Dr. Inge van Roovert-Reijrink
Carla van der Pol M.Sc.

Chick quality
Besides optimizing hatchability, an important aim of a hatchery is to deliver perfect day-old chickens. Chick quality is expressed in many ways, most important of which are number of second grades, hatchling yolk free body mass, chick length, navel quality, and first week growth and mortality. During the incubation process, eggshell temperature, carbon dioxide concentration, and post hatch environment are the most important drivers of embryo development and, therefore, chick quality and subsequent performance. In the current article, all of these factors are discussed as well as how they depend on incubator design. 

Eggshell temperatures affect chick quality
Several studies aimed to determine the optimal eggshell temperature (EST). Lourens et al. (2005) showed that an EST lower than 37.8°C (100°F) during the first week of incubation or higher than 37.8°C  during the third week of incubation resulted in a higher percentage of second grade chickens (up to 5% more) and shorter chick length (up to 5 mm smaller). Molenaar et al. (2011) showed that an EST of 38.9°C from day 7 of incubation onwards increased the incidence of ascites in later life in comparison to an EST of 37.8°C. 
EST in the hatcher phase also has a large impact on chick quality. Maatjens et al. (2014) applied 3 ESTs from day 19 of incubation onward and found higher relative heart weights at hatch for 36.7 and 37.8°C than for 38.9°C (0.69% and 0.66% vs. 0.58%, respectively). These results suggest that it may be even better to incubate at a temperature slightly below 37.8°C than above it; yolk free body mass of the 36.7°C incubated chicks was 0.65 g higher at hatch than that of 38.9°C incubated chicks. All these studies show the relevance of an optimal EST during incubation to obtain superior chick quality. 

Incubator design and eggshell temperatures 
EST is influenced by embryonic heat production and heat transfer capacity of the air, which depends on air temperature, air velocity, and relative humidity. All of these are greatly dependent on incubator design. The machine needs to have enough heating (prior to about day 9 of incubation) and cooling (after day 9, when embryonic heat production increases) capacity to maintain the right temperature. Air velocity should be uniform, like in HatchTech’s incubators with the laminar concept, to reduce variation in heat transfer capacity and, thereby, ESTs. Relative humidity affects the air’s heat transfer capacity because humid air transfers heat better than dry air. To maintain optimal ESTs throughout incubation, it would be ideal to maintain a high relative humidity during the whole incubation process. However, this is impossible because an egg needs to lose at least 10%, but preferably around 12%, of its weight at day 18 of incubation to maximize hatchability. Therefore, a balance must be found between heat transfer capacity created by relative humidity (to maintain uniform ESTs) and egg weight loss control. When the design of an incubator and all its settings are correct, EST should always be close to 37.8°C or below 37.8°C after day 19 of incubation to obtain superior chick quality. Facts&Figures HatchTech Magazine

CO2 concentrations affect chick quality
In commercial hatcheries, the CO2 concentration is often maintained below a maximum level to supply enough fresh air to the developing embryos. Early on in life, CO2 concentration does not seem to have a large impact on chick quality. De Smit et al. (2006) showed that a gradual increase of CO2 to a concentration above 1% during the first 10 days of incubation had a positive effect on relative growth during the first week of life, but did not result in higher body weights at slaughter age. Everaert et al. (2007) demonstrated that chicken embryos can tolerate high CO2 concentrations (4%) between day 10 and 18 of incubation without an effect on chick quality. 
CO2 concentrations become more crucial in the hatching phase. In practice, some hatchery managers try to reduce the hatch window (time between the first and last chicken hatched) by reducing the inlet of fresh air into the hatcher, increasing CO2 concentrations in the hatcher above 0.8%. This triggers the embryo to hatch earlier than if the maximum CO2 concentration is below 0.35%. However, Maatjens et al. (2014) found lower navel quality at 12 hours post hatch for chicks exposed to 1.0% CO2 from day 19 of incubation onward compared to chicks exposed to 0.2% CO2. This emphasizes the importance of maintaining CO2 concentrations at a lower level to guarantee superior chick quality. . 
A combination of high EST and a high CO2 concentration during the hatching process might be even more detrimental for chick...