Egg producers can suffer economic losses of around 10% due to eggs becoming cracked during routine handling. Cracking is not only a waste and a financial loss. It also increases the risk of eggs becoming infected with disease-causing organisms – a risk that is likely to increase with the move from battery cages to more extensive housing systems.
A new measure of eggshell strength
Together with other European scientists, researchers in Scotland have found a new way to measure eggshell strength known as ‘dynamic stiffness’. In breeding programmes, it can be used to select for hens that lay eggs which are less prone to cracking. Our work shows that dynamic stiffness is a strong indicator of an eggshell’s tendency to crack in the packing plant. The fact that it is predictive is an important feature because previous measurements of eggshell strength were destructive, i.e. they involved breaking the eggs. Being non-destructive, dynamic stiffness allows a direct link between egg strength and the chance of it cracking under commercial conditions.
A materials scientist sees an eggshell as a bio-ceramic with measurable properties. For instance, it will oscillate at a natural or resonant frequency if it is tapped. In 1998, scientists at the Catholic University in Leuven, Belgium, came up with an acoustic technique to measure this frequency. From this, they derived a measure of eggshell strength that they called ‘dynamic stiffness’. Critically, cracked eggs produce quite different measurements, and the researchers proposed that the principle of resonance frequency analysis could be used to develop a quick, on-line, non-destructive check to identify damaged eggs at packing plants. After further work, it was found that acoustic resonance frequency analysis could be installed on-line for rapid and non-destructive checking of eggs at a rate of around 200 eggs/hour. This is an improvement over traditional candling, which is unsuitable for modern, high-throughput packing machines.
The heritability of dynamic stiffness
Recently, we have found another important application for this knowledge of dynamic stiffness. It might help poultry breeders to screen for eggshell quality, and so select birds whose eggs are less likely to break. Firstly, we needed to demonstrate that an egg’s dynamic stiffness is genetically determined, and also to ensure that selection for increased stiffness would not bring any detrimental traits in laying hens. Finally, it was necessary to show that dynamic stiffness could, in fact, predict the probability of eggs cracking under commercial conditions.
In order to do this, two eggs from each of 1500 hens from a pedigree population were acoustically tested to measure the genetic and environmental contributions to dynamic stiffness. The contributions form ‘heritability’, a measure that indicates the relative importance of genetic influences in dynamic stiffness. The larger the heritability, the easier it is to change the measured trait by genetic selection. Other egg and production traits were measured from the same hens, and their genetic correlation with dynamic stiffness was also calculated. The genetic correlation values indicate which other traits might be changed – and in which direction – if selection for dynamic stiffness was to be implemented.
It appears that the heritability of dynamic stiffness was far higher than any of the normal measures of eggshell quality. Half of the variation is determined by genetics. Fortunately, we found no evidence of undesirable genetic correlations with other production traits, except for a small negative correlation with egg production. This is also the case with conventional measures of eggshell quality, confirming that there is some trade-off between higher shell quality and slightly reduced egg production. Combined with the simplicity of the measurement, these results mean that this is a suitable technique for selection programmes.
Dynamic stiffness as part of a breeding programme
The final but vital piece of the jigsaw was to prove that selection for higher dynamic stiffness would reduce the number of cracked and damaged eggs in practice. Because the dynamic stiffness measurement is non-destructive, we were able to follow the passage of selected eggs through the collection and on-line packing process in a large commercial plant. Eggs from the front of layer cages had their dynamic stiffness measured. Those that were not cracked were marked and replaced in the same place. The same eggs were re-assessed by the acoustic resonance tester at the end of the automatic conveyance and packing process to identify which were still intact and which had cracked. The figure shows that eggs with low dynamic stiffness are more likely to crack during their passage through the packing plant, i.e. the cracking probability value will be closer to 1.
We conclude that increasing the dynamic stiffness of eggs laid by hens in a flock by genetic selection will reduce the number of cracked eggs. Currently, the method is being assessed by breeding companies on a larger scale for inclusion in their breeding programmes.
The dynamic stiffness of an egg is a good predictor of its chance of cracking in the packing plant. Genetics accounts for much of the variation in dynamic stiffness and there appear to be no detrimental genetic correlations with this characteristic, and so the selection of hens on the basis of dynamic stiffness looks to be a realistic prospect for the future in order to reduce cracked eggs in the industry.