Body weight at point of lay and the ultra-structural properties of eggshells

E.K. Suawa and J.R. Robert

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Ultra-structural studies have demonstrated that the eggshell is comprised of morphological distinct calcified layers with the mammillary layer being the foundation of the eggshell.

Studies have identified ultra-structural variations in the mammillary layer that can be used as indicators of eggshell quality. Understanding the ultrastructure of eggshells has reinforced the view that the mechanical properties of the eggshell cannot be defined by a simple thickness measurement.

Body weight at point of lay is a major factor influencing subsequent egg size, and this applies to both immature and more mature ages. Larger birds consume more feed while producing larger sized eggs with inferior eggshell quality. The current study was conducted to investigate the relationship between body weight at point of lay and ultra-structural properties of the mammillary layer of the eggshell.

Sixty Hy-Line brown hens were housed individually in cages. Three different groups of body weight (light, medium and heavy – mean body weight at 16 weeks of 1.170 kg, 1.337 kg and 1.507 kg, respectively) were selected. A total of 10 eggshell samples were processed for ultra-structural observation from each group at 10 week intervals during the laying life of the flock: 20, 30, 40, 50, 60, 70 and 80 weeks of age.

A small equatorial piece of eggshell was cut from each eggshell and the inner membrane removed manually. The outer membrane was removed using a Plasma Etcher. Pieces of shell were then mounted, inner surface uppermost, onto an aluminum stub and coated with gold using a sputter coating unit for 5 minutes. Mammillary ultrastructure variables were viewed using a JEOL Neoscope SEM and scored after the method of Solomon (1991).

There was no significant effect of body weight at point of lay on ultra-structural properties except for cap quality, which was best in the light birds, and cuffing and changed membrane, which were best in the heavy birds. Cap quality estimates the extent of contact between the shell membranes and the shell, which is thought to influence shell strength. Cuffing is additional calcite around and between adjacent mammillary cones, which also increases shell strength. Changed membrane may reflect abnormal conditions within the oviduct while the eggshell was being laid down. Alignment tended (P=0.09) to have a lower incidence in the heavy group. As hen age increased, the incidence of ultra-structural features known to be associated with poorer shell quality increased: alignment of mammillae, type A bodies, type B bodies, aragonite, late fusion and pitting; cap size became more variable and cap quality poorer.

The incidence of ultra-structural features that have been shown to be associated with good shell quality, such as early fusion and confluence decreased as the flock increased in age.