Abstract
As herbicide-resistant weed populations become increasingly problematic in crop production, alternative strategies of weed control are necessary. Giant ragweed, one of the most competitive agricultural weeds in row crops, has evolved resistance to multiple herbicide biochemical sites of action within the plant, necessitating the development of new and integrated methods of weed control. This study assessed the quantity and duration of seed retention of giant ragweed grown in soybean fields and adjacent field margins. Seed retention of giant ragweed was monitored weekly during the 2012 to 2014 harvest seasons using seed collection traps. Giant ragweed plants produced an average of 1,818 seeds per plant, with 66% being potentially viable. Giant ragweed on average began shattering hard (potentially viable) and soft (nonviable) seeds September 12 and continued through October at an average rate of 0.75 and 0.44% of total seeds per day during September and October, respectively. Giant ragweed seeds remained on the plants well into the Minnesota soybean harvest season, with an average of 80% of the total seeds being retained on October 11, when Minnesota soybean harvest was approximately 75% completed in the years of the study. These results suggest that there is a sufficient amount of time to remove escaped giant ragweed from production fields and field margins before the seeds shatter by managing weed seed dispersal before or at crop harvest. Controlling weed seed dispersal has potential to manage herbicide-resistant giant ragweed by limiting replenishment of the weed seed bank.
Highlights
As herbicide-resistant weed populations become increasingly problematic in crop production, alternative strategies of weed control are necessary
Floral initiation of selected giant ragweed plants occurred near the end of July, but was typically delayed by several days in the more competitive field margins
Giant ragweed plants in field margins may be competing for light, which causes plants to be etiolated with fewer branches, less stem biomass, and fewer leaves per plant (Jurik 1991)
Summary
As herbicide-resistant weed populations become increasingly problematic in crop production, alternative strategies of weed control are necessary. Giant ragweed seeds remained on the plants well into the Minnesota soybean harvest season, with an average of 80% of the total seeds being retained on October 11, when Minnesota soybean harvest was approximately 75% completed in the years of the study These results suggest that there is a sufficient amount of time to remove escaped giant ragweed from production fields and field margins before the seeds shatter by managing weed seed dispersal before or at crop harvest. A zero-weed threshold may require nonchemical strategies such as hand weeding before seeds shatter or HWSC to prevent seed bank replenishment These systems require that weeds be controlled before seed production or that weeds retain seeds until crop harvest. There is no information on giant ragweed seed production and retention as a factor in late-season weed control strategies for the growing conditions and cropping systems of the midwestern United States. The objectives of this research were to determine: (1) the quantity of seeds produced during the growing season and (2) the level of seed retention through crop harvest in midwestern soybean and field margins
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