Embryonic Thermal Manipulation Affects Ventilation, Metabolism, Thermal Control and Central Dopamine in Newly Hatched and Juvenile Chicks

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.

Effect of thermal manipulation on embryonic development, hatching process, and chick quality under heat-stress conditions

Thermal stress is a risk that threatens poultry welfare and productivity. Thermal manipulation during egg incubation is considered a prevention strategy used to mitigate the detrimental effects of high ambient temperatures on birds. This study aimed to investigate the impact of thermal manipulation, applied to chicken breeder's eggs during the incubation period, on embryonic development, hatching characteristics, and chick quality, as well as posthatch thermotolerance and performance. A total of 1,200 fertile eggs were randomly and equally assigned into 2 groups of 3 replicates (200 eggs/replicate), using a randomized experimental design followed by t test. The first group eggs (G1) were subjected to a commercial setter temperature of 37.5°C with 55% relative humidity (RH) throughout the incubation period (1-18 d) and served as a control, while the second group eggs (G2) were treated the same commercial setter conditions until the 11th day of the incubation, then the eggs were exposed to a higher temperature of 39.5°C with 60% RH for 4 h daily from the 12th to the 18th day of incubation. All eggs in both groups were exposed to the same temperature condition of 37.2°C with 70% RH from the 19th to the 22nd days of the incubation (hatching period). Three hundred hatched female chicks per each treatment group were transferred into a closed-system house and distributed randomly into 20 floor pens (15 birds per pen). At the 8th week of age, birds were exposed to a daily heat challenge by raising the temperature to 35°C for 6 h until the 18th week of the chick's age. According to the results, thermal manipulation at 12 to 18 d of egg incubation positively (P ≤ 0.05) affected several studied traits. It improved some embryonic development traits, such as embryonic weight and tibia length, as well as some hatching parameters, such as hatching time and pipped eggs. It also improved hatched chick quality traits, including the chick's weight, length, and activity. In addition, it enhanced the posthatch chick's thermotolerance and body weight. Hatched chicks of G2 had significantly (P ≤ 0.05) higher total protein, albumin, IgM, glucose, calcium, total antioxidant, and T3 than G1 chicks. They also had significantly (P = 0.001) higher body weight (23%) at the 18th week of age than G1, as well as a lower feed conversion ratio (20.71%) than G1 chicks at 8 to 18 wk of age. Therefore, it is recommended to apply thermal manipulation during egg incubation, particularly at 12 to 18 d, for its positive effects on the pre- and posthatch performance.

Heat stress affects breathing and metabolism of chicks incubated at high temperature

Global warming poses serious implications to animal physiology and a gradual increase in ambient temperature affects all living organisms, particularly fast-growing selected species. We recorded ventilation (V̇_E), body temperature (T_B), oxygen consumption (V̇O₂) and respiratory equivalent (V̇_E/V̇O₂) of 14-day-old (14d) male and female chicks at room air conditions, hypercapnia and hypoxia at heat stress (HS, 32 °C). These chicks had previously been exposed to control (CI, 37.5°C) and high (HI, 39°C) temperatures during the first 5 days of incubation. Under resting conditions, acute HS increased V̇_E in HI females but not in HI males. Hypercapnia combined with heat promoted a potentiation of CO₂-hyperventilatory response in HI females when compared with thermoneutral condition, whereas in HI incubated males a hypoventilation under hypercapnia and heat stress was observed compared to the CI group. Hypoxia associated with heat stress increased V̇_E only in HI females. Our data indicates that females are more sensitive to thermal manipulation during incubation and it seems that the thermal embryonic manipulation, at least during the first days of development, does not improve the adaptive response of chicks to heat stress.

Effects of embryonic thermal manipulation on the immune response to post-hatch Escherichia coli challenge in broiler chicken

Background and Aim

Thermal manipulation (TM), exposure to mild heat shock during embryogenesis, which is a critical developmental period of broiler chickens, improves tissue stability, oxidative stress response, and immune response during heat stress. Thermal manipulation could be more cost-effective than other methods to boost the immune response. This study aimed to evaluate the impact of TM during embryogenesis, concomitant with an Escherichia coli challenge, on body weight (BW), body temperature (Tb), and splenic mRNA expression of cytokines (Interleukin [IL]-1β, IL-2, IL-6, IL-8, IL-12, IL-15, IL-16, IL-18, and interferon [IFN]-γ) in poultry.

Materials and Methods

A total of 740 fertile eggs were procured from a certified Ross broiler breeder. The eggs were divided into two incubation groups: the control and TM groups. The eggs in the control group were kept at 37.8°C air temperature and 56% relative humidity (RH) during incubation; eggs of the TM group were incubated under standard conditions, except for embryonic days 10-18, during which they were incubated at 39°C and 65% RH for 18 h daily. On the 7th day of incubation, eggs with dead embryos were excluded. After hatching was complete, each group was further subdivided into saline-treated or E. coli-challenged groups. The E. coli (serotype 078 with the dose of 1.5 × 105 colony-forming unit/mL) challenge was performed when the birds were 20 days old. Body weight and Tb measurements were taken on post-hatch days 20, 21, 23, and 25. Splenic mRNA expression of cytokines (IL-1β, IL-2, IL-6, IL-8, IL-12, IL-15, IL-16, IL-18, and IFN-γ) was analyzed by real-time quantitative polymerase chain reaction.

Results

Following the E. coli challenge, the TM-treated group's body performance parameters (BW and Tb) were significantly increased compared with the control group. Body weight was higher in the TM group than in the control group (p < 0.05); Tb was lower in the TM group than in the control group (p < 0.05). The mRNA levels of IL and IFN-γ were more stable and moderately induced in the TM group compared with the control group. Thermal manipulation altered the basal mRNA levels of ILs and IFN-γ and changed their expression dynamics after the E. coli challenge.

Conclusion

Thermal manipulation during embryogenesis could boost the immune system response to E. coli.