1. 'returns' imply that the farmer made an investment (like labor, chemicals, etc), and that they've done something to increase the value of the result. 'interests' wouldn't be used.
'interest' in english is usually used to refer to a situation where there is a return, but it is purely financial (like you load someone $100 at 6%, and they pay you back $106% next year).
'benefit' usually means a human kind of benefit. in this context, maybe something like the farmer improved the quality of the crop so the consumers became more healthy or something.