As chickens age, the eggs they lay change in ways that matter to farmers and food producers. Researchers studied brown egg-laying hens at different ages and found that older hens produce eggs with lighter shells and weaker egg whites compared to younger hens. The study discovered that this happens because older hens’ bodies make less of certain proteins and pigments needed to create strong, colorful eggs. Understanding why this happens could help farmers keep egg quality high as their flocks age.
The Quick Take
- What they studied: How chicken age affects egg quality, including shell color, shell strength, and egg white thickness
- Who participated: 192 brown laying hens divided into three age groups: young (165 days old), middle-aged (307 days old), and older (475 days old). Each group had the same diet and living conditions for 3 weeks
- Key finding: Older hens (475 days) produced eggs with lighter-colored shells and thinner, weaker egg whites compared to younger hens (165 days). The egg white quality dropped significantly in the oldest group
- What it means for you: If you buy eggs, this explains why eggs from older hens may look different and have different cooking properties. For farmers, it suggests they may need to adjust their flock management as hens age to maintain egg quality
The Research Details
Researchers took 192 brown laying hens and divided them into three groups based on age: young hens (165 days old), middle-aged hens (307 days old), and older hens (475 days old). Each age group had 8 smaller groups of 8 birds each. All the hens ate the same food and lived in the same conditions for 3 weeks. The researchers then measured the eggs these hens produced, looking at shell color, shell thickness, egg white quality, and yolk size. They also tested the hens’ tissues to understand what was happening inside their bodies at the cellular level.
The researchers used special equipment to measure egg shell color using a color scale (measuring lightness and different color tones). They measured egg white thickness and strength using a standard test called the Haugh unit, which is like a quality score for egg whites. They also looked at specific proteins and chemicals in the eggs and in the hens’ reproductive organs to understand the biological reasons behind the changes.
This research approach is important because it doesn’t just show that eggs change as hens age—it explains why. By measuring both the eggs themselves and what’s happening inside the hens’ bodies, researchers can understand the biological mechanisms. This helps farmers and scientists figure out if there are ways to prevent egg quality from dropping as hens get older, which could save money and reduce food waste
This study has good design features: it used a reasonable number of hens (192), kept conditions consistent across groups, and measured multiple aspects of egg quality. The researchers looked at both the eggs and the biological processes inside the hens, which strengthens their conclusions. However, the study only looked at one breed of brown laying hens, so results might be different for other chicken breeds. The study was relatively short (3 weeks), so it shows what happens at specific ages but doesn’t track individual hens over their entire lives
What the Results Show
As hens aged from 165 days to 475 days old, their eggs showed clear changes in quality. The shells became lighter in color—this was measured on a brightness scale where older hens’ eggs scored higher (meaning lighter). The egg white (albumin) became thinner and weaker, with older hens’ eggs scoring much lower on the Haugh unit test, which measures egg white quality and thickness.
The researchers discovered that these changes happened because older hens produced less of two important substances: protoporphyrin IX (a red pigment that colors the shell) and ovomucin (a protein that gives egg whites their thick, gel-like texture). The genes (instructions in cells) that tell the hen’s body to make these substances were less active in older hens.
Interestingly, the middle-aged hens (307 days) showed some changes but not as severe as the oldest hens. This suggests that egg quality doesn’t drop all at once but gradually gets worse as hens age. The yolk size actually increased slightly in older hens, but this didn’t make up for the loss of quality in the shell and egg white.
The researchers also found signs of cellular stress in older hens. A substance called malondialdehyde, which indicates damage from oxidative stress (a type of cellular damage), was higher in the oldest hens’ blood and reproductive organs. This suggests that aging causes stress at the cellular level, which may be why the hens can’t produce quality eggs as well. Younger hens had better antioxidant defenses (natural protective systems in their bodies), which may help them maintain egg quality
This research confirms what farmers have observed for years: egg quality naturally declines as hens age. However, this study goes deeper by identifying the specific biological reasons. Previous research suggested this happened, but this study provides detailed evidence about which genes and proteins are involved. The findings align with general knowledge about aging in animals—as they get older, their bodies become less efficient at producing high-quality products
This study only looked at one breed of brown laying hens, so the results might not apply to white egg-laying breeds or other chicken types. The study lasted only 3 weeks, so it shows what happens at specific ages but doesn’t show the complete aging process. The researchers didn’t test whether changing diet, supplements, or living conditions could prevent these age-related changes. Additionally, the study was done in a controlled laboratory setting, which may not reflect real farm conditions where hens face more stress and variation
The Bottom Line
Based on this research, farmers should expect egg quality to naturally decline as their flocks age, particularly after hens reach about 475 days old (roughly 15 months). This is normal biology and not a sign of poor management. Farmers might consider replacing older flocks or adjusting expectations for egg quality from older hens. For consumers, understanding that older hens produce different eggs can help set realistic expectations. The evidence is strong (based on direct measurements) but applies specifically to brown laying hens
Egg farmers and producers should care most about this research, as it helps them understand when egg quality will decline and plan their flock management accordingly. Food companies that use eggs in products should be aware that eggs from older hens may have different properties. Consumers who care about egg quality and appearance may want to know the age of their hens. This research is less relevant for people who primarily care about basic nutrition, as older eggs are still nutritious
The changes described in this study happen gradually over months. Younger hens (around 5-6 months old) produce the best quality eggs. By the time hens reach about 15 months old, egg quality has noticeably declined. These changes continue as hens get even older. Farmers can expect to see measurable differences in shell color and egg white quality within weeks of hens reaching middle age
Want to Apply This Research?
- If tracking egg purchases, note the farm’s flock age if available, and track your observations of shell color (light vs. dark), shell strength (cracks easily or not), and egg white thickness (runny vs. thick). Rate these on a simple 1-5 scale weekly to see if you notice patterns related to the eggs’ source
- For farmers using an app: log your flock’s age in weeks and track average egg quality scores (shell color, Haugh unit measurements if available, or visual assessments). This creates a personal record of when quality typically declines in your specific conditions, helping you plan flock replacement timing
- Track egg quality metrics monthly as your flock ages. Create a simple scoring system for shell color (1=very light, 5=very dark) and egg white appearance (1=very runny, 5=very thick). Plot these over time to see your flock’s individual aging curve, which may differ from the research averages based on your specific conditions, feed, and genetics
This research describes normal biological changes in egg-laying hens and is intended for farmers, food producers, and consumers interested in understanding egg quality. The findings apply specifically to brown laying hens and may not apply to other chicken breeds. This information should not be used to diagnose or treat health problems in animals or humans. Farmers with concerns about flock health or egg quality should consult with a veterinarian or poultry specialist. Consumers with food safety concerns should follow standard food handling practices regardless of egg source or age
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.