Weed Seedbank Research Articles

The Effect of Rotation and In-Crop Weed Management on the Germinable Weed Seedbank after 10 Years

Agricultural production systems that reduce the use of in-crop herbicides could greatly reduce risks of environmental damage and the development of herbicide-resistant weeds. Few studies have investigated the long-term effects of in-crop herbicide omissions on weed seedbank community size and structure. A crop-rotation study was sampled 10 yr after a strictly annual rotation and an annual/perennial rotation were exposed to different in-crop herbicide omission treatments. In-crop herbicides were applied either in all annual crops (control), omitted from oats only, or omitted from both flax and oats. Seedbank densities were greatest when in-crop herbicides were omitted from flax and oats, and this treatment also reduced crop yield. Shannon-Wiener diversity differed among crops in the annual crop rotation and among herbicide omission treatments in the perennial rotation. Herbicide omissions changed the weed-community structure in flax and in wheat and canola crops in the annual rotation enough to warrant alternate control methods in some treatments. The magnitude of the effects on the seedbank parameters depended largely on the competitive ability of the crop in which herbicides were omitted. No yield response to omitting herbicides in oats indicated that standard weed management practices have reduced weed populations below yield-loss thresholds.

Integrating Multiple Tactics for Managing Weeds in High Residue No‐Till Soybean

Rolled cover crop mulches can suppress weeds in subsequent cash crops, reduce the need for herbicides, and allow organic no‐till cash crop establishment. This study investigated the weed suppressiveness of a cereal rye (Secale cereale L.) cover crop mulch across varying weed seedbank density. Cereal rye was seeded at two dates in the fall and terminated at five dates in the spring to create biomass ranging from 100 to 1600 g m−2. The first three termination dates included both herbicide (glyphosate) and rolling of the rye, while later three dates were only rolled. Soybean [Glycine max (L.) Merr.] was no‐till planted after rye termination, and weed biomass and soybean yield were assessed. Spring termination date more strongly affected cereal rye biomass than fall planting date; a termination delay of 5 to 15 d compensated for a planting delay of 30 d. Weed biomass generally declined with increasing cereal rye biomass, and this relationship was stronger at higher weed seedbank densities. Supplemental weed control reduced weed biomass compared to no supplemental control and postherbicide was more effective than cultivation. While increasing cereal rye biomass was associated with a decline in soybean yield in 2009, it did not consistently impact soybean stand. Instead soybean stand establishment appeared to be impacted by high cover crop biomass and changing edaphic conditions at planting. Future research should focus on improved technology for direct seeding in high residue environments and developing longer term cropping systems less reliant on tillage and herbicides.

Grass–Legume Mixtures and Soil Fertility Affect Cover Crop Performance and Weed Seed Production

Summer leguminous cover crops can improve soil health and reduce the economic and environmental costs associated with N fertilizers. However, adoption is often constrained by poor weed suppression compared to nonlegume cover crops. In field experiments conducted in organic vegetable cropping systems in north-central New York, two primary hypotheses were tested: (1) mixtures of legume cover crops (cowpea and soybean) with grasses (sorghum–sudangrass and Japanese millet) reduce weed seed production and increase cover crop productivity relative to legume monocultures and (2) higher soil fertility shifts the competitive outcome in favor of weeds and nonlegume cover crops. Cover crops were grown either alone or in grass–legume combinations with or without composted chicken manure. Under hot, dry conditions in 2005, cowpea and soybean cover crops were severely suppressed by weeds in monoculture and by sorghum–sudangrass in mixtures, resulting in low legume biomass, poor nodulation, and high levels of Powell amaranth seed production (> 25,000 seeds m−2). Under more typical temperature and rainfall conditions in 2006, cowpea mixtures with Japanese millet stimulated cowpea biomass production and nodulation compared to monoculture, but soybeans were suppressed in mixtures with both grasses. Composted chicken manure shifted competition in favor of weeds at the expense of cowpea (2005), stimulated weed and grass biomass production (2006), and suppressed nodulation of soybean (2006). In a complementary on-farm trial, cowpea mixtures with sorghum–sudangrass suppressed weed biomass by 99%; however, both common purslane and hairy galinsoga produced sufficient seeds (600 seeds m−2) to replenish the existing weed seedbank. Results suggest that (1) mixtures of cowpeas with grasses can improve nodulation, lower seed costs, and reduce the risk of weed seed production; (2) soybean is not compatible with grasses in mixture; and (3) future costs of weed seed production must be considered when determining optimal cover crop choices.

Weed seedbanks in arable fields: effects of management practices and surrounding landscape

José‐María L & Sans FX (2011). Weed seedbanks in arable fields: effects of management practices and surrounding landscape. Weed Research51, 631–640.SummaryWeed seedbanks are a reserve of weed diversity and can contribute to the prediction of future weed problems in arable fields. Managing seedbanks should therefore help in optimising biodiversity and controlling weed infestations. This study assessed the effects of management system (organic vs. conventional) and landscape complexity on seedbank size and species richness at the edges and centres of Mediterranean dryland cereal fields and examines the relationship between specific management practices and seedbanks. Field edges and organic fields had more species‐rich, denser seedbanks than field centres and conventional fields, and landscape complexity had a limited effect on arable seedbanks. Accordingly, the promotion of low‐intensity farming practices regardless of landscape complexity, especially at field edges, would be an effective measure for conservation purposes in Mediterranean agroecosystems. Nevertheless, the high seed density of organic seedbanks reveals the need for more effective seedbank management. The analysis of the effects of specific management practices highlights the importance of cleaning crop seeds properly to reduce seedbank size and using complex rotations, especially as this tends to conserve species richness while reducing seed abundance.

Effect of a Living Mulch on Weed Seed Banks in Tomato

Weeds that emerge between rows in fresh market tomatoes after the critical period of competition are not suppressed by the crop and can produce large quantities of seed. A living mulch planted between rows might limit weed seed production. Buckwheat was seeded between tomato rows after the critical period in 2007 and 2008 in field studies near Lafayette, IN. Weeds were allowed to emerge after the critical period (CP), controlled throughout the growing season (no seed threshold [NST]), or mowed to limit seed production (MOW). Buckwheat and MOW plots were mowed twice after the critical period in 2007 and once in 2008. Seed banks were sampled after the critical period and in the following spring. Tomato yields were not reduced by growing buckwheat between rows. Seed bank densities for common purslane and carpetweed, which escaped mowing due to their prostrate habits, increased in all treatments. Germinable seed bank densities were 306 seeds m−2or less in the NST and buckwheat treatments but 755 seeds m−2or more in the CP treatments for species with erect habits in both years. Seed bank densities were lower in the MOW treatment than in the CP treatments in 2007 but not in 2008. In a parallel experiment conducted in adjacent plots, buckwheat was seeded at five rates (0, 56, 112, 168, and 224 kg seed ha−1). Plots were mowed and emergent weeds sampled as described for the intercrop experiment. Weed densities before mowing decreased linearly with buckwheat seed rate. After mowing, no relationship was detected between seed rate and weed densities. This study supports the hypothesis that a living mulch planted after the critical period can be used to limit seed bank growth without reducing tomato yields, but additional research is needed to better understand the effect of mowing on living mulch growth and weed suppression.

Effects of insect population density and storage time on grain damage and weight loss in maize due to the maize weevil Sitophilus zeamais and the larger grain borer Prostephanus truncatus

In the period 2007 to 2010, at localities Backi Maglic and Krivaja studies of weed seedbank were performed under soybean crop. Results showed that weed seed separated at Backi Maglic in all years dominated in the top soil layer, while the lowest quantities were separated from the deepest layer, and that number of weeds declined annually. At Krivaja, the lowest weed seed stocks were found in the top layer, and the highest in the deepest layer. At this locality, number of weed seeds kept rising from season to season, which was the consequence of application of different cultural practices. Key words: Soybean, weed seedbank, Amaranthus retroflexus L., Chenopodium album L.,Datura stramonium L.

Study of weed seedbank in soybean crop

In the period 2007 to 2010, at localities Backi Maglic and Krivaja studies of weed seedbank were performed under soybean crop. Results showed that weed seed separated at Backi Maglic in all years dominated in the top soil layer, while the lowest quantities were separated from the deepest layer, and that number of weeds declined annually. At Krivaja, the lowest weed seed stocks were found in the top layer, and the highest in the deepest layer. At this locality, number of weed seeds kept rising from season to season, which was the consequence of application of different cultural practices. Key words: Soybean, weed seedbank, Amaranthus retroflexus L., Chenopodium album L.,Datura stramonium L.

National-scale regulation of the weed seedbank by carabid predators

1. Assuring future food productivity and security will require that better use is made of pest regulation provided by naturally occurring ecological services. However, empirical evidence of large-scale regulatory effects that might be employed in agriculture is still relatively scarce. 2. Using data from 257 conventionally managed arable fields at the UK national scale, we examine whether changes in the long-term store of weed seed in the seedbank are consistent with regulation by seed predatory carabid beetles. 3. We test three expectations of a simple conceptual model for carabid seed predation. The relationships we estimate are consistent with the model and suggest that carabid predation of weed seeds shed onto the soil surface changes the amount of seed returned to the seedbank bringing about seedbank change and regulation. 4. Granivorous and omnivorous carabids regulated seedbank abundance, with effects being observed on monocotyledon seedbank abundance, in all crops, and on total seedbank abundance, in spring maize and winter oilseed rape; effects that were robust across fields with differing pesticide management and between regions of the UK. 5. We found evidence of density dependence, with increasing amounts of seed rain leading to stronger regulation of the seedbank. 6. Our results also suggest that correlations between seed predators and seed rain abundance, which might be used to infer important effects of seed predators, do not provide sufficient evidence to indicate regulation of the weed seedbank. 7. Synthesis and applications. A major challenge for the future is to manage ecological, pest control services in place of current pesticides with little or no additional risk to productivity and food security. Our work shows that carabid seed predators have regulatory effects on the seedbank that appear general and robust across a range of current cropping and farm management situations at the national scale. Environmental Stewardship methods already exist across Europe to enhance carabid numbers in farmland. This means that carabid seed predators fit within a working framework that could be used to promote integrated pest management alongside or even in place of herbicides.

Modelling rotations: can crop sequences explain arable weed seedbank abundance?

Bohan DA, Powers SJ, Champion G, Haughton AJ, Hawes C, Squire G, Cussans J & Mertens SK (2011). Modelling rotations: can crop sequences explain arable weed seedbank abundance? Weed Research51, 422–432.SummaryWe investigated the effects of crop sequences on monocotyledon, dicotyledon and total weed seedbank abundance. Using seedbank data sampled from the conventionally cropped part of the GB farm‐scale evaluations of genetically modified, herbicide‐tolerant (GMHT) crops, we asked whether it is possible to identify crop sequence effects, to identify their duration and to simplify crop sequences into crop management classes with similar effects on weed seedbanks. This work showed that it is possible to detect historical effects of past crops, sown in sequence, on weed seedbanks for up to 3 years and that crop sequences may be simplified to crop management classes describing the season of sowing, crop type and weed target for herbicide application. Model estimates for the seedbanks were validated against an independent, follow‐up seedbank data set. The analysis provided abundance estimates that ranged over 3 and 1.7 orders of magnitude for the monocotyledon and dicotyledon weed seedbanks for different crop sequences. This work yields a methodology for estimating seedbank abundance in current crop sequences, potentially allowing sequences to be identified that better reconcile the competing needs for weed control to maintain crop productivity and the demand for increased farmland biodiversity.

Bacterial Communities Associated with Chenopodium album and Stellaria media Seeds from Arable Soils

The bacterial community compositions in Chenopodium album and Stellaria media seeds recovered from soil (soil weed seedbank), from bulk soil, and from seeds harvested from plants grown in the same soils were compared. It was hypothesized that bacterial communities in soil weed seedbanks are distinct from the ones present in bulk soils. For that purpose, bacterial polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) fingerprints, made from DNA extracts of different soils and seed fractions, were analyzed by principal component analysis. Bacterial fingerprints from C. album and S. media seeds differed from each other and from soil. Further, it revealed that bacterial fingerprints from soil-recovered and plant-harvested seeds from the same species clustered together. Hence, it was concluded that microbial communities associated with seeds in soil mostly originated from the mother plant and not from soil. In addition, the results indicated that the presence of a weed seedbank in arable soils can increase soil microbial diversity. Thus, a change in species composition or size of the soil weed seedbank, for instance, as a result of a change in crop management, could affect soil microbial diversity. The consequence of increased diversity is yet unknown, but by virtue of identification of dominant bands in PCR-DGGE fingerprints as Lysobacter oryzae (among four other species), it became clear that bacteria potentially antagonizing phytopathogens dominate in C. album seeds in soil. The role of these potential antagonists on weed and crop plant growth was discussed.

The Selective Memory of Weed Seedbanks after 18 Years of Conservation Tillage

A conservation tillage study provided the opportunity to test whether tillage effects on the germinable weed seedbank would be consistent across different crop rotations and to investigate the potential residual effects of herbicide treatments terminated 12 yr earlier. Our objective was to measure the effects of tillage (moldboard plow [MP] vs. chisel plow [CP] vs. no-till [NT]), crop rotation (2-yr barley–red clover followed by 4-yr barley–canola–wheat–soybean rotation, compared to a cereal monoculture), and of a prior weed management factor (three intensity levels of herbicide use) on the density, diversity, and community structure of weed seedbanks. Species richness, evenness (Shannon'sE), and diversity (Shannon'sH′) of spring seedbanks varied little across treatments and over time. Total seedbank density generally increased as tillage was reduced, with some variations due to weed management in 1993 and crop rotation in 2006. Crop rotations generally had smaller seedbanks with fewer species than the monoculture. In 1993, seedbanks with minimum weed management were twice as dense as those with intensive or moderate weed management (approximately 6,000 vs. 3,000 seed m−2). By 2006, seed density averaged 6,838 seed m−2across intensive and moderate weed management regardless of tillage, but was nearly twice as large in NT (12,188 seed m−2) compared to MP (4,770 seed m−2) and CP (7,117 seed m−2) with minimum weed management (LSD0.005= 4488). Species with abundant seedbanks responded differently to treatments. Barnyardgrass and green foxtail had larger seedbanks in the monoculture than in the rotation. Common lambsquarters and pigweed species had large seedbanks in tilled treatments in the rotation, whereas yellow foxtail and field pennycress contributed to the large seedbanks observed in NT treatments. The latter two species were also associated with residual effects of weed management treatments (terminated 12 yr earlier) in NT. The differential seedbank response of weed species, attributed in part to contrasting weed emergence patterns and agronomic practice effects on seed rain, explained some of the weak treatment effects observed for total seedbank density and diversity. The large weed seedbanks observed in NT plots after 18 yr confirms the importance of seed rain and seedbank management for the sustainability of NT systems.

Ecophysiological study on weed seed banks and weeds in Cambodian paddy fields with contrasting water availability

Weed infestations are a major cause of yield reduction in rice (Oryza sativa) cultivation, particularly with direct‐seeding methods, but the relationship between weed dynamics and water availability in Cambodian paddy fields has not been documented previously. We surveyed the weed abundance and weed seed banks in the soil of paddy fields with inferred differences in their water regime in 22 farm fields in three provinces of Cambodia in the 2005 and 2006 rainy seasons. We studied rain‐fed lowland fields in upslope and downslope topographic positions and fields at different distances from the irrigation water source inside an irrigation rehabilitation area. The weed seed banks were estimated by seedling emergence in small containers and weed abundance and vigor were estimated by a simple scoring system. The estimated weed seed bank in the top 5 cm of soil ranged from 52.1 to 167 × 103 seeds m−2 (overall mean of 8.5 × 103 seeds m−2) and contained a high proportion (86%) of sedge species, such as Fimbristylis miliacea L. and Cyperus difformis. Several fields had particularly large seed banks, including one near the reservoir. No clear difference was found in the weed seed banks between the irrigated fields that were located close to (upstream) and distant from (downstream) the water source or between the irrigated and rain‐fed lowland fields, but the weed scores were larger in the rain‐fed fields and the downstream fields within the irrigated area. A water shortage during the late growing season in 2005 led to a proliferation of weeds in some fields and an associated increase in weed seedbank size in 2006. However, the weed scores in 2006 were more strongly associated with that year's water conditions than with the weed seedbank size.

Influence of Winter Annual Weed Removal Timings on Soybean Cyst Nematode Population Density and Plant Biomass

Soybean cyst nematode (SCN) is one of the most yield limiting pathogens in U.S. soybean production. Henbit and purple deadnettle are winter annual weeds shown to facilitate SCN reproduction after crop harvest in the eastern Corn Belt. These weeds, along with volunteer soybean that germinates in autumn after harvest, are common to postharvest soybean production fields and provide an opportunity for SCN reproduction and population increase outside of the typical soybean production season. The objective of this experiment was to determine if autumn removal of these weeds and volunteer soybean can influence the winter weed seedbank, plant biomass, and SCN population densities. Microplots were established with or without Lamium spp. and volunteer soybean, and four winter weed removal timings (none, October, December, and May). Dry weights of autumn Lamium spp. were reduced 50% in October when grown in competition with volunteer soybean. SCN juveniles were found in henbit roots at higher densities in October (42 per gram of root) than December (5 per gram of root) and were also found in the roots of volunteer soybean (14 per gram of root) in October. SCN egg population densities were 50% lower in August after the summer fallow period. The results of this experiment suggest that autumn removal of winter annual weeds and volunteer soybean did not reduce SCN populations.

Influence of Cropping Systems on Soil Aggregate and Weed Seedbank Dynamics During the Organic Transition Period

Agronomic management during the 3‐yr transition period to organic certification influences soil quality and the weed seedbank. We studied two cropping systems during the transition period and the first certified organic season. A 4‐yr rotation of corn, soybean, wheat/alfalfa, corn (C‐S‐W/A‐C) [Zea maysL.,Glycine max(L.) Merr.,Triticum aestivumL.,Medicago sativaL.], produced under a more complex management that included manure and cover crop residue, was compared to a perennial based corn, alfalfa, alfalfa, corn (C‐A‐A‐C) rotation. We compared soil aggregate size distribution and bulk density after Year 1 and on completion of the transition period. Weed seedbank populations were quantified through two seasons in the greenhouse. Weed surface density and aboveground weed biomass were quantified in the field. Over the course of the study, the percentage of large soil macroaggregates (>2000 μm size class) had 2.7 and 3.4‐fold increase for the C‐A‐A‐C and C‐S‐W/A‐C treatments, respectively. The C‐S‐W/A‐C system generated a 4.5‐fold increase in aggregates of this class when wheat that was interseeded with alfalfa was harvested as forage. Bulk density decreased 14 and 6% for the C‐S‐W/A‐C and the C‐A‐A‐C systems, respectively. There was a 60 to nearly 300% increase in total weed seeds germinated in the greenhouse for the C‐S‐W/A‐C system. This same system had a 60 to more than 500% decreased weed seedbank density in the field. We conclude from this study that either strategy can improve soil quality while the weed seedbank was better managed in the more complicated C‐S‐W/A‐C system.

Weed seedbank responses to 12 years of applications of composts, animal slurries or mineral fertilisers

De Cauwer B, Van Den Berge K, Cougnon M, Bulcke R & Reheul D (2010). Weed seedbank responses to 12 years of applications of composts, animal slurries or mineral fertilisers. Weed Research50, 425–435.SummaryFertiliser amendments can impact weed populations in a variety of ways. This study evaluated the effects of 12‐year‐long applications of different fertilisation systems on size and composition of the weed seedbank in a conventionally managed maize monoculture field. Fertilisation systems included all factorial combinations of two dairy cattle slurry rates, three vegetable, fruit and garden waste compost rates, and three synthetic N fertiliser rates. Soil samples were taken in each subplot in May 2008 after sowing and prior to herbicide application. Residues recovered from soil samples were tested for weed seedling emergence to characterise soil seedbanks. Total weed seedbank density was affected by mineral N fertilisation, but not by compost or animal slurry application. Weed seedbank composition was related to compost amendment and mineral N fertilisation. Annual compost amendments reduced seedbank density of some persistent species (e.g., Chenopodium album and Solanum nigrum) irrespective of mineral N fertilisation. Compost is a promising tool for incorporation into integrated weed control strategies aimed at reducing weed seedbank persistence.

Vineyard Weed Seedbank Composition Responds to Glyphosate and Cultivation after Three Years

This research compared effects of the weed control practice, soil cultivation, and the conventional practice, glyphosate application on weed seedbank, in a vineyard system. The experiment was conducted in a commercial wine-grape vineyard in the Napa Valley of northern California from 2003 to 2005. The annual treatments were “winter–spring glyphosate,” “spring cultivation,” “fall–spring cultivation,” and “fall cultivation–spring glyphosate,” and were applied “in-row,” under the vine. Composition of the weed seedbank collected in 2002 before treatment establishment did not differ among treatments. After 3 yr of weed treatments, detrended correspondence analysis indicated that the composition of spring cultivation and winter–spring glyphosate tended to differ from each other, but the remaining two treatments showed little differentiation. As determined by linear discriminant analysis, the specific weed species were associated with seedbanks of certain treatments. These were Carolina geranium, annual bluegrass, brome grasses, California burclover, and scarlet pimpernel, which do not pose problems with regard to physical aspects of grape production. Although ‘Zorro’ rattail fescue was ubiquitous among treatments, its distribution between depths in the cultivated treatments indicated that tillage provided some homogenization of seedbank along the vertical soil profile. The seedlings from the seedbank study were not congruent with those measured aboveground in the field, suggesting that both treatment and microclimatic effects in the field may have influenced germination, and thus, aboveground composition.

Relationship between seedbank and aboveground botanical composition during spring

Poor pasture persistence and ingress of weedy species are major concerns for farmers The seedbank gives an indication of what weedy species may establish in pastures and compete with sown species It was hypothesised that the aboveground composition reflects the seedbank The seedbank was sampled and botanical assessments were undertaken during OctoberNovember 2009 in Northland Waikato Taranaki and North Canterbury Aboveground botanical composition did not reflect the weed seedbank composition in any of the regions assessed While cover of grasses was greater than legumes and herbs aboveground weedy herbs were most dominant in the seedbank followed by grasses then legumes There was no emergence of sown grass from the seedbank highlighting the lack of a persistent sown grass seedbank which may contribute to poor pasture persistence

Managing Weeds in Potato Rotations Without Herbicides

Managing weeds without herbicides requires an integration of methods and strategies and a change in how weeds are perceived. Weeds should be managed in a holistic, intentional and proactive manner. Understanding the interactions between the cropping system and the weed community and managing the cropping system to prevent and discourage weeds and maintain a low weed seedbank is necessary for successful weed management in organic systems. Cultural practices, including a well-planned crop rotation, planting cover crops, sanitation practices, and optimum row spacing and timing of planting are important aspects of managing weeds in organic systems. Multiple, well timed shallow cultivations or flaming can eliminate many early season weeds. Once emerged, many crops are fast growing and produce a canopy able to suppress weeds. New technologies for detecting crop rows and weeds coupled with precision cultivation, flaming, and application of nonselective organic herbicides are being developed and hold promise to reduce the need for hand weeding.

Reducing the germinable weed seedbank with soil disturbance and cover crops

Mirsky SB, Gallandt ER, Mortensen DA, Curran WS & Shumway DL (2010). Reducing the germinable weed seedbank with soil disturbance and cover crops. Weed Research50, 341–352.SummaryEcologically based weed management relies heavily on a greater integration of cultural and mechanical control tactics than conventional weed management. As such, management outcomes are more dependent on biotic interactions among the crop, weed and methods of management. In this study, we assessed the influence of soil disturbance and cover cropping on the germinable seedbank of Chenopodium album, Abutilon theophrasti and Setaria spp. across a range of initial weed seedbank densities. Cover crop system treatments ranged from greater reliance on tillage at one extreme, to a greater reliance on cover cropping to accomplish weed suppression at the other. In general, seedbank losses to germination were highest when soil disturbance was associated with cover cropping. Of the five cover crop systems (1 year duration), the summer fallow and the yellow mustard/buckwheat/winter rape mixture consistently decreased the weed seedbank. The magnitude of decline in these high disturbance systems reached complete depletion of the Setaria spp. germinable seedbank and an 85% and 80% reduction for C. album and A. theophrasti respectively. Both systems included tilling the soil three to four times throughout the growing season. In contrast, treatments which involved less soil disturbance had weed escapes that set seed in one or more site years. Cover crop systems that stimulated weed seed germination and where weeds were either suppressed by the cover crop or where subsequent tillage provided control, resulted in the greatest weed seedbank declines.

Increased weed diversity, density and above-ground biomass in long-term organic crop rotations

AbstractWhile weed management is consistently a top priority among farmers, there is also growing concern for the conservation of biodiversity. Maintaining diverse weed communities below bioeconomic thresholds may provide ecosystem services for the crop and the surrounding ecosystem. This study was conducted to determine if weed diversity, density and biomass differ within and among organic and conventional crop rotations. In 2007 and 2008, we sampled weed communities in four long-term crop rotations near Mead, Nebraska using seedbank analyses (elutriation and greenhouse emergence) and above-ground biomass sampling. Two conventional crop rotations consisted of a corn (Zea mays) or sorghum (Sorghum bicolor)–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 or sorghum–soybean–wheat sequence and a green manure-based alfalfa (Medicago sativa)–alfalfa–corn or sorghum–wheat sequence. Species diversity of the weed seedbank and the above-ground weed community, as determined by the Shannon diversity index, were greatest in the organic green manure rotation. Averaged across all sampling methods and years, the weed 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 layer of the organic animal manure rotation was 1.4×, 3.1× and 5.1× greater than the organic green manure, diversified conventional and conventional rotations, respectively. The grass weed seedbank density in the tillage layer of the organic green manure rotation was 2.0×, 6.1× and 6.4× greater than the organic animal manure, diversified conventional and conventional rotations, respectively. The above-ground weed biomass was generally greatest in the organic rotations. The broadleaf weed biomass in sorghum and wheat did not differ between organic and conventional rotations (CRs), but grass weed biomass was greater in organic compared to CRs for all crops. The above-ground weed biomass did not differ within CRs, and within organic rotations the grass weed biomass was generally greatest in the organic green manure rotation. The weed seedbank and above-ground weed communities that have accumulated in these rotations throughout the experiment suggest a need for greater management in long-term organic rotations that primarily include annual crops. However, results suggest that including a perennial forage crop in organic rotations may reduce broadleaf weed seedbank populations and increase weed diversity.