Genetic selection to enhance egg pr

Genetic selection to enhance egg production increases susceptibility
of laying hens to environmental temperatures. It is well
known that optimal temperature for laying hens is between 18
and 22 °C, with a relative humidity between 50% and 75% during
the laying period. Under lower or higher temperatures than optimum,
the cost of thermoregulation is near the maximum sustainable
threshold of energy mobilization and therefore less energy
will be available for egg production (Blem, 2000). There is
considerable evidence that body weight, feed intake, egg
production, egg weight and shell thickness are adversely affected
by high ambient temperatures (Smith, 1974; Deaton et al., 1982;
Mashaly et al., 2004; Mack et al., 2013) while cold exposure has
also a negative impact on egg production and egg quality of laying
hens and quails (Spinu and Degen, 1993; Şahin et al., 2003). Thus,
extreme conditions for laying hens result in major economic losses.
Previous studies showed that effect of higher or lower ambient
temperatures not only on these performance parameters, but also
blood parameters; such as blood acid–base status, triiodothyronine
levels, and heterophil: lymphocyte ratio (H:L) (Franco-Jimenez
and Beck, 2007; Melesse et al., 2011) and immunological
parameters of laying hens (Spinu and Degen, 1993; Hangalapura
et al., 2003). Earlier reports suggested that environmental stress
also diminishes antioxidant status of chickens by increasing malondialdehyde
(MDA), decreasing glutathione peroxidase (GSH-Px),
catalase (CAT), and superoxide dismutase (SOD) activity (Şahin
et al., 2003; Özkan et al., 2007; Lin et al., 2008).
Prenatal temperature conditioning is one of the strategies in
order to reduce the potential negative effect of heat or cold stress.
The theory or assumptions underlying this type of conditioning
that to induce long-lasting memory based on epigenetic adaptation.
It was hypothesis that the set point or response of the thermoregulation
system could be altered during the development
and maturation of hypothalamus–hypophysis–thyroid- and adrenal-axis
(Piestun et al., 2008). Because thyroid and adrenal glands
are functional by the end of the 7 d of incubation (De Groef et al.,
2008), the mid- or late-incubation days are critical for prenatal
conditioning (Jenkins and Porter, 2004; Yahav et al., 2004a). Several
studies have reported that broiler embryos exposed to short
term (changing from 3 to 12 h per day) either higher or lower
cyclic or constant incubation temperatures during the critical
periods in embryonic development had improved temperature
resistance in later life (Yahav et al., 2004b; Collin et al., 2005;
Yalçın et al., 2005; Piestun et al., 2011). Our previous studies also
showed that exposing broiler embryos to 1 or 2 °C higher or lower
incubation temperatures from day 10 to 18 showed adaptive response
and improved temperature tolerance when exposed to
higher or lower ambient temperatures, respectively, from day 21
to 42 (Yalçın et al., 2008a,b, 2012a; Akşit et al., 2013).
It would be hypothesized that prenatal temperature conditioning
may also be beneficial tool for laying hen embryos to
adapt to lower or higher environmental temperatures later in life.
Glatz (1997) reported that constant higher incubation temperature
(0.5–2 °C) from day 10 to end of incubation resulted in lower body
weights at 18 wk and smaller eggs over the laying period. Walstra
et al. (2010) demonstrated that laying hen embryos exposed to
40 °C for 4 h/d from 14 to 18 d of incubation responded to higher
environmental temperatures only until 8 d of age. It was shown
that laying chicks exposed to higher incubation temperature had
more mature pattern of glucocorticoid release at 23 d under high
ambient temperature (Wilsterman et al., 2015). To our knowledge,
the long-term effect of prenatal temperature conditioning on response
of laying hens to heat or cold stress during the laying
period has not been studied. Therefore, the objectives of this study
were to evaluate (1) the effect of lower or higher incubation
temperatures from 10 to 18 d of incubation on hatching and laying
performance and (2) the response of prenatal temperature conditioned
hens to cold or heat stress during the laying period.
Plasma triiodothyronine (T3) and thyroxine (T4) levels and oxidant
and antioxidant enzyme activities of laying hens were also
investigated