Having the same issue in my server even if the zombies population start is High and, pop multiplier is very high and peak is set to 3.0, it borked the server
it needs to be solve ASAP, servers are too affected by this one (Full Wiped server)
Grass is my go-to option as it's available everywhere and can be fed to pretty much any animal.
I don't know how much food itself affects the acceptance, but petting the animal and spending time with it definitely works.