Dr. Conny
C. M. Maatjens

What is the importance of temperature during incubation?
During incubation, temperature drives chicken embryonic metabolism, development, and growth. Incubation conditions need to be adjusted to meet embryonic requirements to obtain optimal chick quality and hatchability. To obtain optimal chick quality, embryo temperature, rather than incubator temperature, needs to be controlled. Eggshell temperature (EST) can be used as a non-invasive method to determine embryo temperature.

It was previously found that a high EST of 38.9°C in the second or third week of incubation resulted in higher third week embryonic mortality, lower hatchability, and lower chick quality at hatch, expressed as a shorter chick length and lower yolk free body mass (YFBM) compared to a constant EST of 37.8°C throughout incubation. These negative effects of a high EST in the second part of incubation might be explained by a higher metabolic rate, which is the energy expenditure of the embryos, due to the high incubation temperature. It appears that at a high EST the balance between metabolic rate and O2 availability is disturbed, which may result in higher embryonic mortality and impaired embryonic development. Lowering EST might restore the balance between metabolic rate and O2 availability and may postpone or even prevent the embryos from experiencing O2 shortage as incubation progresses.

What is the importance of CO2 during incubation?
Besides temperature, the CO2 concentration during incubation seems to affect embryonic development. Several studies suggest potential effects of different CO2 concentrations applied during the second or third week of incubation on embryonic development, hatching
parameters,changes in acid-base balance, physiological responses of broiler and layer embryos, and post-hatch growth of broiler chicks. In the current thesis the effect of CO2 and the interaction with EST was investigated during the hatching phase to investigate which of the two factors had the largest effect on embryonic development.

Aims of the thesis
Based on the potential effects of (lower) EST during the last week of incubation and of CO2 during only the hatching phase, as described above, the following three aims were derived for this thesis:

1. To investigate effects of EST during the last phase of the incubation process, with special attention for EST below the general accepted optimal EST of 37.8°C.
2. To examine from which day of the incubationprocess onward EST should be changed from 37.8°C.
3.  To investigate whether CO2 concentrations are interacting with EST during the hatcher phase.

Materials and methods
In the first experiment, 600 Ross 308 eggs were individually monitored from embryonic day (E)19 until hatch. Effects of an EST of 36.7, 37.8, and 38.9°C in combination with a CO2 concentration of 2,000 or 10,000 ppm were investigated.

In the second experiment, 2,870 Ross 308 eggs were individually monitored from E15 until hatch. During that experiment, effects of an EST of 35.6, 36.7, 37.8, and 38.9°C starting from E15, E17, or E19 onward were investigated. In addition, 900 chicks were placed in a broiler grow-out facility to investigate effects on first week post- hatch development and growth performance. Both experiments were performed in climate respiration chambers with low air flow, where the climate could carefully be controlled. In both experiments, eggs were selected for an egg weight of 62 to 65 grams.

What is the effect of EST in the hatching phase on chick quality and physiology?
From the first experiment, it was concluded that time until hatch was longer at an EST of 36.7°C compared to 37.8 and 38.9°C, which might be caused by the lower metabolic rate of embryos that were exposed to an EST of 36.7°C. Although time until hatch was prolonged at an EST of 36.7°C, hatch of fertile (HOF) was not affected by EST. A similar result was found in experiment 2, where it was concluded that time until hatch was longer at an EST of 35.6°C, followed by 36.7, 37.8, and 38.9°C. The effect of EST on total incubation duration was mainly caused by the effect of EST on time until IP, possibly because of the lower metabolic rate at an EST below 37.8°C which reduces O2 requirement and subsequently ensures that the embryo can use fatty acids as an efficient energy source for a longer period than at a higher EST. Consequently, the energy produced from fatty acids can be used for growth and development, resulting in a higher YFBM at an EST of 36.7°C compared to 38.9°C in experiment 1, and a higher YFBM for an EST of 35.6 and 36.7°C compared to 37.8 and 38.9°C in experiment 2. The higher YFBM at a lower EST may suggest that protein from
yolk and albumen is used for growth and development which was supported by the lower residual yolk weight at an EST of 35.6, 36.7, and 37.8°C compared to 38.9°C. In the first experiment, a high EST of 38.9°C was found to affect chick metabolism, which was demonstrated by the lower sugar stores (glycogen) in the liver at hatch and lower lactate levels (which are a sign of sugar breakdown
in the body) at 12h after hatch compared to an EST of 37.8°C. This...