Young breeder flocks are producing day old chicks that are in some ways different from day old chicks produced by older flocks. In some ways the chicks from young breeder flocks have to be considered as neo-natals. However, the reasons for this difference in off spring is not clear,
In broiler breeders, young breeder flocks are typically flocks younger than 31-32 weeks of age, where in layer flocks the treshold is more or less an age of approximately 27 weeks. Although the egg weight and with it the chick weight is obviously lower in young breeder flocks, this doesnt seem to explain all the differences that characterize younger breeder flocks compare to older breeder flocks. Also the quick change in characteristics once the flock has reached the critical age indicates that not only egg weight is a factor in the differences between young and "normal" breeder flocks:
Eggs from young breeder flocks contain smaller embryos with less viable cells than eggs from older breeder flocks
Young breeder flocks produce eggs that contain embryos that are not only smaller, but also are in an earlier stage of embryonic mortality
Probably because of the difference in development, eggs from young breeder flocks are more sensitive for prolonged storage times.
During incubation, more early embryonic death takes place in eggs from young breeder flocks
During incubation, the heat production of eggs from young breeder flocks is lower
Also day old chicks from young breeder flocks produce less heat, both per chick and per gram of chick.
Day old chicks from young breeder flcoks are more sensitive for lower temperarue, and cant resist lower enviromnental temperatures for a longer time than chicks from older breeder flocks.
When too cold, day old chicks from young breeder flocks do not "scream" that much, but are more quietly suffering from that low temperature, giving less clear indications for problems.
Mortality is usual higher in young breeder flock chicks, especially when the houses and the floors are not on temperature.
Over the years many attempts have been made to get more control over these characteristics, for instance through feeding profiles for the breeders, but the success has been limited. The most important management tool seems to be to make sure that the embryos and day old chicks stay on the correct temperature.