What is the reason for the increase in total egg weight of laying ducks after a feed change?

Here is the situation: a farmer we work with was using another company's feed and the laying rate reached 80%, then switched to our company's feed and the laying rate dropped by nearly 5%, and after three days suddenly switched back to the previous feed, and the laying rate went back up. How would you all explain this? Neither feed change was done gradually.
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There are two reasons for the drop in egg weight after a feed change: the energy or protein level in the feed is on the low side. The egg weight increased when the feed was switched back, and the reason is the same.
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Won't changing feed cause stress?