Poor eggshell quality leads to increased numbers of cracked and damaged eggs. Cracked eggs encourage bacterial infections and are 3 to 93 times more likely to cause Salmonella outbreaks than intact eggs.

The impact that Salmonella can have on the industry was clearly demonstrated by the outbreaks that received much media attention in the US in late 2010. In simple monetary terms, it has been calculated that, if there were 10% fewer cases of human Salmonella infections across the EU, for example, it would save some Euro 400 million (US$525.9 million) per annum.

Confidence and waste  

While to the consumer, eggs are an excellent balanced source of nutrition, the shell is critical to consumers’ impression of the product. Cracked or damaged shells not only increase the risk from pathogens, but also harm eggs’ image.

 Egg quality is one of the primary concerns of breeders and farmers of laying hens, not only for those reasons shared with consumers but also because good egg quality contributes to the profitability of the commercial hen.

 Egg breakage causes significant waste directly, but most importantly it reduces the lifespan of laying hens. Increases in the period over which flock egg quality remains high would have impressive effects in environmental impact over the lifecycle of production.

There are thought to be some 370 layer hens in the EU, which is home to major poultry breeding companies. In the early 2000s, it was estimated that the annual financial loss to the UK egg industry alone due to breakages was more than GBP8 million (US$12.4 million) per annum. Additionally, the combined cost to packing centres and retail outlets of 1% breakages was calculated at GBP9.4 million per annum.

Work into better understanding shell quality has been funded by SABRE – Cutting Edge Genomics for Sustainable Animal Breeding. The initiative was a Euro 23 million pan-European project utilizing the latest techniques in genetic science to develop more economically sustainable production systems for cattle, pigs and chickens.

Multi-focus  

 The research project, carried out jointly across a number of European countries, has sought to both better measure aspects of shell quality which contribute to safety and to identify genetic markers which are associated with existing measurements of egg quality. The project, which brought together researchers at the Roslin Institute and the University of Glasgow in the UK, Lohmann Tierzucht in Germany, Spain’s University of Granada, MTT in Finland and France’s INRA, has also identified genes expressed in the shell gland to better understand the formation of eggs and to compare with the chromosomal location of the genetic markers.

The cuticle is a protective coating which prevents bacterial penetration through the gas exchange pores in the egg shell. It has never been quantified. The researchers established a method using dyes and reflectrometry, which allowed it to be established that the degree of cuticle coverage was moderately determined by the genetics of the hen that laid it. It was also established that the variation observed in the cuticle was correlated with bacterial penetration of the shell.

The egg shell is predominantly composed of calcium carbonate crystals. Crystal size determines the properties of materials, so size was measured using X-ray diffraction. It turned out that the size of crystals in an egg shell appears to be highly dependent on the genetics of the hen that laid the egg. The size of crystals appears to be an important component in shell thickness, which is a major part of shell strength.

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Selection  

By using crosses between commercial pure line hens, the study identified regions of the genome that controlled shell strength on chromosome 2, 3, 6, 14 and Z, which were then densely genotyped using SNPs. The SNPs with the highest significance values were tested on pure line hens and found to have significant associations with shell quality traits. This means that pure line hens can be selected using these markers.

The team also reports that large scale transcriptomic studies have identified many of the transporter molecules responsible for the prodigious secretion of proteins and minerals required for shell formation.

The project used a combination of approaches to increase understanding of the genetics and physiology of the formation of the eggshell and delivered a number of tangible tests and assays that will contribute directly to the genetic improvement of egg quality and safety. New tools have been developed to measure aspects of egg shell structure which underlie egg safety and quality and molecular tools to improve the efficiency of selection. This will allow breeders to select hens that lay better and healthier eggs.

Roslin Institute’s Dr Ian Dunn commented: “The results of the work carried out by partners in Germany, Scotland, Spain, France and Finland have given us both greater insight and practical tools to improve egg quality and safety.

“The research on tools to measure cuticle coverage quantitatively is, perhaps, the most tangible in terms of producing something that can be immediately applied and is being further tested and developed for application to breeding. The genetic markers, each of which has a small effect, when added together can make a significant contribution to improved quality and safety, and are likely to be utilised as cheaper methods of genotyping are introduced.

Perhaps most exciting is the results on the genetics of crystal size which strongly suggest we are getting close to an understanding of the basic components that determine the variability in eggshell structure. This may however have to wait a little while until we fully understand how the size of calcite crystals are determined in the hen’s oviduct, what that means for shell quality will, perhaps, have to wait until x-ray crystallography becomes more accessible!”