Cereal rye mulch biomass and soybean density affect weed suppression and community assembly in an organic no-till system

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Uriel D. Menalled and Matthew R. Ryan, School of Integrative Plant Sciences - Soil and Crop Sciences, Cornell University, Ithaca, NY, Guillaume Adeux and Stéphane Cordeau, Agroécologie, INRAé, Dijon, France, Richard G. Smith, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH, Steven Mirsky, SASL, USDA-ARS Sustainable Agriculture Systems Laboratory, Beltsville, MD

Background/Question/Methods
Organic no-till crop production, where cover crops are mechanically terminated and used a surface mulch, can provide soil health benefits and requires less labor and fuel compared with traditional, tillage-based, organic crop production. However, without the use of synthetic herbicides or soil tillage, organic no-till crop production is highly dependent on effective ecological weed management. In addition to reducing weed-crop competition, ecological weed management aims to reduce weed populations, prevent immigration and proliferation of highly adapted species, and increase crop tolerance to weed competition over time. Weed community assembly theory is an integral part of ecological weed management and can contribute to optimizing production systems by understanding how traits are filtered by different management practices. Using data from a factorial field experiment conducted in Pennsylvania and Maryland in two years, we quantified the effects of increasing crop density and cereal rye (Secale cereale L.) mulch biomass on weed suppression in soybean [Glycine max (L.) Merr.] that was no-till planted and managed organically. In each treatment, weed biomass was sampled, separated by species, dried and weighed. A trait-based approach was then used to quantify the relationship between management practice intensity and weed life cycle, emergence timing, seed weight, height, and specific leaf area.

Results/Conclusions
Total weed biomass was affected by the interaction between soybean density and mulch biomass, suggesting synergistic weed suppression when the two management practices were combined. Increased soybean density and mulch biomass also affected weed communities by constraining their trait functional dispersion and shifting trait composition. Changes in mulch biomass had a larger effect on annual weed suppression and weed community composition than changes in soybean density. In an applied context, farmers who can produce high cover crop mulch biomass for weed management should expect an increased proportion of perennials and weeds with later emergence, heavier seeds, and shorter stature. However, increased soybean density reduced perennial weed biomass and may be an effective strategy for managing perennial weeds. Results illustrate the potential for weed community assembly theory to help guide ecological weed management and enable farmer adoption of organic no-till crop production.