Multiple factors influence incubation and chick quality, making it a complex process with numerous challenges. Temperature regulation during incubation is critical, as excessively high or low temperatures can result in poor chick quality, such as reduced body weights and increased susceptibility to diseases like ascites. Humidity control is important, affecting the hatching process, where improper humidity can lead to malformed chicks or difficulties during the hatch. Ventilation within incubators is essential for oxygen supply and the removal of excess carbon dioxide; inadequate ventilation results in hypoxia, which adversely affects embryonic development. Egg position, turning angles, and airflow influence several processes, including gas exchange and heat transfer between the eggs and the external environment, egg water loss, adhesion of the embryo to the extra-embryonic membrane structures (corium, amnion, and allantois), and nutrient availability.

Egg handling before and during incubation, including storage conditions, significantly impacts hatchability and chick quality. Prolonged or poor storage conditions can reduce hatchability and chick vitality. Researchers have studied the potential benefits of cold exposure during incubation, such as lowering ascites, but it can also introduce risks, such as increased occurrences of hock burns and pododermatitis. Hatchery practices, including the use of multi-stage versus single-stage incubators, play a pivotal role, with the former requiring more complex management but offering better control over the incubation environment. The design of the hatchery facility itself, including insulation, ventilation and hygiene practices, is fundamental to maintaining an optimal environment.

Lastly, transport conditions for chicks post-hatch are crucial, as poor conditions can lead to moisture loss and temperature stress, reducing chick quality and subsequent performance. The design of an incubator is paramount to achieving high hatchability and healthy chicks. A well-sealed cabinet with no thermal leakage, combined with effective ventilation, humidity control, temperature regulation, egg turning, monitoring and stage programs, ensures that every egg within the incubator experiences the same optimal environment. Such meticulous design considerations lead to uniform embryo development, a reduced hatch window, and improved chick quality and welfare. There is a lack of studies examining the combined effects of CO₂ concentration and humidity during incubation in single-stage incubators, but recent research underscores the crucial roles of these factors in determining the success of a hatch. This review aims to provide a comprehensive understanding of the correct balance of CO₂ and humidity, drawing on current scientific literature.

Enhancing hatchability and chick quality

CO₂ levels play a crucial role in optimising incubation conditions to enhance hatchability and chick quality. Studies have shown that controlled increases in CO₂, particularly during the early stages of incubation, significantly improve hatch outcomes. Elevated CO₂ levels (up to 1.2%) during the first week of incubation, especially when the single-stage incubator is well-sealed and eggs have high fertility, are essential to achieving the required levels of CO₂ and humidity by day six. Researchers have found that these elevated CO₂ levels improve chick quality by accelerating organ development, increasing hormone secretion, and shortening the hatching window without compromising hatchability. For instance, controlled CO₂ environments resulted in higher relative weights of critical organs like the heart and intestine and increased levels of corticosterone and thyroid hormones, which are vital for embryo development. Similarly, non-ventilated intervals during early stages of incubation, allowing CO₂ levels to rise, have been linked to higher hatchability, earlier hatching times, and improved chick weight. These benefits can be attributed to the physiological impact of hypercapnic conditions, which enhance embryonic survival, particularly during the final days before hatching.

the early stages of incubation

Recent studies have shown that controlled CO₂ levels during the early stages of incubation (up to 7 days) can significantly influence embryonic development. Elevated CO₂ levels (around 1.2%) during this period have been found to accelerate development, shorten hatch time, and improve chick quality.
Elevated CO₂ acts as a metabolic stimulant, increasing the rate of cellular respiration and energy production in the developing embryo. This leads to faster organ development, particularly in the cardiovascular and respiratory systems. High CO₂ levels can enhance the dissolution of calcium carbonate from the eggshell, providing the developing embryo with essential calcium for skeletal development. This process is further supported by maintaining high humidity levels, which prevent excessive water loss from the egg.

Based on extensive experience with EmTech’s 20-stage program, CO₂ levels typically reach up to 1.2% by day 7, and sometimes as early as day 6, when eggs have higher fertility. After reaching this peak, the CO₂ set point is gradually reduced, reaching no more than 0.4% by the time of transfer. The hatcher itself follows a 10-stage program that smoothly increases CO₂ concentration to a maximum of 0.7%, with levels not exceeding 0.3% by hatching time. This controlled environment accelerates embryonic organ development, shortens the hatch window, and leads to a more synchronised hatch without compromising quality. Additionally, the hatcher program is designed to concentrate CO₂ to 0.7% until piping, then reduce it to 0.3% or less at the time of the hatch, thereby reducing embryo mortality and enhancing post-hatch performance.

Role of humidity on incubation and chick development

Humidity is another crucial parameter in the incubation process, influencing the rate of water loss from the egg and the development of the air cell. Proper humidity levels ensure that the eggs lose water at a rate that promotes healthy embryonic development. High humidity levels towards the end of the incubation period can lead to excessive moisture retention, resulting in chicks with unhealed navels or increased incidences of yolk sac infections. Conversely, low humidity can cause desiccation, leading to weaker chicks that struggle to hatch. The recommended relative humidity for incubation is between 50%-55%. Water loss from the egg is inevitable because of the presence of pores in the eggshell. The rate of water loss is determined by two factors: the water vapour conductance of the shell and shell membranes and the difference in water vapour pressure between the contents of the egg and the egg’s microenvironment. The water lost from the egg is replaced by air, creating the air cell at the blunt pole of the egg. During internal pipping, the embryo punctures the chorioallantoic and internal shell membranes and can then begin pulmonary ventilation, re-breathing the air cell gas. This allows it to begin the transition from diffusive respiration through the chorioallantoic to convective breathing through the lungs.

Exploring the combined influence of CO₂ and humidity

Studies have shown that eggs incubated under high humidity (80%) and elevated CO2 levels during the first week of incubation have higher hatchability rates compared to those incubated under standard conditions. High levels of CO₂ during the early incubation phase help regulate blood pH and enhance oxygen delivery to embryonic tissues. Balanced hydration is achieved as high humidity levels reduce water loss from the egg, maintaining the consistency of the amniotic fluid within the egg, which is crucial for enzymes and other biomolecules to function optimally during development.The combination of high humidity and elevated CO₂ creates an acidic environment that enhances the dissolution of calcium carbonate from the eggshell. This increased availability of calcium is vital for the development of a strong skeletal system in the embryo. High humidity reduces the risk of dehydration, ensuring that muscle and tissue development occurs without any metabolic hindrance. The presence of elevated CO2 increases the metabolic rate of the embryos, leading to quicker utilisation and absorption of yolk sac nutrients. This results in chicks with better weight and overall condition.

Impact on calcium mobilisation from the eggshell

The eggshell is primarily composed of calcium carbonate (CaCO3), which serves as a crucial source of calcium for the developing embryo and is essential for the formation of the chick’s skeletal system and other vital structures. The presence of high CO₂ and humidity creates an acidic environment inside the egg, which can enhance the dissolution of calcium carbonate. Carbon dioxide (CO₂) combines with high humidity (H2O) to form carbonic acid (H2CO3), {CO2 + H2O → H2CO3} which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-), {H2CO3 → H+ + HCO3-}. The increase in H+ ions creates a mildly acidic environment that facilitates the dissolution of calcium carbonate from the eggshell { CaCO3 + H+ → Ca2+ + HCO3-}. The dissolved calcium ions (Ca2+) are then absorbed by the embryo through the chorioallantoic membrane, a vascular structure that facilitates nutrient and gas exchange between the embryo and eggshell.

Summary and recommendations

CO₂‘s Role:
Controlled CO₂ levels, especially during early and late stages of incubation, improve chick quality by accelerating organ development, optimising hormone levels, and reducing hatch time. Specific CO₂ concentrations (up to 1.2%) during early incubation stimulate metabolic activity, enhance calcium mobilisation, and support uniform embryonic development.

Humidity’s Role:
Proper humidity management is crucial for controlling water loss from the egg, ensuring a balanced air cell formation while supporting healthy embryonic development. The recommended humidity levels vary throughout the incubation period, with higher levels towards the end helping to prevent dehydration and aiding in the final stages of hatching.

Combined Effects:
The synergistic effects of maintaining CO2 levels around 1.2% and humidity at 55% during incubation lead to significantly improved hatch rates and chick quality. This combination enhances gas exchange, supports calcium absorption from the eggshell, and promotes healthier and more uniform chick development.

Optimise CO2 Levels:
Hatcheries should maintain CO2 levels at approximately 1.2% during the first 7 days of incubation and from internal pipping to hatching. This range has been shown to enhance embryonic development, reduce hatch time, and improve chick quality without compromising hatchability.

Maintain Proper Humidity:
Incubators should be calibrated to keep humidity levels around 55% during the early stages of incubation and increase to 65%-70% during the final days (days 18-21). Proper humidity control is vital to ensure the right amount of water loss and to prevent issues like dehydration or fluid retention.

Combine CO2 and Humidity Control:
For the best hatchability and chick quality outcomes, combine elevated CO2 levels (up to 1.2%) with high humidity (80%) during the first week of incubation. This strategy enhances metabolic activity, improves calcium mobilisation, and promotes more uniform embryonic development.

Monitor Environmental Conditions:
Continuous monitoring and adjustments of both CO2 and humidity levels are necessary to maintain optimal conditions throughout the incubation period. This will help prevent any adverse effects related to improper environmental control, such as increased embryonic mortality or poor chick quality.

Incubator Types:
Ensuring that the incubator is well-sealed with no thermal leakage and utilising a well-designed stage program, such as the 20-stage program for setters and the 10-stage program for hatchers, maximises hatchability and chick quality.

References are available from the author on request