ABSTRACT
Modern commercial egg layers have been selected to produce either white or brown eggs. White egg-laying strains are derived from White Leghorns. A composite of selected strains (derived from White Leghorns, Rhode Island Reds, Rhode Island Whites, Light Sussex, and Barred Plymouth Rocks) are used to produce the brown egg laying strains. From the first quarter of the 20th century, egg production per hen has increased from 175 to 320 per calendar year. In layer selection, hen-housed production (HHP) is the most important trait, which is often based on the number of eggs laid when all birds were housed together until a fixed age. This means that HHP is confounded with sexual maturity and viability. Some breeders adjust egg records for sexual maturity and mortality. The act of adjusting records for sexual maturity and mortality is termed the hen day rate (HDR). In addition to HHP/HDR, breeders also select for body weight, sexual maturity, egg quality traits (i.e., shell strength, shell shape, shell color, albumen quality, and absence of blood or meat spots), egg weight, and FCR. Further processing of eggs has gained attention, and breeders have paid attention to percent solids and lipid content of the egg. Selection for high egg producing birds, coupled with housing layers in cages, has led to increases in aggressive behavior, cage fatigue, skeletal problems, and cannibalism.
