WEED SEED BANK EMERGENCE ACROSS THE CORN-BELT

Field experiments, conducted from 1991 to 1994, generated information on weed seedbank emergence for 22 site-years from Ohio to Colorado and Minnesota to Missouri. Early spring seedbank densities were estimated through direct extraction of viable seeds from soil cores. Emerged se edlings were recorded periodically, as were daily Values for air and s oil temperature, and precipitation. Percentages of weed seedbanks that emerged as seedlings were calculated from seedbank and seedling data for each species, and relationships between seedbank emergence and mic roclimatic variables were sought. Fifteen species were found in 3 or m ore site-years. Average emergence percentages (and coefficients of var iation) of these species were as follows: giant foxtail, 31.2 (84%); v elvetleaf, 28.2 (66); kochia, 25.7 (79); Pennsylvania smartweed, 25.1 (65); common purslane, 15.4 (135); common ragweed, 15.0 (110); green f oxtail, 8.5 (72); wild prose miller, 6.6 (104); hairy nightshade, 5.2 (62); common sunflower, 5.0 (26); yellow foxtail, 3.4 (67); pigweed sp ecies, 3.3 (103); common lambsquarters, 2.7 (111); wild buckwheat, 2.5 (63), and prostrate knot-weed, 0.6 (79). Variation among site-years, for some species, could be attributed to microclimate variables though t to induce secondary dormancy in spring. For example, total seasonal emergence percentage of giant foxtail was related positively to the Is t date at which average daily soil temperature at 5 to 10 cm soil dept h reached 16 C. Thus, if soil warmed before mid April, secondary dorma ncy was induced and few seedlings emerged, whereas many seedlings emer ged if soil remained cool until June.