Eating poo - real reasons why and effective management

Does anyone know if there is a real proven reason for dogs to be eating poo ?
Is it really due to mineral deficiency or is it a learned mimick of their mum when they were pups ?
Maybe instinct, stronger in females ?

Nifty no longer eats hers but I am careful to pick up directly after delivery ( )
She will go back to sniff the area and eventually lick the grass if I don’t distract her, but gets a biscuit on perfect no-poo behaviour

Problem is I would like her to be with me more when I am with the horses but what a complete nightmare around horse poo !
Would it “help” to just let her eat it until she makes herself sick, hoping she’ll eventually go off it ?
Worms are always a worry, despite the horses being regularly done.

Anyone got any advice on cohabitation with horses ?