I think it will depend on some degree to what area of the country your are in, and the duration of your duck season...
In California, I have had friends try to grow Japanese Millet, and the blackbirds ate it out before duck season even began...
We have had far better luck with watergrass or smartweed (as seed heads form with water already on, and to some degree the stress of the water is what makes them seed out)... Smartweed is easier to grow and probably takes less water and probably produces seeds later into the season. Smartweed produces more seeds and seems to be a preferred food when available. Birds feeding at night is a huge issue with both of these, and cold or fog greatly improves shooting success over either of these choices.
We have one pond that produces both plants well (if we can get water on during the late summer which is not an option in our current drought)... Smartweed tends to favor the high ground, and watergrass tends to grow better in the lower areas... Both do better after disking...
My ideal situation when growing these plants would be to have them covering the entire pond with no water showing... The birds do NOT need an opening to land in this stuff, and later in the season will in fact avoid the openings (due to being shot over them so much).
