eh... how does that work?
duke taxes lords, ruler taxes dukes. no good reason to have a separate tax structure bypassing hierarchy just because it's food.
and if you have that, you'll reinforce city lord = duke mindset which is bad. because every man and dog will have duke+city lord tax rural lord and skip trading altogether.
and the other thing is... doesn't really make sense to have food taxed once a gold tax, as opposed to everyday. (though it would encourage those with large stores sell it quickly)