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.

In the midwestern United States, biotypes of giant ragweed resistant to multiple herbicide biochemical sites of action have been identified. Weeds with resistance to multiple herbicides reduce the utility of existing herbicides and necessitate the development of alternative weed control strategies. In two experiments in southeastern Minnesota, we determined the effect of six 3 yr crop-rotation systems containing corn, soybean, wheat, and alfalfa on giant ragweed seedbank depletion and emergence patterns. The six crop-rotation systems included continuous corn, soybean-corn-corn, corn-soybean-corn, soybean-wheat-corn, soybean-alfalfa-corn, and alfalfa-alfalfa-corn. The crop-rotation system had no effect on the amount of seedbank depletion when a zero-weed threshold was maintained, with an average of 96% of the giant ragweed seedbank being depleted within 2 yr. Seedbank depletion occurred primarily through seedling emergence in all crop-rotation systems. However, seedling emergence tended to account for more of the seedbank depletion in rotations containing only corn or soybean compared with rotations with wheat or alfalfa. Giant ragweed emerged early across all treatments, with on average 90% emergence occurring by June 4. Duration of emergence was slightly longer in established alfalfa compared with other cropping systems. These results indicate that corn and soybean rotations are more conducive to giant ragweed emergence than rotations including wheat and alfalfa, and that adopting a zero-weed threshold is a viable approach to depleting the weed seedbank in all crop-rotation systems.

Economic assessment of alternative crops and crop rotations helps farmers determine those most appropriate for their farms. Th e objective of this research was to evaluate the economic net return and associated fi nancial risk for crops and crop rotations common to the midwestern United States, based on two 3-yr experiments in southern Minnesota where herbicide-resistant giant ragweed (Ambrosia trifi da L.) was present. Crop rotations included corn (Zea mays L.), soybean [Glycine max L. (Merr.)], wheat (Triticum aestivum L.), and alfalfa (Medicago sativa L.). Crop rotations were continuous corn (CCC), soybean-corncorn (SCC), corn-soybean-corn (CSC), soybean-wheat-corn (SWC), soybean-alfalfa-corn (SAC), and alfalfa-alfalfa-corn (AAC). Average crop yields during the study period were utilized along with average prices received and estimated production costs for each crop in Minnesota during this period to evaluate economic net return. Average net return of the SWC, SCC, CCC, CSC, SAC, and AAC rotations was US$11, $170, $247, $258, $368, and $919 ha -1 yr -1 , respectively. Th e AAC rotation was stochastically dominant to all other crop rotations for risk-averse decision makers. One or 2 yr of alfalfa stochastically dominated corn, soybean, and wheat, regardless of crop rotation, largely due to more stable alfalfa prices over the study period coupled with above-average yield and lower production costs. Th ese results confi rm that rotations containing alfalfa have the potential to provide a substantial economic net return to farmers while mitigating the risk of herbicide-resistant giant ragweed (Ambrosia trifi da L.) infestations.

Diversification of agroecological systems to enhance agrobiodiversity is likely to be critical to advancing environmental, economic, and social sustainability of agriculture. Temperate-zone agroecological systems that are currently organized for production of summer-annual crops can be diversified by integration of fallow-season and perennial crops. Integration of such crops can improve sustainability of these agroecological systems, with minimal interference with current agricultural production. Importantly, these crops can provide feedstocks for a wide range of new bio-products that are forming a new agricultural bioeconomy, potentially providing greatly increased economic incentives for diversification. However, while there are many fallow-season and perennial crops that might be used in such a "bioeconomic" strategy for diversification, most are not yet well adapted and highly-marketable. Efforts are underway to enhance adaptation and marketability of many such crops. Critically, these efforts require a strategic approach that addresses the inherent complexity of these projects. We outline a suitable approach, which we term "sustainable commercialization": a coordinated innovation process that integrates a new crop into the agriculture of a region, while intentionally addressing economic, environmental and social sustainability challenges via multi-stakeholder governance. This approach centers on a concerted effort to coordinate and govern innovation in three critical areas: germplasm development, multifunctional agroecosystem design and management, and development of end uses, supply chains, and markets. To exemplify the approach, we describe an ongoing effort to commercialize a new fallow-season crop, field pennycress (Thlaspi arvense L.).

Core Ideas Delayed pre‐plant tillage effectively controls herbicide‐resistant giant ragweed. Spring tillage did not affect total giant ragweed emergence. Early tillage treatments suppressed giant ragweed emergence the week following tillage