Each district within a Region has its own Farm interface. The competition here is much less intense than that for Silver Mines, which are shared by the entire Region. There are farms of varying output, each of which can be occupied for around 4 hours.
Grain is critical for recruiting reserve units and keeping your army full. Seize the highest output Farms in the middle any time you see them available!
With high levels of Prestige you can occupy more than one Farm!