Effects of Hen Egg Shell and Other Calcium Sources Upon Egg Shell Strength and Ultrastructure

Abstract

The suitability of egg shell as a calcium source in layer diets was examined by comparing pulverized and crushed sterilized egg shells against oyster shell, limestone grit, and pulverized limestone, as calcium supplements. These calcium sources were fed in appropriate amounts to obtain diets with 3% total calcium. Breaking strength of the eggs, determined at 90 and 150 days of feeding, showed that eggs from the hens fed the diets containing oyster shell, pulverized egg shell and limestone grit were significantly stronger (P<0.01) than eggs from hens fed diets containing pulverized limestone as the calcium source. Crushed egg shells as a dietary source of supplemental calcium promoted egg shell quality equivalent to that obtained with pulverized limestone. The results of a second experiment indicated that oyster shell produces a greater improvement in the shell quality of the second, third and subsequent eggs in the clutch than it does upon the first egg in the clutch. These results indicated that the presence of particles of calcium carbonate in the diet are more important for hens in very high production than for those in low egg production.Scanning electron micrographs revealed that most of the changes responsible for differences in shell breaking strength apparently take place in the palisade layer, while the mammillary layer remains approximately constant. It appeared that the palisade layer contributes most of the strength in the shell. As the palisade layer decreased in thickness and in concentration, there was a closely predictable decrease in breaking strength of the eggs. Evidence was obtained indicating a poor correlation between breaking strength and total shell thickness. Breaking strength is a superior measure of egg shell quality because it takes into consideration both the thickness and the concentration of the palisade layer around the crystalline mammillary portion of the egg shell.