To know the stage of development of the embryos is essential to determine the correct time to achieve the best
hatchability and to perform In-Ovo injection. Moreover, embryonic development is directly associated to many
aspects of the incubation process.
Before In-Ovo injection is taken into consideration, it is important to remember that many studies have shown
that eggs transferred too early (around day 17 of incubation) from Incubators to Hatchers tend to have their
hatchability reduced when compared to eggs transferred later (day 18 or 19), whether they are injected or not.
The differences (water loss, humidity, heat and gas exchange) in the environment of these two types of
equipment are the main responsible for differences in hatchability.
Normally, a positive effect on hatchability is observed if the embryo remains in the incubator for longer time, and
is transferred to the hatchers later. Also, difference in embryonic development may be observed in different
types of incubators due to their different water loss and temperature standards, and to determine the correct
time for In-Ovo injection - to achieve maximum hatchability and good immune response against diseases - the
physiological characteristics of the embryo are much more important to be considered than the number of hours
of incubation.
The optimal timing for In-Ovo Injection is normally recommended to be between 17.5 and 19.2 days of
Incubation (count starts from Egg Set Time). The lower limit – 17.5 days – is normally related to the period the
yolk sac is starting to enter the abdomen and the head of the bird is positioned under the right wing. The upper
limit – 19.2 days – is limited by the percentage of eggs presenting external pipping, since the presence of
multiple holes in the egg can contribute to egg cracking during transfer process. Eggs cracked on the side or the
small end generally result in embryos stuck to the exposed (and dried) membrane or the egg shell, therefore
reducing hatchability. It is not recommended to perform In-Ovo injection in flocks with more than 1% of pipped
eggs.