gene improved the productive performances of broilers in a tropical climate.

Alleviating heat stress effects in poultry: updates on methods and mechanisms of actions

Heat stress is a threat that can lead to significant financial losses in the production of poultry in the world's tropical and arid regions. The degree of heat stress (mild, moderate, severe) experienced by poultry depends mainly on thermal radiation, humidity, the animal's thermoregulatory ability, metabolic rate, age, intensity, and duration of the heat stress. Contemporary commercial broiler chickens have a rapid metabolism, which makes them produce higher heat and be prone to heat stress. The negative effect of heat stress on poultry birds' physiology, health, production, welfare, and behaviors are reviewed in detail in this work. The appropriate mitigation strategies for heat stress in poultry are equally explored in this review. Interestingly, each of these strategies finds its applicability at different stages of a poultry's lifecycle. For instance, gene mapping prior to breeding and genetic selection during breeding are promising tools for developing heat-resistant breeds. Thermal conditioning during embryonic development or early life enhances the ability of birds to tolerate heat during their adult life. Nutritional management such as dietary manipulations, nighttime feeding, and wet feeding often, applied with timely and effective correction of environmental conditions have been proven to ameliorate the effect of heat stress in chicks and adult birds. As long as the climatic crises persist, heat stress may continue to require considerable attention; thus, it is imperative to explore the current happenings and pay attention to the future trajectory of heat stress effects on poultry production.

Embryonic modulation through thermal manipulation and in ovo feeding to develop heat tolerance in chickens

Healthy chickens are necessary to meet the ever-increasing demand for poultry meat. Birds are subjected to numerous stressful conditions under commercial rearing systems, including variations in the environmental temperature. However, it is difficult to counter the effects of global warming on the livestock industry. High environmental temperature is a stressful condition that has detrimental effects on growth and production performance, resulting in decreased feed intake, retarded growth, compromised gut health, enhanced oxidative stress, and altered immune responses. Traditional approaches include nutritional modification and housing management to mitigate the harmful effects of hot environments. Currently, broiler chickens are more susceptible to heat stress (HS) than layer chickens because of their high muscle mass and metabolic rate. In this review, we explored the possibility of in ovo manipulation to combat HS in broiler chickens. Given their short lifespan from hatching to market age, embryonic life is thought to be one of the critical periods for achieving these objectives. Chicken embryos can be modulated through either temperature treatment or nourishment to improve thermal tolerance during the rearing phase. We first provided a brief overview of the harmful effects of HS on poultry. An in-depth evaluation was then presented for in ovo feeding and thermal manipulation as emerging strategies to combat the negative effects of HS. Finally, we evaluated a combination of the two methods using the available data. Taken together, these investigations suggest that embryonic manipulation has the potential to confer heat resistance in chickens.

Growth performance and hepatic antioxidants responses to early thermal conditioning in broiler chickens

There are little data about antioxidants' status responses to early thermal conditioning (TC) on broiler chickens. Therefore, the present study was conducted to investigate the different time ages of thermal conditioning on antioxidants responses and the growth rate of broiler chicks. A total of two hundred forty-one-day-old male broiler chicks (Cobb 500) weighed on average 51.5 ± 0.5g were randomly distributed into four equal groups (60 chicks each), and chicks of each group were ranked in five replicates. The first group reared under the ambient temperature, while the second, third and fourth groups (TC3, TC5 and TC7) were subjected to early-age thermal conditioning at 39°C for 6 h on the third, fifth and seventh day of age respectively. Broilers were fed ad-libitum, and drinking water was a free choice during the experimental period. At the end of the experimental period that lasted 5 weeks, all experimental groups were exposed to heat challenge at 36°C for 6 h. Early-age thermal conditioning did not affect growth performance. Plasma corticosterone elevation in TC5 (as a post-challenge response) was the lowest among the experimental groups. Hepatic malondialdehyde significantly increased in TC5 and TC7 groups both at the post-conditioning stage and at the end of the experimental period. Hepatic glutathione, glutathione S-transferases, catalase and superoxide dismutase significantly decreased by early-age thermal conditioning compared with non-conditioned broilers. Microscopic examination of the liver sections from broilers chickens in TC5 and TC7 groups showed all the basic features of normal liver tissue, while the control and TC3 groups showed few necrotic areas. It could be concluded that early-age thermal conditioning at 39°C for 6 h on the fifth day of age could improve the antioxidant defence system of broilers without any adverse effects on growth performance.

Effect of thermal conditioning on serum electrolytes, metabolites, corticosterone and expression of CRH gene in selected chicken strains

Early age thermal conditioning has been found to improve thermotolerance in birds. This study assessed the effect of perinatal thermal conditioning on serum parameters, corticosterone, free fatty acid, globulin and expression of corticotropin-releasing hormone (CRH) gene in five chicken strains; using fifty chicks per strain of Cobb 500 (C500), Ross 308 (R308), Shika Brown (SB), Normal Feathered Nigeria Indigenous (NF) and FUNAAB Alpha (FA). Twenty-five chicks per strain were conditioned at 40 ± 1 °C for 3 h on day 6. On day 10, both conditioned and unconditioned chicks were challenged acutely at 40 ± 1 °C for 15 min, without feed and water. Body weight and feed intake data were collected before and after the heat exposures. Blood samples were collected to determine serum electrolytes, metabolites and corticosterone levels. Brain tissue samples were collected from the 10-day-old conditioned and unconditioned chicks, from which RNA were extracted, synthesized into cDNA and subjected to qPCR. Serum parameters were significantly affected (p < 0.05) by strain, thermal conditioning and their interactions. Calcium and glucose concentrations were highest in NF while FA had highest in sodium. Calcium, glucose and phosphorus were higher in conditioned birds. NF had the highest free fatty acid while FA had the lowest. C500 had the highest globulin levels. Thermal conditioning significantly lowered corticosterone levels in conditioned birds. CRH was shown to be overexpressed in C500. From this research, it can be concluded that early age thermal conditioning affects body temperature regulation in chickens and enhances thermotolerance.

Mitigating the detrimental effects of heat stress in poultry through thermal conditioning and nutritional manipulation

The poultry industry faces several obstacles and challenges, including the changes in global temperature, increase in the per capita demand for meat and eggs, and the emergence and spread of various diseases. Among these, environmental challenges are one of the most severe hurdles impacting the growth and productivity of poultry. In particular, the increasing frequency and severity of heat waves over the past few years represent a major challenge, and this is expected to worsen in the coming decades. Chickens are highly susceptible to high ambient temperatures (thermal stress), which negatively affect their growth and productivity, leading to enormous economic losses. In the light of global warming, these losses are expected to increase in the near future. Specifically, the worsening of climate change and the rise in global temperatures have augmented the adverse effects of heat on poultry production worldwide. At present, the world population is approximately 7.9 billion, and it has been predicted to reach 9.3 billion by 2050 and approximately 11 billion by 2100, implying a great demand for protein supply; therefore, strategies to mitigate future poultry challenges must be urgently devised. To date, several mitigation measures have been adopted to minimize the negative effects of heat stress in poultry. Of these, thermal acclimation at the postnatal stage or throughout the embryonic stages has been explored as a promising approach; however, for large-scale application, this approach warrants further investigation to determine the suitable temperature and poultry age. Moreover, molecular mechanisms governing thermal conditioning are poorly understood. To this end, we sought to expand our knowledge of thermal conditioning in poultry, which may serve as a valuable reference to improve the thermotolerance of chickens via nutritional management and vitagene regulation. Vitagenes regulate the responses of poultry to diverse stresses. In recent years, nutritionists have paid close attention to bioactive compounds such as resveratrol, curcumin, and quercetin administered alone or in combination. These compounds activate vitagenes and other regulators of the antioxidant defense system, such as nuclear factor-erythroid 2-related factor 2. Overall, thermal conditioning may be an effective strategy to mitigate the negative effects of heat stress. In this context, the present review synthesizes information on the adverse impacts of thermal stress, elucidating the molecular mechanisms underlying thermal conditioning and its effects on the acquisition of tolerance to acute heat stress in later life. Finally, the role of some polyphenolic compounds, such as resveratrol, curcumin, and quercetin, in attenuating heat stress through the activation of the antioxidant defense system in poultry are discussed.

Effects of Herbal Adaptogen Feed-Additive on Growth Performance, Carcass Parameters, and Muscle Amino Acid Profile in Heat-Stressed Modern Broilers

Heat stress has strong adverse effects on poultry production and, thereby, threats its sustainability, which energized scientists to search for innovative and effective solutions. Here, we undertook this study to evaluate the effects of in-feed herbal adaptogen (stress response modifier) supplementation on growth performances, meat quality, and breast amino acid profile in chronic cyclic heat-stressed broilers. Day-old male Cobb 500 chicks (n = 720) were randomly assigned, in environmental chambers (n = 12, 24 pens), to three diet-treatments: a three-phase corn-soybean based diet fed as such (Control, C), or supplemented with the herbal adaptogen at 500 g/1000 kg control diet (NR-PHY-500) or at 1 kg/1000 kg control diet (NR-PHY-1000). From d29 to d42, birds from 9 chambers were exposed to cyclic heat stress (HS, 35°C from 9:30 am-5:30 pm), however, the rest of the chamber were maintained at thermoneutral conditions (24°C, TN), which creates 4 experimental groups: C-TN, C-HS, NR-PHY-500HS, and NR-PHY-1000HS (6 pens/group, 168 birds/group). HS altered growth performance via depression of feed intake and body weight. Adaptogen supplementation stimulated feed intake and averaged 65.95 and 83.25 g better body weight and 5 and 10 points better FCR at low and high dose, respectively, compared to heat-stressed birds. This increase in body weight was mirrored in enhanced weights of body parts (breast, tender, wings, and legs). Adaptogen supplementation modulated also breast amino acid profile, pH, color, and quality. Together, these data suggested that adaptogen supplementation could be a promising solution to alleviate heat stress, however further in-depth investigation for its mode of action and its underlying mechanisms are warranted.

Effects of hypoxic conditions during the plateau period on pre- and posthatch broiler performance

Adequate ambient temperature and oxygenation are necessary to maintain normal embryonic development of broilers; however, hypoxia challenge during incubation can aid in improving regulatory plasticity and lead to different phenotypes later in life. This study aimed to examine the effects of moderate hypoxia (O₂ 17%) during the plateau phase on the embryonic physiological parameters and on posthatch performance (growth rate, feed consumption and feed conversion) up to the age of poultry marketing. The study included examined embryos exposed to O₂ 17% for 12 h per day (h/d) from E16 through E18 (designated as 12H), or O₂ 17% continuously, from E16 through E17 (designated as 48H) and a standard incubation control group (21% O₂). Physiological and morphological parameters of embryos and hatched chicks were measured. Male Chicks from all 3 treatment groups were raised under recommended temperature regime, and body weight, feed intake and FCR were recorded on a weekly basis. The intermittent hypoxia protocol (12H), allowed embryos to properly adapt to the shortage of oxygen, compensate for the gap in body mass that developed following the first exposure window, and hatch with characteristics similar to those of the control embryos. In contrast, while the 48H embryos were able to adapt to the hypoxic stress, the prolonged exposure prevented them from catching up with both control and 12H embryos. Broilers that were subjected to hypoxia showed hatchling body weights and growth rates similar to those of controls, throughout the entire growth phase. During the fifth wk, lower feed consumption was observed in the 12H and 48H groups and became significantly lower than the control chicks in the sixth wk of growth. Following hypoxia exposure, chicks managed to reach normal body weight with less feed, with the 12H group demonstrating lower and more efficient FCR during the last 2 wk of growth. Broiler embryos reacted to plateau-phase hypoxia challenge with numerous physiological and metabolic modifications. The prudent alterations in metabolism and cardiovascular system during exposure to hypoxia and posthatch, resulted in more efficient energy utilization in broilers, which may have a long-lasting enhancing effect on posthatching thermotolerance and sustainability in chicks reared under sub-optimal environmental conditions.

Environmental stress and livestock productivity in hot-humid tropics: Alleviation and future perspectives

Tropical environments are characterized by persistently high temperature and relative humidity and the harsh environmental conditions pose a serious limitation on the optimal performance of the animals raised in this region. Heat stress causes deleterious effects on welfare, immunology and physiology of farm animals with a resultant impact on their productivity as the use of body resources is re-organized and the metabolic priorities of animals shift away from production, growth, health and reproduction. It is imperative to understand the mechanisms involved in the thermoregulation of animals under tropical conditions in order to develop appropriate strategies for their improvement. This review focuses on the available data on the increasing global temperature and the adverse impact of tropical conditions on animals' adaptive mechanism affected during thermal stress on production performance, intestinal and ileal microbiome, physiological responses, antioxidant system, metabolic responses, cellular and molecular response, adaptive mechanism strategies to heat stress and also strategies to palliate environmental stress on livestock under humid tropical conditions including environmental manipulation, genetic opportunity, epigenetic and feeding modification. Overall, the present review has identified the disturbance in the physiological indices of tropical livestock and the need for concerted efforts in ameliorating the adverse impacts of high ambient temperature aggravated by high humidity on livestock in tropical environments. Further research is needed on genotype-by-environment interaction on the thermotolerance of different livestock species in the tropics.

Heat stress management in poultry

High ambient temperature is one of the major causes of economic losses in the livestock industry. The poultry industry is an integral part of the livestock industry. It faces severe losses due to heat stress (HS). The adverse effects of HS can be seen on production performance, body temperature, intestinal health, appetite hormone regulation, immune responses and oxidative characteristics. It is important to monitor these parameters to identify the HS possessions during rearing so that timely action can be taken to minimize the adverse effects of high ambient temperature. Furthermore, the application of productive methods on farms is equally important. Several strategies have been suggested by researchers. Providing a suitable environment with selective rearing systems along with proper ventilation and hygiene is the basic requirement for all types of livestock reared for animal protein. Supplementation of appropriate feed additive could be useful for improving intestinal absorption and minimizing adverse effects of HS. Selection for breeding heat resistant birds also provide merits for improving the germplasm of the strains. Early age thermal conditioning also helps in developing resistance for HS. The most recent advancement is the supplementation of active substances during incubation. It is expected that these methods may have a potential impact on the poultry industry for creating thermotolerance in the newly hatched chicks. This review highlights the major issues concerning chicken health and suggests the measures to be adopted following the increase in environmental temperature.

Early age thermal manipulation on the performance and physiological response of broiler chickens under hot humid tropical climate

Initial brooding temperature is critical for post-hatch growth of broiler chickens. A study was conducted to investigate the early age thermal manipulation (EATM) on the performance and physiological responses broiler chickens under hot humid tropical climate. A total of 260 unsexed day-old Arbor-acre broiler chicks were assigned to five thermal treatments of brooding temperature regimens having 4 replicates of thirteen birds each. The heat treatments were: initial brooding temperature of 35 °C for the first 2 days, and then decreased subsequently, gradually to 22 °C at 21 d of age (CT), initial temperature of 35 °C, sustained for the first 4 days and then decreased gradually (conventionally) (FD), initial temperature of 35 °C for the first 7 days (SD), the birds in CT, but the brooding temperature was raised to 35 °C again for another 3 days from day 7 (SD3), initial brooding temperature of 35 °C for the first 10 days (TD). Data were collected on daily feed intake and weekly body weights. Blood samples were collected from 8 birds per treatment weekly for the determination of plasma uric acid, triglycerides, triiodothyronine (T₃) and creatinine kinase. Data obtained were laid out in a Completely Randomized Design (CRD). Results showed that the final weights of the birds in FD were higher (P < 0.05) than those of the other treatments at the finisher phase. Feed intake of the birds in FD was higher than those of SD3 and TD. FCR of broiler chickens in CT, SD, SD3 and TD was higher than that of FD. The rectal temperature, plasma MDA and blood glucose of the thermally challenged birds in FD was generally better (P < 0.05) than those of the other treatments. It was concluded that EATM can be used to improve performance and also protect broiler chickens from acute heat stress at market age.

Expression of digestive enzyme and intestinal transporter genes during chronic heat stress in the thermally manipulated broiler chicken

Heat stress has a serious impact on nutrient digestion and absorption in broiler chickens. This study aimed to investigate the effects of chronic heat stress (CHS) on the mRNA expression of digestive enzymes and nutrient transporter genes in thermally manipulated (TM) broiler chickens. The evaluated genes encompassed pancreatic lipase, trypsin, amylase, maltase, and alkaline phosphatase as well as certain glucose transporter (GLUT2, SGLT1), amino acid transporter (y+LAT1, CAT1), and fatty acid transporter (FABP1, CD36) genes in the jejunal mucosa. Thermal manipulation was carried out at 39°C and 65% relative humidity for 18 h daily from embryonic days (ED) 10-18, while CHS was induced by raising the temperature to 35°C for 7 D throughout post-hatch days 28 to 35. After 0, 1, 3, 5, and 7 D of CHS, the pancreas and jejunal mucosa were collected from the control and TM groups to evaluate the mRNA expression by relative-quantitative real-time qRT-qPCR. Thermal manipulation significantly decreased the cloacal temperature (Tc) and the hatchling weight, and improved weight gain in broilers during post-hatch life and CHS. In addition, TM decreased the mortality rate during CHS. During CHS, the mRNA expression levels of SGLT1, GLUT2, FABP1, and trypsin were significantly decreased after 1 D in control chickens, and this lower expression persisted until day 7, after which it further decreased. In contrast, in TM chickens, SGLT1, GLUT2, and FABP1 expression decreased after 3, 5, and 7 D of CHS, respectively, while no significant change in trypsin expression was observed throughout the CHS period. Moreover, it was found that TM significantly modulated the mRNA expression dynamics of CD36, alkaline phosphatase, y+LAT1, CAT1, lipase, amylase, and maltase during CHS exposure. The findings of this study suggest that, in broiler chickens, TM has a long-lasting impact on nutrient digestion and absorption capabilities as well as Tc, mortality rates, and BW during CHS.

Heat stress management in poultry farms: A comprehensive overview

Heat stress causes significant economic losses in poultry production, especially in tropical and arid regions of the world. Several studies have investigated the effects of heat stress on the welfare and productivity of poultry. The harmful impacts of heat stress on different poultry types include decreased growth rates, appetites, feed utilization and laying and impaired meat and egg qualities. Recent studies have focused on the deleterious influences of heat stress on bird behaviour, welfare and reproduction. The primary strategies for mitigating heat stress in poultry farms have included feed supplements and management, but the results have not been consistent. This review article discusses the physiological effects of heat stress on poultry health and production and various management and nutritional approaches to cope with it.

L-Leucine increases the daily body temperature and affords thermotolerance in broiler chicks

OBJECTIVE

Heat stress poses an increasing threat for poultry production. Some amino acids have been found to play critical roles in affording thermotolerance. Recently, it was found that in ovo administration of L-leucine (L-Leu) altered amino acid metabolism and afforded thermotolerance in heat-exposed broiler chicks.

METHODS

In this study, two doses (35 and 70 μmol/egg) of L-Leu were administered in ovo on embryonic day 7 to determine their effect on rectal temperature (RT), body weight (BW) and thyroid hormones at hatching. Changes in RT, BW, and thermotolerance in post-hatched chicks were also analyzed.

RESULTS

It was found that in ovo administration of L-Leu dose-dependently reduced RT and plasma thyroxine (T4) level just after hatching. In post-hatched neonatal broiler chicks, however, the higher dose of L-Leu administered in ovo significantly increased RT without affecting BW gain. In chicks that had been exposed to heat stress, the RT was significantly lowered by in ovo administration of L-Leu (high dose) in comparison with the control chicks under the same high ambient temperature (HT: 35°C±1°C, 120 min).

CONCLUSION

In ovo administration of L-Leu in a high dose contributed to an increased daily body temperature and afforded thermotolerance under HT in neonatal broiler chicks.

The effects of early age thermal conditioning and vinegar supplementation of drinking water on physiological responses of female and male broiler chickens reared under summer Mediterranean temperatures

The effects of early age thermal conditioning (ETC), vinegar supplementation (VS) of drinking water, broilers' gender, and their interactions on respiratory rate, body temperature, and blood parameters (biochemical, hematological, and thyroid hormones) of broiler chickens reared under high ambient temperatures were determined. A total of 1100 1-day-old chicks were divided into four treatments: the "control" which were non-conditioned and non-supplemented; "heat-conditioned" which were exposed to 38 ± 1 °C for 24 h at 5 days of age; "vinegar supplemented" which were given drinking water supplemented with 0.2% of commercial vinegar from 28 to 49 days of age; and "combined" which were both heat conditioned and vinegar supplemented. All groups were exposed to the natural fluctuations of summer ambient temperature (average diurnal ambient temperature of about 30 ± 1 °C and average relative humidity of 58 ± 5%). ETC and broiler gender did not affect the respiratory rate or body temperature of chronic heat-exposed chickens. VS changed the body temperature across time (d35, d42, d49) (linear and quadratic effects, P < 0.05) without changing respiratory rate. Heat-conditioned chickens exhibited lower levels of glycemia (P < 0.0001) and higher hematocrit and red blood cell counts (P < 0.05). Furthermore, the greatest effects of VS, alone or associated with ETC, were the lowering of cholesterol and triglyceride blood concentrations. A significant (P < 0.05) effect of ETC, gender, and ETC×gender on T3:T4 ratio was observed. Finally, some beneficial physiological responses induced by ETC and VS, separately or in association, on chronically heat-stressed chickens were observed. However, the expected cumulative positive responses when the two treatments were combined were not evident.

Thermal manipulation during Pre and Post-Hatch on thermotolerance of male broiler chickens exposed to chronic heat stress

The aim of this study was to evaluate the effects of thermal manipulation (TM) during pre and post-hatch periods on thermotolerance of male broiler chickens exposed to chronic heat stress (CHS) during the finisher phase (34 ± 2°C, 6 h/day). Seven hundred fertile eggs of Ross 308 were assigned to the following groups: 1) control group incubated and housed in standard conditions, 2) pre-hatch treatment (PRE), the eggs were exposed to 39.5°C and 65% RH for 12 h, d from embryonic d 7 to 16 and after hatching the chicks where housed in standard conditions; 3 and 4) post-hatch TM at d 3 (PO3) and post-hatch TM at d 5 (PO5), which had the same incubation conditions as control and exposed to 36 to 38°C for 24 h at 3 and 5 days of age, respectively. TM in PRE group resulted in delay in the hatch time (6 h) along with reduction in body weight compared to control (P = 0.02). TM caused a significant reduction of facial surface temperature (FST) until d 28 (P < 0.02), but not significant during CHS. Body weight gain was suppressed in PO3 and PO5 groups at d 14 (P = 0.007) and compensated at d 28. However, TM led to higher BWG (P = 0.000) but lower FCR (P = 0.03) and mortality at the first week of CHS compared to control. European production efficiency index was higher in TM-treated chickens compared to control (P = 0.01). TM reduced the blood concentration of uric acid, total protein, T3, and T4 in which thyroid hormones in PO3 and PO5 treatments showed more reduction rather than other groups. In PRE group, chickens had lower abdominal fat pad than control (P = 0.0001). The relative weight of heart was decreased in TM groups (P = 0.001). It was concluded that TM may induce thermotolerance in growing broilers, possibly through the modification of physiological parameters of broilers especially during the first week of CHS.

Thermal manipulation during late embryogenesis: Effect on body weight and temperature, thyroid hormones, and differential white blood cell counts in broiler chickens

The effects of thermal manipulation (TM) at 38.5°C and 40°C for 6 h at embryonic day (ED) 16, 9 h at ED 17, and 12 h at ED 18 on body weight (BW) and cloacal body temperature (T^b) during the first wk and later at post-hatch d 10, 14, 21, 28, and 42 were evaluated. Furthermore, chicks' ability to cope with a thermal challenge (TC; 41°C for 6 h) at post-hatch d 14 and 42 was also evaluated. A chick's response to TC was measured by determining the cloacal body temperature; the plasma thyroid hormones (thyroxin (T₄) and triiodothyronine (T₃)); the packed cell volume (PCV); the heterophil (H), lymphocyte (L), monocyte, basophil, and eosinophil percentages; and the heterophil-to-lymphocyte ratios (H/L). Thermal manipulation did not affect the hatchability. However, the body weight of TM chicken was higher compared with controls at marketing age (post-hatch d 42). At post-hatch d 14 and 42, no significant changes in T^b were observed among the different treatment groups. However, during TC at d 14 and 42, the T^b of TM chicks was lower compared with the controls. During TC, a significant increase in plasma T₄ and a significant decrease in plasma T₃ of TM chicks compared with controls were reported. Furthermore, during TC, a significant increase in the PCV and heterophil, monocyte, and H/L ratios, and a reduction in the lymphocyte percentages also were observed in TM chicks compared with the controls. Results of this study showed that chicks subjected to heat manipulation during late embryogenesis respond better to heat stress later in the growth and development period.

Incubation temperature alters thermal preference and response to heat stress of broiler chickens along the rearing phase

The current study aimed to investigate whether embryonic temperature manipulation may alter thermal preference throughout the rearing phase of broiler chickens and how this manipulation may affect response to thermal challenge, metabolism, growth rate and feed intake rate. Eggs were exposed to a constant incubation temperature [machine temperatures: 36°C (Low), 37.5°C (Control), and 39°C (High); eggshell temperature of 37.4 ± 0.08°C, 37.8 ± 0.15°C, and 38.8 ± 0.33°C, respectively] from d 13 till hatching. Low treatment chickens showed lower plasma T3 and GH levels at d 1 of age and lower T3 level at d 42 of age compared to the Control treatment. Preferred ambient, rectal temperature, T4 level, growth rate, food intake rate, and response to thermal challenge were not altered in these chickens. On the other hand, High-treatment chickens exhibited high preferred ambient temperature and rectal temperature during the first 2 wk post-hatch, lower plasma T3 level at d 21 and 42 and a delayed increase in respiratory movement in response to thermal challenge compared to the Control treatment. However, chickens subjected to the Control and High treatments did not differ in T4 and GH level and performance. We conclude that exposure to high temperature during late embryonic development has long-lasting effects on the thermoregulatory system of broiler chickens by affecting the heat tolerance of these chickens. Moreover, the preferred ambient temperature of the chickens from heat-treated eggs correspond to those recommended for the strain under study, whereas for the cold-treated and control-chickens it was 1°C below, indicating that incubation temperature might have consequences on the ambient temperature chickens require during the rearing phase.

Dietary but not in ovo feeding of Silybum marianum extract resulted in an improvement in performance, immunity and carcass characteristics and decreased the adverse effects of high temperatures in broilers

A total of 360 fertile eggs from a broiler breeder strain (Ross 308) were used for in ovo feeding (IOF) of 0, 100 and 200 mg/kg Silybum marianum water extract at 17.5 d of incubation. After hatching, 240 chicks were transferred to the experimental cages. The diets consisted of two types; one of them without extract and the other one containing 100 mg/kg of Silybum marianum extract. Then chicks were exposed to elevated temperatures 4°C above optimum from 7 to 28 d of age for 4 h per d and after 28 d they were kept at optimum temperature. The chicks were divided into 6 treatments with 4 replicates as a completely randomised 2 × 3 factorial design. There was no effect of IOF of Silybum marianum extract on hatchability, body weight (BW) of hatched chicks or hatched chick BW/initial egg weight ratio. Chickens fed on the diet containing the extract had the highest feed intake, daily weight gains (DWGs), final BW and lowest feed conversion ratio. At 28 d, the weights of heart, spleen and bursa of birds in the treatment groups given extract were higher, but abdominal fat weights were lower. At 42 d in those fed extract in the diet, the weights of spleen and bursa were higher and abdominal fat weights were lower than the other groups. No differences were observed between groups in body temperature before application of the higher temperature but at 21 d and 28 d the body temperature of chicks given the Silybum marianum extract was lower than the other treatments during the elevated temperatures. In birds fed on the extract in the diet, HDL and cholesterol concentrations were lowest at 28 d, whereas blood glucose levels were higher in the other treatments. At 42 d the cholesterol concentration was significantly lower in birds given extract in their diet. These results indicated that dietary feeding of Silybum marianum extract resulted in an improvement in performance, immunity and carcass characteristics and decreased the adverse effects of the higher temperatures, specifically in reducing blood fat reduction, but IOF of the extract did not have an effect.

Thermal manipulation during embryogenesis has long-term effects on muscle and liver metabolism in fast-growing chickens

Fast-growing chickens have a limited ability to tolerate high temperatures. Thermal manipulation during embryogenesis (TM) has previously been shown to lower chicken body temperature (T_b) at hatching and to improve thermotolerance until market age, possibly resulting from changes in metabolic regulation. The aim of this study was to evaluate the long-term effects of TM (12 h/d, 39.5°C, 65% RH from d 7 to 16 of embryogenesis vs. 37.8°C, 56% RH continuously) and of a subsequent heat challenge (32°C for 5 h at 34 d) on the mRNA expression of metabolic genes and cell signaling in the Pectoralis major muscle and the liver. Gene expression was analyzed by RT-qPCR in 8 chickens per treatment, characterized by low T_b in the TM groups and high T_b in the control groups. Data were analyzed using the general linear model of SAS considering TM and heat challenge within TM as main effects. TM had significant long-term effects on thyroid hormone metabolism by decreasing the muscle mRNA expression of deiodinase DIO3. Under standard rearing conditions, the expression of several genes involved in the regulation of energy metabolism, such as transcription factor PGC-1α, was affected by TM in the muscle, whereas for other genes regulating mitochondrial function and muscle growth, TM seemed to mitigate the decrease induced by the heat challenge. TM increased DIO2 mRNA expression in the liver (only at 21°C) and reduced the citrate synthase activity involved in the Krebs cycle. The phosphorylation level of p38 Mitogen-activated-protein kinase regulating the cell stress response was higher in the muscle of TM groups compared to controls. In conclusion, markers of energy utilization and growth were either changed by TM in the Pectoralis major muscle and the liver by thermal manipulation during incubation as a possible long-term adaptation limiting energy metabolism, or mitigated during heat challenge.

Cyclic variations in incubation conditions induce adaptive responses to later heat exposure in chickens: a review

Selection programs have enabled broiler chickens to gain muscle mass without similar enlargement of the cardiovascular and respiratory systems that are essential for thermoregulatory efficiency. Meat-type chickens cope with high ambient temperature by reducing feed intake and growth during chronic and moderate heat exposure. In case of acute heat exposure, a dramatic increase in morbidity and mortality can occur. In order to alleviate heat stress in the long term, research has recently focused on early thermal manipulation. Aimed at stimulation of long-term thermotolerance, the thermal manipulation of embryos is a method based on fine tuning of incubation conditions, taking into account the level and duration of increases in temperature and relative humidity during a critical period of embryogenesis. The consequences of thermal manipulation on the performance and meat quality of broiler chickens have been explored to ensure the potential application of this strategy. The physiological basis of the method is the induction of epigenetic and metabolic mechanisms that control body temperature in the long term. Early thermal manipulation can enhance poultry resistance to environmental changes without much effect on growth performance. This review presents the main strategies of early heat exposure and the physiological concepts on which these methods were based. The cellular mechanisms potentially underlying the adaptive response are discussed as well as the potential interest of thermal manipulation of embryos for poultry production.

Thermal manipulation of the embryo modifies the physiology and body composition of broiler chickens reared in floor pens without affecting breast meat processing quality

Selection in broiler chickens has increased muscle mass without similar development of the cardiovascular and respiratory systems, resulting in limited ability to sustain high ambient temperatures. 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 identify 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 concentrations. At d 34, chickens were exposed to a moderate heat challenge (5 h, 32°C). Greater O2 saturation and reduced CO2 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 concentration ratio at d 28 and comb temperature during the heat challenge on d 34 (h(2) > 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 highlights the value of 2 new heritable characters involved in thermoregulation for further broiler selection.

The effects of early-age thermal manipulation and daily short-term fasting on performance and body temperatures in broiler exposed to heat stress

This study was conducted to investigate the effects of thermal manipulation at 5 days of age and short-term fasting during the warmest part of the day on responses to prolonged heat stress of broilers. A total of 240-day-old Ross 308 female broiler chicks were divided into three groups: control, thermal manipulation (chicks were exposed to 36 °C for 24 h at 5 days of age) and short-term fasting during the warmest part of the day (10.00-17.00 h). Prolonged heat stress was induced daily from 28 to 42 days by heating until the ambient temperature reached 32-35 °C between 10.00 and 17.00 h. Both thermal manipulation and short-term fasting resulted in a decrease in rectal temperatures and haematocrit values at 35 and 41 days of age. Thermal manipulation improved body weight, feed consumption and feed conversion. However, short-term fasting caused a reduction in body weight and a deterioration in feed conversion. Short-term fasting lowered the percentages of carcass, whereas thermal manipulation highered breast yield. Both thermal manipulation and short-term fasting decreased heart mass and abdominal fat.

Durability of induced heat tolerance by short-term heat challenge at broilers marketing age

A trial was conducted to determine the influence of short-term exposure to high ambient temperature at 28 and 35 days of age on deep body temperatures (Tb) and subsequent growth of birds until 42 days of age. A total of 90 day old chicks were reared in stainless steel battery cages and were assigned at random into 18 pens of 5 birds each, with 9 pens containing males and another 9 pens containing females. Three treatment groups, each represented by 3 male and 3 female pens, were represented by T1 without any heat exposure, T2 with heat exposure starting at day 28 and T3 with heat exposure starting at day 35. Heat stress was defined as 180 min exposure to 35 +/- 1 degrees C. Tb and body weights were measured at 35, 37 and 39 days of age immediately following heat exposure. Heat stress resulted in higher Tb and Onset of heat stress at 28 days resulted in significantly lower Tb than onset of heat stress at 35 days. Lower Tb in T2 than T3 permitted recovery in body weight at 42 days. Sexes responded similarly to heat stress.