Interrelationships of egg size, chick size, posthatching growth and hatchability

Several interrelationships are apparent between egg weight, chick weight, chick growth and other parameters. The hatchability of intermediate size eggs is better than that of very large or very small eggs. Decreased hatchability, hatchling weight, chick growth and chick viability have been associated with the first few eggs produced by young pullets. Incubation time is positively correlated with egg size. Embryo weight is not correlated with egg weight during the first half of the incubation period. Thereafter the correlation increases and reaches a maximum at the time of hatching (0.5–0.95). Chick weight is primarily determined by initial egg weight, normally being 62–78% of egg weight, and is secondarily determined by weight loss during incubation, shell and residue weight, strain, incubation time and conditions, breeder age and chick sex. The correlation of egg weight to posthatching chick weight decreases with increasing age of the chick. A 1 g change in egg weight has been shown to result in a corresponding change of 2–13 g in broiler weight at 6 to 8 weeks of age. This effect is much greater in eggs from young breeders than in those from old breeders. The effects of egg weight on chick feed conversion have been variable with no clear conclusion. Improved growth and uniformity may be obtained by growing chicks separately according to hatchling size. Therefore, sorting eggs by weight prior to incubation might be advantageous in some production operations to improve broiler or pullet uniformity and efficiency.