At Alltech's recent 25th International Animal Health and Nutrition Symposium, Dr. Sally Solomon, a professor at the University of Glasgow, reviewed the structure of the reproductive tract of the hen followed by a detailed review of shell formation. Specific comments were provided on initiation of mineralization based on the fibers extending from the shell membranes followed by deposition of calcium carbonate in calcitic form. 

Development of the mammillary bodies which fuse followed by the subsequent production of the palisade and vertical crystalline layers of the shell must be sequenced to achieve optimal shell quality. Appropriate function of the tubular shell gland and the shell gland pouch, in which the majority of mineralization occurs, is critical to achieving quality. It is self evident that damage to this delicate secretory tissue by infectious bronchitis virus or possibly Mycoplasma spp. will have a detrimental effect on shell quality. 

Dr. Solomon noted that shell thickness is not a measure of quality. She defines “effective thickness” which relates to appropriate micro-structure and suggested that ultra structural analysis, resonance assay and dynamic force measurements are more effective criteria of shell quality. Defective shells generally have disorganized mammillae (incomplete mineralization and erosions). This leads to micro-cracks which cannot be detected with conventional egg crack detectors. 

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Dr. Sally Solomon presents at Alltech's recent 25th International Animal Health and Nutrition Symposium.

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Dr. Solomon claims that shell quality has in fact declined in commercial eggs over the past 15 years, attributed to genetic selection. In contrast the quality of broiler hatching eggs has improved. This is probably an indirect effect of selection for hatchability since shell quality is an important determinant of embryonic development.

Dr. Solomon considers that genetic selection is the most significant factor in obtaining future improvements in shell quality. Environmental control including temperature and humidity to which flocks are subjected and the absence of disease are local farm-related determinants of quality. Dietary factors including trace minerals in addition to calcium and phosphorus will influence shell quality in either a positive or negative direction. The presence of toxins such as DDT and mercuric anti-fungal treatment of seed will seriously impact shell formation as occurred in free living species during the 1950s and 1960s.

Generally the research conducted by Dr. Solomon relates to relatively narrow aspects of shell formation and apart from general principles there is little that can be applied to short term improvement and quality in the context of specialty eggs. This said, an understanding of mechanisms associated with shell formation is critical to diagnosing problems encountered in the operations of certain of our Franchisees. 

At this stage it does not seem appropriate for defective product to be submitted to her laboratory or to a suitably equipped researcher in the U.S. for diagnostic evaluation. Since we cannot influence the genetic factors which appear to be most involved in quality, other than banning certain strains, our efforts to improve shell quality should be directed at assuring that franchisees supply flocks with adequate levels of macro and micro nutrients, are effectively vaccinated against IB and are housed and managed to preclude thermal stress.

The age and maintenance of cages, conveyor systems and the setting of crack detectors on graders probably have an effect on shell quality which exceeds genetic and nutritional factors.