The difference in hatchability among strains was
highly significant and was related to the age of the
parent flock. Management at the breeder farm as well
as at the hatchery should be adjusted according to the
strains, because every strain responded differently to
hatchability. Fertility of an egg and embryonic mortality
during the hatching process are the most important
characteristics to determine hatchability, and it is
known to be different for different strains. As the flock
ages, for example, there was more infertility and early
embryonic mortality in eggs from Ross 308 compared
with Cobb 500 (Deeming and Van Middelkoop, 1999).
Among different strains, difference was also found in
egg weight and components like the yolk and albumen
percentage, yolk:albumen ratio, shell percentage,
and incubation time (Suarez et al., 1997; Joseph and
Moran, 2005b). As revised by Decuypere and Michels
(1992), there is a significant difference in the tolerance
limits for temperature fluctuation during incubation
not only between strains but also within strains. Besides,
genetic strain and parent flock age influence daily
embryonic metabolism during the early and latter
days of incubation, which coincides with the incidence
of greater embryonic mortality during this period of incubation
(Hamidu et al., 2007).