Increased weed diversity, density andabove-ground biomass in long-termorganic crop rotations

While weed management is consistently a top priority among farmers, there is also growing concern for the conservationof biodiversity. Maintaining diverse weed communities below bioeconomic thresholds may provide ecosystem services forthe crop and the surrounding ecosystem. This study was conducted to determine if weed diversity, density and biomassdiffer within and among organic and conventional crop rotations. In 2007 and 2008, we sampled weed communities in fourlong-term crop rotations near Mead, Nebraska using seedbank analyses (elutriation and greenhouse emergence) andabove-ground biomass sampling. Two conventional crop rotations consisted of a corn (Zea mays) or sorghum (Sorghumbicolor)–soybean (Glycine max)–sorghum or corn–soybean sequence and a diversified corn or sorghum–sorghum or corn–soybean–wheat (Triticum aestivum) sequence. Two organic rotations consisted of an animal manure-based soybean–corn orsorghum–soybean–wheat sequence and a green manure-based alfalfa (Medicago sativa)–alfalfa–corn or sorghum–wheatsequence. Species diversity of the weed seedbank and the above-ground weed community, as determined by the Shannondiversity index, were greatest in the organic green manure rotation. Averaged across all sampling methods and years, theweed diversity index of the organic green manure rotation was 1.07, followed by the organic animal manure (0.78),diversified conventional (0.76) and conventional (0.66) rotations. The broadleaf weed seedbank density in the tillage layerof the organic animal manure rotation was 1.4r, 3.1r and 5.1r greater than the organic green manure, diversified conventionaland conventional rotations, respectively. The grass weed seedbank density in the tillage layer of the organicgreen manure rotation was 2.0r, 6.1r and 6.4r greater than the organic animal manure, diversified conventional andconventional rotations, respectively. The above-ground weed biomass was generally greatest in the organic rotations. Thebroadleaf weed biomass in sorghum and wheat did not differ between organic and conventional rotations (CRs), but grassweed biomass was greater in organic compared to CRs for all crops. The above-ground weed biomass did not differ withinCRs, and within organic rotations the grass weed biomass was generally greatest in the organic green manure rotation. Theweed seedbank and above-ground weed communities that have accumulated in these rotations throughout the experimentsuggest a need for greater management in long-term organic rotations that primarily include annual crops. However, resultssuggest that including a perennial forage crop in organic rotations may reduce broadleaf weed seedbank populations andincrease weed diversity