Would depend on a number of things, mainly what will be the main type of birds using it?
Corn is always a good thing but can get expensive to plant. Just about everything will eat/use the corn, especially if you can get some native grasses (crab, panic) or smart weeds to grow in conjunction with it.
Chufa can be like cocaine to mallards once they find it. But wouldn't expect your more herbacious ducks such as gadwall and wigeon to use it due to large "nut" it produces that has to be rooted up before eaten.
Jap millet always a good thing, can plant as late as Aug. and be ready in 45-60 days. Biggest problem might be blackbirds eating it out if they find it . . . But ducks really like it and might be the best overall choice.
Browntop millet good too, takes a little dryer soil than Jap but produces nice seed heads.
Chiwapa millet . . . big, hardy stalk that grows much taller than the other millets and can produce one heck of a seed head. Guys in the Central flyway like it alot.
Soy beans are good too but they deteriorate the fastest of any grain crop once wet.
I'd look at doing a mix of Jap, browntop in that all ducks like that combo. But you'll want something "tall" in there, whether it be for food purposes or for shelter because they need that too. Corn works but so would Egyptian wheat. And there are many others.
Have you considered doing moist-soil management? You apparently have a great water supply so you have good re-flood opportunities (a bonus) to control non target species. A field slam full of smartweed, barnyard grass, panic grass, spike rush, sedges, crab grass, etc. would provide excellent food and nutrition for the birds, along with providing a substrate that would promote late season aquatic invertaebrates that arn't as common in agricultural areas. Birds really like/need these in the later part of the season. And a properly managed ms unit will cost about a third of what a corn impoundment will and also can be "manipulated" after the plants all mature. Just some thoughts here . . .
There's lots of great information on several duck chat sites under "Habitat Forum" that I would recommend you visiting. I know I've gotten lots of help from those guys and gained a wealth of knowledge from them because most either do it for a living or have years of experience managing their own properties. Good luck with it!