CHANGES IN FUNCTIONAL QUALITIES OF TURKEY EGGSHELLS IN STRAINS SELECTED FOR INCREASED EGG-PRODUCTION OR GROWTH

Much past research has been devoted to physical eggshell qualities, i.e., weight, deformation, or egg density, but little research has investigated the functional eggshell qualities when expressed in terms of eggshell conductance to gases. Conductance is a measure of the ease with which gases diffuse across the pores of the shell and is (by Fick's law) determined by the number and geometry of the individual pores. The objective of the present study was to determine changes that have occurred in functional eggshell characteristics (eggshell conductance) following selection for egg production or growth over nearly 30 generations. Major changes have occurred in the physical dimensions of eggs from selected lines of turkeys compared with their respective randombred controls. Egg weights increased 11% due to selection for growth and decreased 14% due to selection for egg production. Other measurements such as length and width increased allometrically with egg weights. Selection for egg production has decreased the physical dimensions (egg weight, volume, or surface area) of eggs, and selection for growth has increased the same measurements, but adjustments have not been made by selected hens to increase or decrease the functional characteristic (conductance) allometrically (conductance constants). An interaction was observed in eggshell conductance constants as the hens aged, suggesting that genetic selection may affect functional characteristics. This suggests that the energetics of embryonic development may be different in eggs from turkey hens of various ages. Energetics may result in poor hatchability and poult quality. It can be concluded that as the hens aged the conductance and conductance constants did not change allometrically with increasing egg weight. This may suggest a reason for increased embryonic mortality seen in the selected lines compared with their randombred control lines.