Try this...when its time to harvest...make everyone a farmer..watch them and as soon as they finish dropping the food in the bin..drag them back to the patch for more.
the patch can produce 800 per harvest..but your people will not get all of it by their selves. I managed to get around 775 per harvest if I helped them out... Plus your compensating for your older villagers. Just don't touch them till after they deliver the food. Good Luck!
I haven't found that it makes harvest faster or bigger to put alot of people on it...I kept 5 farmers watering and the rest all researchers...