1. most cracks that occur in a battery cage are produced when the egg drops on to the floor at oviposition;

2. they are located predominantly in the two polar thirds of the shell;

3. their incidence is determined mainly by the effective mass of the cage floor, the mean drop height and the mean attitude of the eggs at emergence from the cloaca (broad or narrowpole first) ;it is affected only slightly by the mean shell thickness and the overall mean shell curvature (egg size);

4. if a hen is kept on a floor with a high effective mass, an indirect estimate of her mean drop height can be obtained quickly, easily and with high precision from observations made on her eggs after they have been laid; the observations required are the incidence of eggs with poorly formed shells, the type, location and incidence of cracks in eggs with well‐formed shells and the mean weight of the eggs;

5. the incidence of cracking on a heavy cage floor can be used to predict the incidence on a normal floor;

6. the incidence of cracking is heritable and responds rapidly to downward selection.

It is suggested that breeders should test the technique described, with a view to adopting it as a method of selecting for low crack incidence.  相似文献