Composition of Eggs by Commercial Size Categories: Effects of Genetic Group, Age, and Diet

Abstract

Effects of genetic group, age, and dietary protein level on composition of eggs within the commercial size categories were studied in two Leghorn control strains formed in 1950 and 1958, two corresponding strains selected for high egg production and related traits, their reciprocal crosses, and a commercial Leghorn stock.Mean hen-housed egg production to 385 days was 154, 184, 197, and 191 eggs, and 385-day egg weight was 59.4, 63.1, 64.4, and 62.0 g for the control strains, selected strains, strain crosses, and commercial stock, respectively. In the eggs graded as large, wet and dry yolk weights, percent of yolk, and dry yolk/albumen ratio increased while wet albumen weight and percent of albumen decreased as dietary protein level increased from 15 to 19%. No such effects were observed in the medium or extra large eggs.Within each egg size category, wet and dry yolk weight and percent of yolk increased and wet and dry albumen weight, percent of albumen, albumen percent solids, and shell weight decreased with age. These trends were reversed by forced molting between the sixty-fourth and seventy-first weeks of age but they resumed afterwards.There was no heterosis for any egg composition trait and no genetic group effects on yolk percent solids. Within the same commercial size category, eggs of the control strains had lower albumen percent solids than those of selected strains or the commercial stock and contained more yolk and less albumen and shell. These differences reflect changes in the composition of the eggs from hens that have been under selection for increased production since the control strains were formed.