It mostly comes back to how much stubble, residue, and water is left during the migration. The entire state of IL seems to get a fall chisel plow these days, the AR delta mows down the rice stubble and drains the fields (unless leased out for hunting), and most of it gets pesticide applications so the invertebrates the ducks eat during warmer weather are severely diminished.
The one crop that seems to get left for stubble is wheat in the prairie pothole region (helps catch snow and retain moisture), but that isn't where wintering ducks are, and the pintails nest in it the following spring only to get their nests plowed under during planting season.
Modern monoculture* agriculture is apocalyptic for most wildlife.