The size/weight of an egg depends mainly on the breed and age of a laying hen.  Of course, there are several factors that can directly or indirectly affect egg size once the commercial egg-type laying hen begins egg production.  Normally, in a healthy hen as her laying cycle continues her eggs get bigger.  The increase in size of the egg associated with aging of the hen has a direct influence on the thickness of the shell.  This is true for hens that lay eggs with a white or brown shell.  As the hen ages and egg size increases the thickness of the shell decreases. 

Also, the intensity of color in the shell of an egg from a brown egg-layer also decreases with age of the hen.  The purpose of this article is to briefly review why the age of the laying hen directly influences the thickness and color of the eggshell.  It is essential to have an understanding of “what is normal and expected” as related to eggshell quality and pigmentation as a flock of white or brown egg-type layers age.  This understanding will assist production managers, nutritionists and research scientists in identifying, preventing and solving problems in eggshell quality and pigmentation as the layer flock ages.

Size and quality

It is commonly accepted that simply increasing the calcium level of the diet will not overcome the decline in eggshell quality associated with aging of a commercial egg-type layer flock.  As commercial egg-type white and brown layers get older the size of their eggs increase.  The initial increase in egg size from a pullet entering egg production is fairly rapid.  Within a few weeks the increase in egg size slows down and gradually continues to increase throughout the egg production cycle.

A major contribution to the area of research involving factors influencing eggshell quality was a publication by Dr. Roland in 1975 (Poultry Science: 54: 1720-1723).  Prior to this publication, the exact relationship between eggshell calcium content and decline in eggshell quality, as related to hen age, was not understood.  At that time it was thought that the reasons for the decline in eggshell quality as the hen aged was due to: (1) an age-related decrease in the ability of the hen to mobilize her skeletal calcium for shell formation, (2) her ability to absorb calcium from her digestive tract declined with age, and (3) the hen’s genetic potential for egg production increased at a faster rate than her ability to maintain adequate shell deposition.  Dr. Roland and his research team proved that the decline in eggshell quality as the hen aged was not directly related to the above three reasons.  

The research discovery that Dr. Roland and his research team made and published in 1975 answered the question of why the quality of the eggshell declined with age of the hen and thus allowed for more rapid advances to be made in improving the quality and integrity of the eggshell.  These researchers found that as the hen aged, the increase in egg size was accompanied by no proportionate increase in calcium deposition on the eggshell. 

Simply stated, approximately the same amount of shell material (calcium carbonate) that covered a small egg from a young pullet covered a large egg from an older hen.

In 1979 Dr. Roland published another article (Poultry Science: 58: 774-777) that greatly expanded the knowledge in the area of eggshell quality when 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.  This finding and further research in this area of eggshell quality by Roland and his research team eventually led to the discovery that in a flock of layers the quality of a hen’s eggshell at the end of the laying cycle was directly related to the shell quality of her eggs at the beginning of the cycle and the number of eggs a hen laid during the production cycle had no effect on the quality of her eggshells.

Size and color

The major reason for the decrease in eggshell pigmentation as a commercial brown egg-type layer ages has just recently been discovered and reported in 2007.  Just as Dr. Roland concluded in 1975 that the decline in eggshell 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 eggshell, the research data collected and reported by the authors of this present article (Poultry Science: 86: 356-363) provide evidence that the decrease in eggshell color as a brown hen ages is also directly attributed to an increase in egg size without an accompanying increase in pigment deposition onto the surface of the eggshell. As a result, more shell surface is covered with a given amount of pigment as the hen ages and lays larger eggs.

These researchers used brown egg-type layers and measured the change in brown color in their eggshells for 10 months using a computer-based color machine vision system.  The actual eggshell 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. 

Their research data also showed that hens laying eggs with lighter shell color (less pigmentation) during the early part of the laying cycle laid lighter colored eggs later in the laying cycle. These data, relating eggshell color changes with hen age, are in agreement with those reported by Dr. Roland about eggshell 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 eggshell 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 eggshell pigmentation between the first month and the last month of the 10-month experimental period.  Therefore, the larger eggshell surface area due to the increase in egg size resulted in lighter colored eggs and the decline in eggshell pigmentation should be expected in brown layers as their laying cycle continues.  However, it must be kept in mind that there are numerous factors that have a negative effect on eggshell pigmentation.  These factors must be understood and controlled so that the impact that they have on the normal decline in eggshell pigmentation in a flock of brown egg-layers will be minimal.

Summary

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 eggshells become thinner.  This leads to a decline in eggshell quality with age of the flock. The age-related decline in eggshell quality is not due to a decrease of calcium carbonate deposition onto the shell.  Instead, the decline in eggshell quality is due to the same amount of calcium carbonate being deposited onto a larger egg that has more surface area than a smaller egg from a younger bird.  

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.

#1 in series of 3 articles by Dr. Richard Miles on egg quality and pigmentation.