The significance and importance of the pre-incubation and incubation temperatures for broiler chickens has been elucidated by altering normal incubation conditions to study the effects on embryo development, according to Y. Piestun of the Volcani Center in Israel and co-authors there and North Carolina State University in the US.

In their paper in Poultry Science, they continue that only recently has convincing evidence that temperature could influence the sex ratio of avian offspring become available.

The researchers found in their experiments that thermal treatments pre-incubation or during the sex determination period of incubation had, in general, a positive effect on hatchability, growth performance and secondary sexual characteristics of broiler males and females,

The objective of their study was to elucidate the effects of temperature before or during (or both) the sex determination period of incubation on hatchability, apparent sex ratio, growth and development post-hatching, and secondary sexual phenotypic characteristics.

Two experiments were conducted in winter and summer using Cobb 500 fertile eggs that had been stored for four and nine days, respectively.

Four treatments of 180 eggs each were applied: control, pre-heating (Pre) 30.2°C for 12 hours before incubation, heating (38.1°C) the embryos between embryonic day 0 (E0) and E5 (M) of incubation, and a combination of both (Pre+M).

All three thermal treatments increased early embryonic deaths but improved hatchability in both experiments.

The point of 50 per cent hatchability was achieved more rapidly in the treated eggs. The bodyweights of males and females at 35 days of age in both experiments were numerically or significantly greater in the broilers that had been exposed to thermal treatments, which was coincident with a similar trend for increased relative breast muscle weight.

Secondary sexual characteristics (comb, wattles, testes in males) were also affected by thermal treatments, being heavier in most cases, which may be attributed to the finding that the three thermal treatments resulted in numerically or significantly increased plasma testosterone concentration in both sexes and experiments.

Differences in the level of significance between the experiments probably related to the length of storage period and the season in which each experiment took place.

Piestun and co-authors concluded that thermal treatments pre-incubation or during the sex determination period of incubation had, in general, a positive effect on hatchability, growth performance and secondary sexual characteristics of broiler males and females, probably caused by the increase of plasma testosterone concentration in both sexes.

Reference

^{Piestun Y., S. Druyan, J. Brake and S. Yahav. 2013. Thermal treatments prior to and during the beginning of incubation affect phenotypic characteristics of broiler chickens posthatching. Poult. Sci. 92(4):882-889. doi: 10.3382/ps.2012-02568}