In the second half of the previous century, special types of chicken breeds were developed to provide either meat or eggs. The specialized breeds are efficient producers. The disadvantage is that the male chicks of the layer breeds are not economically suitable for producing meat. In the current practice, male chicks of layer breeds are killed right after hatch.
This thesis reports a number of studies in layer chickens on sex dependent egg characteristic and the mechanisms of sex determination and offspring sex ratio adjustment. The developed knowledge in these fields is relevant for two proposed alternatives for the killing of male day-old chicks: 1) Sexing eggs before incubation, so that only female eggs may be incubated, and 2) Lowering the sex ratio in order to reduce the number of male eggs.
The studies showed that male and female unincubated eggs do not differ sufficiently in egg characteristics to allow sexing of freshly laid eggs. The characteristics measured included the yolk concentrations of testosterone, oestradiol, androstenedione, progesterone, dihydrotestosterone and glucose as well as egg mass, width and length. However, the studies also showed that chicken hens are able to manipulate their sex ratio. After feed restriction or treatment with corticosterone, the rate of decrease in hen body mass and egg mass were significant predictors of the primary offspring sex ratio. The studies provided three different pieces of evidence that meiotic drive in the pre-ovulatory follicle is the underlying mechanism of sex ratio adjustment.The animal experimental models developed in this research can be used to elucidate the underlying mechanisms of sex ratio bias in the ovary in more detail. Understanding the regulation of these mechanism could lead to practical methods to change the sex ratio in the poultry industry.