In the commercial layer industry, approximately 50 per cent of chicks are killed immediately after hatch, because they are males and have slow growth rate and inferior meat characteristics when compared to male broilers, according to Kristen J. Navara and Jeanna L. Wilson of the University of Georgia in a report of research sponsored by the US Poultry & Egg Association.

They proposed that, if the poultry industry could manipulate layer hens such that a majority of chicks were female, productivity would dramatically increase. While attempts have been made to estimate embryo sex and eliminate eggs that produce males, it would be even more fruitful if hens produced more female-bearing eggs from the start.

The overall aim of this work was to identify an optimal treatment to stimulate hens to skew sex ratios of offspring towards females. The researchers initially intended to use the reproductive hormone, progesterone. However, they say they quickly found that progesterone was nearly impossible to utilise as a treatment for these purposes because progesterone interrupted the laying cycle in 77 per cent of the hens injected.

As a result, they chose to use the stress hormone, corticosterone, which has previously been shown to skew sex ratios towards females in avian species.

Specific objectives were to (1) confirm previous findings that corticosterone stimulates offspring sex ratio skews towards females and determine the lowest effective dose, (2) determine the optimal treatment levels of corticosterone for sex ratio manipulation towards females, and (3) identify potential adverse effects on reproductive success of females resulting from short-term corticosterone treatment.

Treatment of hens with a high dose of corticosterone five hours prior to ovulation resulted in the production of 83 per cent male offspring. This result is the opposite of that expected, because work in other avian species has shown that long-term treatment stimulates a female-bias. The Georgia researchers explained, however, that their experiment differed from previous work in that they provided a single, higher short-term dose, which could explain why the results differed in direction.

A three-fold lower dose of corticosterone did not exert a similar effect, and so they determined that the lowest effective dose of corticosterone was their high dose.

There was also no effect of the same high-dose treatment given at four hours prior to ovulation, suggesting that the optimal treatment regimen is a high dose at five hours prior to ovulation. There was no adverse effect on the fertility of eggs compared to either control-treated hens or uninjected hens.

The researchers found that an injection of testosterone exerted similar effects, producing a male-biased sex ratio (73 per cent) and very little impact on fertility or the laying cycle.

Navara and Wilson concluded that they established optimal treatment and timing for producing a significantly biased, offspring, sex ratio in White Leghorns. While the skew was not towards females, the effects of a corticosterone inhibitor may be explored in attempt to produce a female-biased sex ratio, they added