USDA Reports for Sept 17-23, 2021: Wheat:  Net sales of 290,100 metric tons (MT) for 2021/2022 were down 19 percent from the previous week and 30 percent from the prior 4-week average.   Corn:  Net sales of 370,400 MT for 2021/2022 primarily for Guatemala (138,400 MT), Mexico (102,600 MT, including decreases of 5,900 MT), Canada (87,600 MT, including decreases of 200 MT), Japan (77,400 MT, including 88,600 MT switched from unknown destinations and decreases of 11,800 MT), and Costa Rica (33,000 MT, including 30,000 MT switched from unknown destinations), were offset by reductions primarily for unknown destinations (110,900 MT).  Exports of 676,200 MT were primarily to Mexico (262,200 MT), China (140,400 MT), Japan...