Pigs productivity is about manure and slurry (liquidmanure). Overall pigs productivity 0% doesn't mean anything, for example when you empty pig barn slurry tank... pigs productivity goes to zero, OH MY! Heh yep, no need to worry, its just an empty slurry tank ready for pigs to take a piss into. Note: pigs ESC animal menu has no productivity statistics, you can only see it on JTSage's Production Inspector mod.
Selling pigs for profit. Pigs in real life and in Farming Simulator games are grown for human food, giants in their child friendly manner never mention this but pigs are slaughtered so you and I can eat bacon, pork chops, ground beef / mince beef etc, yummy. You should only sell old fat pigs because pigs price is calculated based on its weight, in FS22 there is no weight just the age, so choose the oldest pig to sell for highest profit, newborn piglets are the cheapest and would make no sense to sell them (unless you needed empty space for the barn and even then, why not still sell the oldest ones). Highest price in easy economy for oldest pigs I've ever seen is $1,200 for 33 month old pig (but I have limited experience).
About pigs prices to sell, see FS22 Animals Guide Selling page for details.
Pig barn, Hogg barn (mod from, hmm cant remember who, that huge $250k costing long building with feed silos at the end) buildings info box shows food AMOUNT / 200k liters, it does not differentiate between food sources. Neither does ESC animals menu, it only shows total capacity, base food, grain, protein and root crops.