Abstract
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−2increased in a linear fashion from 1989 to 1994, with annual additions averaging from 330 seeds m−2for velvetleaf in corn to 2,500 seeds m−2for velvetleaf without competition from corn. Five-year cumulative seed production was 1,480 seeds m−2in plow-disk and 1,810 seeds m−2in no-till corn. In no-till corn, 42 velvetleaf seedlings m−2emerged the 1st year after the 1989 seed rain, but only 35 seedlings m−2emerged 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−25 yr after a single seed rain (290 seeds m−2) by velvetleaf in plow-disk corn to 1,020 seeds m−2following 5 consecutive yr of seed rain where 12,580 seeds m−2were 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.
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