Diversity - Intensity of Cover Crop Systems: Managing Weed Seed Bank - Soil Health

Diversity - Intensity of Cover Crop Systems: Managing Weed Seed Bank - Soil Health

Summary

This project is addressing two related issues on low-input and organic small-scale diversified vegetable farms: recurrent and ubiquitous weed problems, and the need for long term soil improvement. Cover crop species and the frequency, timing, and depth of soil disturbance can affect soil quality and contribute to weed management by imposing stresses at multiple points in weed life cycles. Different cover cropping strategies are being investigated with simultaneous goals being the depletion of the weed seed bank and the maintenance or improvement in soil health. Cover cropping practices that can address these issues are critical to sustain production without herbicides, and are needed if reliance on cultivation is to be reduced or eliminated.

Objectives/Performance Targets

1) Experiment Station and on-farm research will yield a decision-aid matrix including weed species or functional groups, timing and intensity of disturbances, and diversity of cover crops deployed; management strategies will be identified based on their ability to reduce the germinable portion of the weed seed bank while maintaining or improving soil health.

2) Fifteen farmers will implement intensified/diversified cover cropping strategies on particular fields to reduce the seed bank of problematic weed species.

This Northeast Region SARE Project (LNE01-141; USDA 2001-38640-10221) was established to evaluate different cover cropping opportunities in vegetable cropping systems. Strategies for weed management and soil improvement can become antagonistic on farms attempting to reduce or eliminate farm chemical use, as herbicides are replaced with repeated and intensive soil cultivation. Such soil disturbance regimes can badly damage soil structure, decrease soil organic matter levels, and decimate soil biological activity, thus pitting short term non-chemical weed control against long-term soil health. Cover crops can substantially offset or reverse losses of soil quality by reducing the need for recurrent intensive cultivation, by adding crop residues to rebuild soil structure and organic matter, as well as by maintaining soil cover thus reducing erosion losses. Weed control from cover crops is derived from their competitive ability during growth, their function as a physical barrier or mulch when killed and left on the surface, as well as allelopathic properties of certain incorporated and surface residues.

Considering the multiple points at which cover cropping practices may contribute to these goals, we initiated a cropping systems comparison in 2001 featuring the following: (a) A conventionally-managed 2-year rotation of broccoli and winter squash (“Conv.); (b) an organic, land-limited system, also a 2-year rotation of broccoli and winter squash, but with winter cover crops (e.g., rye/hairy vetch) planted following harvest of the cash crops (“Fall CC”); (c) an organic, 4-year rotation of broccoli, winter squash, cereal/red clover, and red clover sod (“2-Yr CC”); and (d) an organic, 4-year rotation including broccoli, cover crop/summer fallow/cover crop, winter squash, and cover crop/summer fallow/cover crop (“Alt. Yr. CC”).

Accomplishments/Milestones

Soil quality differences among treatments are becoming more apparent each year. Soil quality testing in pre-tillage broccoli plots in 2004 show that Water Holding Capacity has increased as the intensity of cover cropping increases. At the same time, Bulk Density is decreasing in the intensely cover-cropped system, indicating an improving environment for root growth as compared to the conventional system.

After the first season the density of germinable common lambsquarters seeds was greater following winter squash in each system (4060 m-2 to 10 cm depth) compared to following broccoli (1100 m-2 to 10 cm depth). The decline in the seedbank due to the disturbance-intensive cover cropping practices (d, above) was evident in comparison to the sod-based cover cropping system (c, above), with mean densities of 1200 and 4600 germinable common lambsquarter seeds m-2, respectively.

Since the experiment began there has been a decline in the relative abundance of marsh yellow cress (Rorippa islandica (Oeder) Borbas) and a corresponding increase in common lambsquarters. Interestingly, this shift in the weed community was evident the Fall CC and 2-Yr CC systems, but not in the Conv. and Alt. Yr. CC systems. In the red-clover- based, 2-Yr CC system, despite efforts to preempt seed production by timely mowing, there is significant common lambsquarter seed rain in this system. Also, seeds are buried by tillage during establishment of the red clover cover crop; lack of subsequent soil disturbance, which is comparatively more frequent in the Alt. Yr. CC system, apparently encourages proliferation of common lambsquarters. This systems comparison highlights the challenge offered by crops likely to have high levels of seed rain, e.g., winter squash, and promises to refine our ability to recommend cover cropping practices based on weed management requirements.

Impacts and Contributions/Outcomes

Results from the cover cropping systems trial were featured at local, regional and national meetings in 2004. The project was well received by 40 growers and agriculture professionals at the Rogers Farm field day in Stillwater, Maine. Posters featuring the project were presented at the National SARE Conference in Burlington and the American Society of Agronomy national meeting in Seattle. A two hour session was dedicated to the the systems study at the Maine Organic Farmers and Gardeners Farmer-to-Farmer Conference in Bar Harbor, Maine. Lastly, the projects’ data sets were featured in a presentation at the North Central Weed Science Society annual meeting in Columbus, OH.

Grower participants are eager to design a second phase of the cover cropping systems experiment.

Collaborators: