I think that stuff (Hygromycin B?) is supposed to be used continuously to be effective.
Cat food really isn't a great protein booster long term, there's a lot of fat in it which can effect overall condition and thus laying.
When you feed other foods, more than 10% of daily volume,
you are...
They may well need more protein.
Layer feed is minimal in protein(~16%), the cracked corn is diluting that protein - maybe significantly depending on how much in relation to layer feed.
Either free feed a higher protein feed or stop with the cracked corn.
Oyster shell should be free fed in a...