A single diet offered during the early hours of the light period with some source of calcium top-dresses during the afternoon is the standard feeding program for layers worldwide. A new method has been proposed that adapts the normal behavior of the hen to the requirements of modern farming.
In my later years of stay in Spain, I happened to be contracted as a consultant by a very good nutrition company who was blessed in having their own research farm of very high standards. Lamentably, what they had in abundance in research resources hey lacked in common sense.
I have always been an advocate of high-quality piglet feeds. In fact, I abandoned my dreams for an academic career only because the then top piglet nutrition company offered me the position of nutritionist in the U.S.
Zinc oxide is an old friend of pigs, but an enemy to the environment and toxic to broilers when used at pharmacological doses. Copper sulphate is an even older friend to both pigs and poultry, but still not the best thing to dump into the fields through manure when used at growth-promoting levels.