Weed Control for More Sustainable Soybean Production

Objective 1. Compare weed control of Canada thistle in an organic production system using competitive soybean varieties and bacterial biocontrol to current cultivation- and labor-intensive methods.

Field experiments were conducted in 1996 and 1997 to evaluate the efficacy of the bacterial biocontrol, PST, the competitive soybean variety "Kato" and two cultivations in an integrated weed management (IWM) system for the control of Canada thistle in organic soybean production. PST reduced Canada thistle survival by 33%, 69%, 91%, and 93% in 1996 and by 11%, 32%, 68%, and 70% in 1997 at 35, 51, 73 and 107 days after planting (DAP) respectively. Average height of surviving Canada thistle shoots was an average of 63% less in 1996 and 57% less in 1997 in plots treated with PST. PST reduced seedhead production on surviving Canada thistle plants by an average of 100% in 1996 and 85% in 1997. Kato had no significant impact on Canada thistle survival, height or seed production when compared to the non-competitive soybean variety "Evans" for both years. Two cultivations in Kato reduced Canada thistle height by 42% at 35 and 42% at 51 DAP in 1996 for plants not treated with PST. Two cultivations did not reduce Canada thistle height in 1997 for either variety. Two cultivations had no impact on the number of Canada thistle plants producing seed in each plot in both years but caused a 38% and 83% reduction in the total number of seed heads produced per plant at 51 and 73 DAP respectively in 1996. Soybean yield did not differ between plots treated or untreated with PST, which suggests the biocontrol agent is suited to soybean production. Soybean yield did not differ between plots receiving one or two cultivations. Yield did not differ among soybean varieties in 1997, however Evans yielded 9% more than Kato in 1996. There were no significant interactions between PST, soybean variety or cultivation for Canada thistle survival, suggesting that the combined weed control effects of each measure were additive. However, the combination of Kato plus PST plus two cultivations consistently resulted in the lowest values for seed production.

Objective 2. Compare weed control of Canada thistle in a no-till production system using competitive varieties, bacterial biocontrol, and reduced herbicide rates to current herbicide-intensive methods.

Field experiments were conducted in 1996 and 1997 to evaluate the efficacy of the biocontrol agent PST and the herbicide bentazon in soybean varieties "Kato" and "Evans" for an integrated weed management system to control Canada thistle and other weeds in a no-till soybean production system. PST reduced Canada thistle shoot survival by 5% and 50% at 48 and 105 DAP respectively in 1996 and by 16%, 28% and 60% at 48, 69 and 105 DAP respectively in 1997 compared with controls. Height of Canada thistle shoots that survived treatment with PST was 29% less in 1996 and 36% less in 1997 than untreated plants when averaged over sampling dates. PST reduced seedhead production on surviving Canada thistle plants by an average of 71% in 1996 and 86% in 1997. Bentazon provided the best control of Canada thistle and reduced survival by 30%, 34%, 96% and 97% in 1996 and by 24%, 52%, 88% and 95% in 1997 at 34, 48, 69 and 105 DAP respectively. Height of Canada thistle plants that survived bentazon treatment was always less than controls and bentazon reduced seed production by an average of 70-100%. Height differences in surviving Canada thistles between bentazon and PST treatments were not observed until 69 DAP in both years. Soybean variety was not a significant predictor of Canada thistle survival but Evans reduced Canada thistle height by 15% at 48 DAP in 1996 for weedy control plots when compared to Kato and while not statistically significant, was less for Evans in most treatments in both years. In Evans, surviving Canada thistles produced 43% and 48% fewer seed heads per plant at 48 and 69 DAP respectively in 1996 and 75% fewer seed heads per plant at 105 DAP in 1997 when compared to Kato. Bentazon reduced total weed density and dry shoot weight at 105 DAP by 68% and 84% respectively. Kato reduced total weed density and dry shoot weight at 105 DAP by 43% and 55% respectively when compared to Evans. Kato also enhanced the efficacy of PST by improving reductions in total weed densities and dry shoot weights observed in PST treated plots by 48% and 57% respectively at 105 DAP when compared to Evans. Soybean yield did not differ among treatments or soybean varieties. Combined effects of the competitive soybean variety Kato and other weed control treatments were observed in the general weed population at 105 DAP and provided better control than each treatment alone. Our findings are consistent with the hypothesis that careful combination of weed control measures in an integrated weed management system can improve weed control in no-till soybean production.

Objective 3. Collaborate with member organizations of the Sustainers' Coalition of the Minnesota Institute for Sustainable Agriculture in presentations, discussions and evaluations of research findings at summer field days and winter workshops.

There has been extensive collaboration with farmers from the SFA and the Coordinator of MISA as described in the Methods/Approach and Results sections for objective 4.

Objective 4. Initiate a broad-based cooperative research approach to develop integrated cultural and biological weed management, in order to reduce sediment and herbicide pollution associated with weed control, and support development of new sustainable farming systems for the north-central region.

After several group discussions during the winter of 1996-97, there was agreement between farmers and researchers that we have struggled to develop knowledge collaboratively. Many of the farmers involved in these conversations have seen, first-hand, both the potential and challenges of group learning about complex farming systems. Farmers, researchers and extensionists all agreed there is an urgent need to develop an operational model for such collaborative learning.

As an outcome of many one-on-one conversations with diverse stakeholders to explore their perceptions and interests, we organized a diverse planning team in May 1997 to plan "learning communities" (LCs) composed of farmers and other agriculturalists, for local inquiry in sustainable agriculture. The planning team is composed of farmers, researchers, and the Coordinator of the Minnesota Institute for Sustainable Agriculture. As we have progressed in our work, we believe that a "research cooperative" approach should be more focused on systems-level work oriented toward supporting transition to more biologically-rich and diversified agroecosystems. Moreover, we have shifted the emphasis of our work from research to farmer learning. This shift has been motivated by agreement in our planning group that farmer-participatory research has often been driven by the agendas of academic scientists. Our extensive organizing and planning work have led us to our current perspective. In what follows, we describe our plan of work to develop the LCs. This work plan is presented in the Results section because it is the result of many in-depth conversations of the planning team interacting with regional SFA chapters.

In our work to develop LCs , we will integrate several existing methods for planning and managing multidimensional farm operations. Agroecosystem analysis is a method for systematically learning about interactions between parts of a farm operation. Its premise is that these interactions determine much about the operation, including outcome of pest management efforts. Producers are typically well aware of many important interactions, but a number of others are not considered. AEA helps producers capture, organize and make more effective use of their knowledge of the components of a farm operation. We will link AEA to an adaptive management approach based on use of the inquiry cycle , a systematic method for inquiry into pest management problems by farmers. We believe that this approach will allow producers to align time and labor management with site-specific factors significantly more effectively than is possible with existing methods.

In our application, the key feature of AEA is a process of "pattern analysis," in which the major parts of a farm operation relevant to weed management are mapped and diagramed to organize existing information, reveal key uncertainties, and identify important interactions (e.g., between fertilizing, weed control and tillage operations). Information used in pattern analysis can come from observations, experience, published sources, "precision-farming" tools such as yield monitors, etc. To implement AEA to guide diversified weed management, relevant parts of the operation are identified, and then pattern analysis is carried out. Pertinent issues for pattern analysis will include patterns in time, space, flow and decision making.

Pattern analysis can be used by the producer to select among and integrate with the farm operation a variety of non-chemical weed control methods that may be available. These include: crop rotation, crop variety selection and seeding rate, cover crops, planting timing, soil moisture management, tillage and residue management, mechanical control, fertility management, reduced herbicide rates and others. The outcome of our approach is improved understanding of how an ongoing weed infestation is sustained in a given operation, of the range of applicable non-chemical control options, and of potential conflicts and synergies between these options and other important farm operations. Initial pattern analyses will be crude, but pattern analysis is an ongoing process that will lead to successive refinement of weed management and related farm operations.

Over the next two years we plan to establish two or more learning communities that will work to develop an integrated weed management approach through the method described above. Each LC will use the integrated weed management approach experimentally, assess its practical value, help develop practical and convenient means for using the approach, and help refine it.