Double yolks

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Just curious really. When our girls first started laying practically every egg had a double yolk but now (about 5 months later) none of them do.
They are still getting plenty of greens/mash etc every day, but I have changed the layers pellets for a cheaper brand. So is it their feed that has made the difference or just the fact that they are a bit older?

Young layers often lay double yolkers, then it stops. Sometimes they lay tiny 'pigeon' eggs with no yolks, or other strange phenomena.

ok thanks