SEED PRODUCTION AND SEEDBANK DYNAMICS IN SUBTHRESHOLD VELVETLEAF (ABUTILON-THEOPHRASTI) POPULATIONS

The impact of seed production by subthreshold weed populations on futu re weed problems has impeded the adoption of integrated pest-managemen t principles for weed management. Studies were conducted in fields wit h no velvetleaf history to determine how seedbanks and seedling popula tions change following seed production 1 yr or 5 consecutive yr in plo w-disk and no-tillage corn. Cumulative seed production by 0.19 velvetl eaf plants m(-2) increased in a linear fashion from 1989 to 1994, with annual additions averaging from 330 seeds m(-2) for velvetleaf in cor n to 2,500 seeds m(-2) for velvetleaf without competition from corn. F ive-year cumulative seed production was 1,480 seeds m(-2) in plow-disk and 1,810 seeds m(-2) in no-till corn. In no-till corn, 42 velvetleaf seedlings m(-2) emerged the Ist: year after the 1989 seed rain, but o nly 35 seedlings m(-2) emerged over the next 4 yr. In plow-disk plots, annual emergence averaged 12 seedlings m(-2). Five years after the 19 89 seed rain, the proportion of seeds lost to emergence was about 20% in both tillage treatments. Where velvetleaf seeds were allowed to ret urn to the soil every year, cumulative seedling emergence was lower in plow-disk than in no-till corn, with total emergence of 70 and 360 se edlings m(-2), respectively, after 5 yr. Seedbank numbers ranged from 10 seeds m(-2) 5 yr after a single seed rain (290 seeds m(-2)) by velv etleaf in plow-disk corn to 1,020 seeds m(-2) following 5 consecutive yr of seed rain where 12,580 seeds m(-2) were returned without corn co mpetition in no-till. Seedbank samples in the fall of the 5th year had 69 to 98% fewer seeds than were accounted for by cumulative seed rain and seedling emergence, with greater apparent seed losses in plow-dis k corn than in no-till corn. Over 90% velvetleaf control would be requ ired annually to maintain subthreshold populations for 5 yr following a single seed rain. By comparison, over 95% control would be required annually to maintain subthreshold populations where velvetleaf seed re turn is permitted each year.