3rd International Poultry Meat Congress

e the most important variable to control during incubation (Romijn and Lokhorst, 1951), only
in the last decade embryo temperature (or eggshell temperature (EST) as a reflection of embryo
temperature) is used to control the incubation process in commercial hatcheries (Meijerhof
and Van Beek, 1993; French, 1997). Historically, not the EST, but the air temperature of the
incubator (machine temperature) is controlled and maintained between 36 and 38oC (Decuypere
et al., 2001). However, when machine temperature is maintained throughout incubation at a
fixed level of 36 to 38oC, embryo temperature will increase at later stages of incubation, due to
the increase in embryonic heat production (see below). Studies in the last decade have shown
that a constant EST of 37.5 to 38.0oC, rather than a constant machine temperature, results in
the highest hatchability and chicken quality (Lourens et al., 2005, 2007; Joseph et al., 2006;
Leksrisompong et al., 2007).
The EST is the result of embryonic heat production and heat transfer from the egg to the
environment. Embryonic heat production is very low at the first week of incubation, but increases
exponentially after day 7 to 9 of incubation (Lourens et al., 2007, Molenaar et al., 2010, Nangsuay
et al., 2013). Between day 15 and 18 of incubation, heat production reaches a plateau phase of
approximately 140 to 150mW/egg (Dietz et al., 1998; Lourens et al., 2006a, 2007; Molenaar et
al., 2010; Nangsuay et al., 2013). After internal pipping at day 19 of incubation, embryos changes
their respiration from the chorio-allantoic membrance (CAM) to lung ventilation, resulting in a
strong increase in heat production at that stage (Rahn et al., 1981; Janke et al., 2004; Figure 1).
Figure 1. Embryonic heat production, evaporative heat loss, embryo temperature and machine
temperature in relation to the day of incubation, assuming a constant EST throughout incubation
(kindly provided by A. Lourens).
Besides embryonic heat production, EST is also determined by heat transfer. Heat transfer from
the egg to the surrounding environment is affected by three main factors: 1) machine temperature,
2) air velocity and 3) relative humidity (Meijerhof and Van Beek, 1993; Lourens et al., 2011).
1) To maintain EST between 37.5 and 38.0oC throughout incubation, machine temperature need
to be higher than 37.5 to 38.0oC during the first days of incubation (French, 1997; Lourens
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