Corn grain yield loss due to weed competition and yield loss per unit weed cover in subplots at the Farming Systems Project, Beltsville, Maryland were inversely related to annual rainfall. Rainfall is shown for June through August, which corresponded to the prime corn growing season from approximately pre-sidedress vegetative stage to kernel denting. The 30-year mean rainfall of 289 mm is shown by the dashed reference line. Values of yield loss followed by the same letter are not significantly different (P < 0.05).