Effect of Egg Size on Shell Thickness

It is commonly assumed in the egg industry that increases in egg size cause the decline in shell thickness observed as laying hens grow older. To test this belief, a study of the effect of breed, hen age and egg size on shell quality was recently completed at the University of Manitoba. The research was conducted by Wm. Guenter, A. Golian and C.Bennett.

In the study, egg weight and specific gravity were determined once every ten weeks for all of the eggs laid by 1,200 hens. Eggs were weighed to the gram and categorized as Small, Medium, Large, Extra Large or Jumbo sized eggs. Five salt solutions were used to determine specific gravity of the eggs (specific gravity of 1.070, 1.075, 1.080, 1.085 and 1.090).

Eggs which did not float in any of the solutions were assigned a specific gravity of 1.095. The eggs floating in the 1.070 or 1.075 specific gravity solutions were defined as "thin shelled". Specific gravity measurement is one of the most common methods of testing shell thickness.

No difference in egg weight or specific gravity was observed between the Bovans and DeKalb strains used in the study. For both breeds, average egg weight increased from 54 g at 26 weeks of age to 64 g at 66 weeks of age. The greatest increase in egg weight occurred between 26 weeks and 36 weeks of age. Egg specific gravity for both breeds declined from 1.086 at 26 weeks of age to 1.078 at 66 weeks of age. The decline in specific gravity was fairly constant over this time period.

In the young hens, the effect of egg weight on shell thickness followed the expected pattern. At 26 weeks of age, for example, over 90% of the Jumbo sized eggs were categorized as thin shelled (specific gravity <= 1.075). At this age, almost all of the Jumbo sized eggs were "double yolkers." At 36 weeks of age, the largest eggs still tended to have the poorest shells. Twice as many Jumbo sized eggs were thin shelled as compared to the Large sized eggs at 36 weeks of age (see chart).

The effect of egg size on shell thickness, however, almost totally disappeared by 66 weeks of age. In old hens, egg specific gravity was dramatically reduced in all eggs and the greatest reduction occurred in the smaller egg size grades. By 66 weeks of age, the Jumbo and Large sized eggs had a similar incidence of thin shells (51% versus 44%). It appears that old hens lay thin shelled eggs regardless of egg size.

Because the effect of egg size on shell thickness declined as the hens grew older, increased egg weight accounted for only 5% of the increase in the thin shelled eggs between 36 and 66 weeks of age. Hen age accounted for the overwhelming proportion of the increased number of thin shelled eggs. It was estimated that if average egg weight was held to 60 g instead of 64 g at 66 weeks of age, the percentage of thin shelled eggs would have only declined from 46% to 44% of the eggs laid.

If egg weight causes only a small portion of the decline in specific gravity observed in older flocks. why are producers so convinced that it is important? One obvious answer found in the data from this study is the increase between 36 and 66 weeks of age in the number of the Extra Large and Jumbo eggs laid by the hens. At 36 weeks, the two largest egg size grades accounted for 17% of the eggs laid compared to 51% at 66 weeks of age. By 66 weeks of age, half of the cracked eggs would be expected to be Extra Large or Jumbo sized simply due to the high number of these eggs being laid.

One real concern that producers have about Extra Large and Jumbo sized eggs is the momentum that these large eggs build up as they roll out of the cage. This momentum makes them more likely to crack than Medium or Large eggs of equal shell quality. The larger eggs are also more likely to be squeezed during egg gathering. Even if shell thickness is not greatly affected, it is still important to control egg size in laying hen flocks.

While egg size should not be ignored, excess egg size is not a common cause of poor egg shell thickness in older flocks. The basic trend as hens grow older is for eggs of all sizes to become thin shelled. When investigating thin shelled egg problems it is often necessary to look beyond egg size.

Acknowledgement:

Thanks to Keystone Hatchery for donating the day-old pullets.

Prepared By:

Carlyle Bennett, M.Sc.
Business Development Specialist - Poultry
Livestock Knowledge Centre
Manitoba Agriculture, Food and Rural Initiatives

Communities in English

Communities in Spanish

Communities in Portuguese

Effect of Egg Size on Shell Thickness