WHY BROWN EGG SHELL PIGMENTATION DECREASES WITH AGE OF A LAYER FLOCK
Although shell pigmentation (color) is not an indication of internal egg quality, consumers in many egg markets throughout the world prefer brown over white eggs (e.g., United Kingdom, Italy, Portugal, Ireland, Southeast Asia, Australia and New Zealand). Shell color intensity within each country is highly influenced by consumer preference. For example, the Japan egg market has standards for a uniform dark shell color, whereas consumers in other markets prefer a shell with a uniform light brown color. One would think that because egg shell color is of extreme importance in many egg markets there would have been extensive research studies aimed at explaining reasons for the variation in shell color that occurs among and within layer flocks, especially changes that occur as the laying cycle continues. In spite of the economic losses resulting from variations in shell color, the reasons for changes in color as laying hens age were not completely understood until research conducted at the University of Florida, Gainesville, Florida (USA) helped explain why color loss occurs as laying hens age (Odabasi et al. 2007). The reason why egg shell color declines as laying hens age is similar to why egg shell thickness declines as hens age. Even though the reason why egg shell thickness decreases with the age of the hen will be a topic discussed in a future issue of Milwhite’s Journal it is appropriate to review the reason why at this time to help explain why egg shell color declines as well.
Dr. David Roland, while a faculty member in the Department of Poultry Science at the University of Florida, Gainesville, Florida (USA) and his coworkers, published research data that explained the reason why egg shell quality declined with the age of the laying hen (Roland, et.al., 1975). These researchers reported that as the hen aged, the normal increase in egg size was accompanied by no proportionate increase in the total amount of calcium deposited on the egg shell. Simply stated, approximately the same amount of calcium carbonate that covered a small egg from a young pullet covered a large egg from an older hen. Then, in 1979 Dr. Roland published another paper (Roland, 1979) that greatly expanded the knowledge in the area of egg shell quality. He reported that eggs which had a greater increase in size throughout the entire laying cycle also had a greater decline in their shell quality. Further research led to the discovery that in a layer flock the quality of a hen’s egg shell at the end of the laying cycle was directly related to the shell quality of her eggs at the beginning of the cycle. Also, the number of eggs a hen laid during the production cycle had no influence on the quality of the egg shells.
EGG SIZE AND SHELL COLOR
Just as Dr. David Roland concluded in 1975 that the decline in egg shell quality with age of the hen is a direct result of an increase in egg size without a proportionate increase in calcium carbonate deposition in the egg shell, the research data collected and reported by Odabasi et.al., 2007) showed that the decrease in egg shell color as a brown hen ages is also directly attributed to an increase in egg size without an accompanying increase in the amount of shell pigment deposition. These researchers used brown egg-type layers and measured the change in brown color in egg shells for 10 months using a computer-based color machine vision system.
The actual color and color intensity was determined for three eggs collected each month from each of 240 hens. Data collected during the 10-month experimental period showed that as the hen aged her eggs became lighter in color and the decrease in color was due to less intensity of a red pigment in the shell (above photograph). Their research data also showed that hens laying eggs with less pigmentation during the early part of the laying cycle laid lighter colored eggs at the end of the laying cycle.
These data, relating eggshell color changes with hen age, are in total agreement with those reported by Dr. Roland about egg shell quality. Hens laying eggs with poor shell quality in the early part of the laying cycle also laid eggs with poor shell quality later in the laying cycle. Similarly, in this present study concerning egg shell pigmentation, hens that laid eggs with more pigment on the shell (darker shells) early in their laying cycle continued to lay darker eggs with more pigment towards the end of the laying cycle.
At the end of their 10-month study the Florida researchers corrected the egg pigmentation data for the increase in egg weight and found very little change in egg shell pigmentation occurred between the first and the last month of the 10-month experimental period. Therefore, the larger egg shell surface area due to the increase in egg size resulted in lighter colored eggs and the decline in shell pigmentation is normal and should be expected in brown layers as their laying cycle continues. However, it must be kept in mind that there are other factors that have a negative effect on egg shell color as discussed in a previous issue of Milwhite’s Journal. These factors must be understood and controlled so that the impact they have on the normal decline in egg shell pigmentation as a flock of brown egg-layers get older will be minimal.
It is normal and should be expected that as a flock of white and brown egg-type layers age their eggs get larger and their egg shells become thinner. This leads to a decline in egg shell quality with age of the flock. Similarly, as a flock of brown egg-type layers age and their egg size increases the intensity of the brown color in the egg shell decreases. The explanation for this is because the same amount of pigment, as deposited on a small egg, is being deposited onto a larger egg that has more surface area. So, this means that once corrected for egg weight, there is very little change in the total amount of pigment in the egg shell as the hens get older and the laying cycle continues.
The information presented in this issue of Milwhite’s Journal was compiled by Dr. Orlando Osuna, Director of Health Science at Milwhite, Inc. and Dr. Richard Miles, Professor Emeritus, University of Florida.
Roland, D.A., Sr., D.R. Sloan, and R.H. Harms. 1975. The ability of hens to maintain calcium deposition in the egg shell and egg yolk as the hen ages. Poultry Sci. 54:1720-1723. Roland, D.A., Sr.1979. Factors influencing shell quality of aging hens. Poultry Sci. 58:774-777. Odabasi, A.Z., R.D. Miles, M.O., Balaban, and K.M. Portier. 2007. Changes in brown eggshell color as the hen ages. Poultry Sci. 86:356-363.