Do you have any photos that you can post of the site?
You mentioned a corn field within 20'. This could be a problem due to high fertilizer-based nutrient load run-off adding to the organics that the water fowl will be copiously depositing. I foresee heavy and chronic algae blooms whether you have filtration (bio-conversion0 or not.
This could turn into a real money pit for the HO.
A couple of years ago, I did a pond restoration , (earth bottom approx.175,000 gallons) that was home to about 10 ducks. It is located at the entrance to a medical facility. With only this few ducks, the water had a strong sewage odor even though it was a flow through system. Every morning the maintenance staff had to wash down the bridge that spanned the pond to remove the extensive droppings that occurred overnight. Long story short, water quality was less than desired, ducks were relocated before any restoration could begin and also because of aesthetics and health concerns.
Point that I am trying to make is given the size of this pond it will, in short time, most likely become a 24/7/365 maintenance nightmare.
"Everybody is ignorant, only on different subjects." - Will Rogers