Seed production and seedbank dynamics in subthreshold velvetleaf (Abutilon theophrasti) populations

The impact of seed production by subthreshold weed populations on future weed problems has impeded the adoption of integrated pest-management principles for weed management. Studies were conducted in fields with no velvetleaf history to determine how seedbanks and seedling populations change following seed production 1 yr or 5 consecutive yr in plow-disk and no-tillage corn. Cumulative seed production by 0.19 velvetleaf plants m(-2) increased in a linear fashion from 1989 to 1994, with annual additions averaging from 330 seeds m(-2) for velvetleaf in corn to 2,500 seeds m(-2) for velvetleaf without competition from corn. Five-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 only 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 1989 seed rain, the proportion of seeds lost to emergence was about 20% in both tillage treatments. Where velvetleaf seeds were allowed to return 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 seedlings 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 velvetleaf 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 competition 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-disk corn than in no-till corn. Over 90% velvetleaf control would be required 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 return is permitted each year.