An egg with two yolks is formed when two follicles ovulate almost simultaneously and the two yolks come together in the oviduct, where the egg white, membranes and common shell are also deposited around them. In nature, double-yolked eggs are rather rare, although they have been detected in various bird species. In commercial egg production, double-yolked eggs account for 1 do 2 % of total egg production. They occur more frequently in young hens at the beginning of the laying period and less frequently in mature hens at the end of the laying period. Double-yolked eggs are not suitable for hatching as their fertility and hatchability are poor. The formation of these eggs is influenced by several factors: age of the hen, nutrition, photostimulation and selection. It is known that selection of hens for higher body weight is the most important factor leading to higher production of double-yolked eggs, although selection for multiple ovulations has also been shown to be successful under experimental conditions. A higher proportion of double-yolked eggs is therefore encouraged by feeding high protein feed mixtures, and a positive influence on the production of these eggs can also be achieved by early photostimulation of the pullets and the use of high intensity light. Most breeders sell double-yolked eggs from the farm, but many end up in processing. In some world markets, particularly in Asia, these eggs are highly sought after because of the mythological significance attributed to them.
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