Effects of hen egg shell and other calcium sources upon eggshell strength and ultrastructure

Abstract

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 appropri-ate amounts to obtain diets with 3 % total calcium. Breaking strength of the eggs, determined at 90 and ISO 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 pro-duces 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 pres-ence 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. I t 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 ob-tained 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 mam-millary portion of the egg shell