Approaches to determine the sex prior to and after incubation of chicken eggs and of day-old chicks

Domestic chickens produce eggs that contain at almost equal proportions female and male embryos. For many centuries, domestic chickens were bred as dual-purpose birds for both egg and meat production. In contrast, commercial hybrid chickens are selected for one purpose only; either eggs or meat. Surplus day-old male chicks of the layer type are currently without significant commercial value and are at best used as food for e.g. carnivorous animals. All attempts to raise male chicks of the layer type for meat production have resulted in high consumption of feed and poor meat yield which is difficult to market at competitive prices. Ethical views alongside commercial considerations are now confronted with a serious dilemma regarding the limited and reasonable utilization of newly hatched male hybrid chicks from laying strains. An abundance of scientific and practical efforts have been made in both past and present times to detect the sex in eggs preincubation and in embryos as early as possible. These studies include attempts to determine the sex in eggs prior to and during incubation and in day-old chicks. Various manual and instrumental morphological, enzymatic and molecular methods were used. This paper reviews these methods and discusses their relevance for practical breeding and production purposes.