From 5 a day to 1 a day overnight. What can cause this?

[Freia]

It is strange that hens of different breeds all got egg yolk peritonitis (EYP) and died, because EYP is caused by genetics and hormones. You would think three different breeds would not all have the same problem.

Genetics is my suspicion. All of the affected hens came from the same feed-store, meaning from the same hatchery. Many of my other hens came from local breeders or other stores. So I have 2 variables here: high-production breeds vs heritage breeds and same hatchery vs other hatcheries (or better or worse gene-pools if you wish).

 

[Bullitt]

It is really heritage strain of a breed. Heritage strains are bred to meet the original characteristics of a breed.

Unless you plan to show chickens, I think you will be fine purchasing chicks from a hatchery. You might try buying chicks from two or more hatcheries to see if there is any difference and to reduce the chances of what happened before. Plymouth Rocks would be a good choice.