Waterfowl frequently acquire high-energy agricultural seeds in harvested and unharvested croplands during migration and winter. Estimates of agricultural seed biomass in harvested and unharvested corn, soybean, and grain sorghum fields do not exist or are outdated for the southeastern United States. Therefore, we estimated seed biomass in 105 harvested and 59 unharvested corn, soybean, and grain sorghum fields across 4 climate regions in Tennessee, USA, from September through January 2006 and 2007. We also used estimates of seed biomass to calculate duck-energy days (DEDs) in December and January when migratory waterfowl abundance peaks in the southeastern United States. Mean biomass of corn, soybean, and grain sorghum seed in harvested fields declined 239 kg/ha to 39 kg/ha, 118 kg/ha to 26 kg/ha, and 392 kg/ha to 19 kg/ha, respectively, from postharvest to January. Continuous monthly rates of decline were 64% for corn, 84% for soybean, and 74% for grain sorghum. Agricultural seed biomass in harvested corn and grain sorghum fields dropped below the waterfowl giving-up density (i.e., 50 kg/ha) in 3 months; soybean dropped below this threshold 1 month postharvest. Mean DEDs/ha in harvested corn, soybean, and grain sorghum fields were low (274, 90, and 27, respectively) in January, and DEDs were zero in >85% of fields. In unharvested corn, soybean, and grain sorghum fields, mean DEDs/ha in January were high (69,000, 18,000, and 26,000, respectively), and continuous rates of decline (3%, 7%, and 18%, respectively) were much lower than for harvested crops. Waterfowl biologists in the Southeast should use our estimates of agricultural seed biomass in DED calculations. We also recommend that biologists provide unharvested grain fields and natural wetlands for migrating and wintering waterfowl because seed resources are low in harvested agricultural fields.