The allelopathic potential of a plant has been evaluated on the basis of two indicators: specific activity, which is the specific concentration of the allelochemical to exert a half-maximum effect on a receiver plant (EC50), and total activity in a plant, which is the ratio of the concentration of an allelochemical in the producing plant to its EC50. In the present study, a new indicator, total activity in a soil, which takes into account the effects of a soil on the allelopathy activity, is proposed because allelopathic activity is affected by the presence of soils. The total activity in a soil was calculated by multiplying the “total activity in a plant” with a “soil factor.” In this calculation, we assumed simplified cases for comparison, such that the allelopathic plant materials are evenly incorporated in the soils and the allelochemicals are released from the plant materials to the soils at a constant rate. We conducted bioassay experiments in the presence and absence of soils and cited some published data to calculate the specific activities and total activities in a plant and in a soil. The results indicated that the allelopathies of buckwheat caused by (+)-catechin, Leucaena leucocephala by L-mimosine, Xanthium occidentale by trans-cinnamic acid, and Brassica parachinensis by cis-cinnamic acid were not significant in a volcanic ash soil, an alluvial soil, and a calcareous soil, but the allelopathy of sweet vernalgrass caused by coumarin and Spiraea thunbergii by cis-cinnamoyl glucosides was highly effective in those soils. The allelopathies of Juglans species caused by juglone plus juglone precursors and Mucuna pruriens by L-DOPA would depend highly on the soil types. Although some limitations exist for this approach, the total activity approach would allow for a better quantitative estimation of the allelopathic potential of plant materials in soils.

Canada thistle is a perennial spreading weed that is difficult to control in farming systems with reduced reliance upon herbicides for weed management. Experiments were conducted from 2006 to 2008 at Champaign, IL, to evaluate the combined effects of summer annual cover crops and mowing on Canada thistle growth and survival. Whole plot treatments were fallow, buckwheat, sudangrass–cowpea mixture (MIX), and sudangrass. The subplot treatments were mowing frequencies (0 to 2 times). Cover crop and mowing did not interact to suppress Canada thistle. MIX and sudangrass produced more standing biomass, greater regrowth, and more surface mulch following mowing than the buckwheat. A single season with sudangrass or MIX reduced Canada thistle shoot density and mass to less than 20% of the initial values through two growing seasons. Mowing alone only suppressed Canada thistle shoot density and mass on the site with greater initial density. A sudangrass or MIX cover crop alone or combined with mowing suppresses Canada thistle, but intensive management must continue for several years to eliminate patches.

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−2 or less in the NST and buckwheat treatments but 755 seeds m−2 or 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.

Cover crops represent a potentially important biological filter during weed community assembly in agroecosystems. This filtering could be considered directional if different cover-crop species result in weed communities with predictably different species composition. We examined the following four questions related to the potential filtering effects of cover crops in a field experiment involving five cover crops grown in monoculture and mixture: (1) Do cover crops differ in their effect on weed community composition? (2) Is competition more intense between cover crops and weeds that are in the same family or functional group? (3) Is competition more intense across weed functional types in a cover-crop mixture compared with cover crops grown in monocultures? (4) Within a cover-crop mixture, is a higher seeding rate associated with more effective biotic filtering of the weed community? We found some evidence that cover crops differentially filtered weed communities and that at least some of these filtering effects were due to differential biomass production across cover-crop species. Monocultures of buckwheat and sorghum–sudangrass reduced the number of weed species relative to the no-cover-crop control by an average of 36 and 59% (buckwheat) and 25 and 40% (sorghum–sudangrass) in 2011 and 2012, respectively. We found little evidence that competition intensity was dependent upon the family or functional classification of the cover crop or weeds, or that cover-crop mixtures were stronger assembly filters than the most effective monocultures. Although our results do not suggest that annual cover crops exert strong directional filtering during weed community assembly, our methodological framework for detecting such effects could be applied to similar future studies that incorporate a greater number of cover-crop species and are conducted under a greater range of cover-cropping conditions.

Field and pot studies were conducted in Central New York to determine the potential weed-management benefits of a buckwheat cover crop grown before winter wheat. Specific objectives were to determine buckwheat residue effects on (1) emergence and growth of winter annual weeds; (2) wheat establishment and yield; and (3) emergence of summer annual weeds in the spring following overwinter seed burial. In a field study, buckwheat was sown at two timings (July or August), mowed, and either incorporated or left on the soil surface. Winter wheat was drilled into buckwheat residue in September and weed and crop growth were monitored. In a complementary pot study, four winter annual weeds were sown in soil removed from buckwheat and bare-soil plots at 0 or 15 d after incorporation and monitored for emergence and early growth. To assess buckwheat residue effects on spring emergence from overwintering seeds, seeds of three weed species were buried in buckwheat residue and bare-soil plots in the fall, exhumed in April, and evaluated for emergence. To investigate the mechanism for possible effects of buckwheat residue on overwintering seeds, two levels each of seed treatment (none or fungicide) and fertilization (none or 170 kg ha−1) were applied before burial. Buckwheat residue had no negative effect on wheat yields but suppressed emergence (22 to 72%) and growth (0 to 95%) of winter annual weeds, although effects were often small and inconsistent. Buckwheat residue had no effect on the emergence of buried weed seeds in spring. However, fungicide treatment enhanced the emergence of Powell amaranth seeds by 12.5 to 25.5% and of barnyardgrass seeds by 0 to 12%. Our results suggest that buckwheat residue can contribute to weed management in wheat cropping systems, but that further studies investigating the mechanistic basis for the inconsistent selective effects of buckwheat residue on weeds are needed before buckwheat use can be optimized.

Buckwheat residues can suppress both emergence and growth of weeds, but the mechanisms of this suppression are not well understood. The main objectives of this research were to evaluate the possible role of (1) low initial nitrogen (N) availability and (2) fungal pathogens in this suppression for three sensitive weed species: Powell amaranth, shepherd's-purse, and corn chamomile. Growth chamber experiments were conducted comparing weed emergence and growth in bare soil or soil with freshly incorporated buckwheat residue at multiple rates of N fertilization with or without fungicide seed treatment. In the absence of N or fungicide addition, emergence of all weed species was reduced 40 to 70%, and dry weight was reduced 85% in buckwheat residue compared with bare soil. For all three weed species, suppression of growth by buckwheat residue was completely overcome with the addition of N. For shepherd's-purse and corn chamomile (2005 only), suppression of emergence was also overcome with the addition of N. In 2006, treatment of corn chamomile seeds with fungicide resulted in a higher emergence in buckwheat residue than in bare soil. In contrast, suppression of Powell amaranth emergence was not overcome with N fertilization or fungicide treatment. The results suggest that buckwheat-mediated changes in N dynamics account entirely for suppression of weed growth but that the mechanisms responsible for suppression of emergence by buckwheat residue vary by species. Fungal and N effects account for suppression of emergence of corn chamomile and shepherd's-purse, but the mechanism of suppression for Powell amaranth remains obscure.

Currently there are no herbicides registered for direct application to buckwheat for broadleaf weed control. This 4-yr Lithuanian study examined weed control using several rates of clopyralid alone or combined with a single rate of desmedipham. Most applications were applied at the 1-leaf stage of crop growth, however, one rate of clopyralid was applied pre-emergence (PRE) in 2 of the 4 yr. Buckwheat injury was evident within a few days after application (or emergence) with all treatments, but by harvest, no symptoms were evident. In the 2 yr with greatest weed densities, densities were reduced with increasing clopyralid rates alone or with desmediphan. However, weed biomass and density were similar to the nontreated control at harvest. Common lambsquarters, scentless mayweed, wild buckwheat, and narrowleaf hawksbeard were species that were best (> 40% density reduction) controlled with clopyralid. With few exceptions, buckwheat yield in all herbicide-treated plots was similar to the nontreated control. Because yield was not increased with these herbicides, other weed control benefits, such as reduced interference with harvesting equipment or less dockage due to weed seed contamination, must be carefully weighed against the costs of herbicide and application and crop injury that reduced early-season vigor.

Buckwheat is a broadleaved annual species that is often used as a summer cover crop for its quick growth, weed suppressive ability, and ease of management. Tartary buckwheat is a species related to buckwheat, with many of the same traits valued in buckwheat as a cover crop. However, Tartary buckwheat has been reported to grow more vigorously than buckwheat, especially in cool conditions, which might fill a unique niche for vegetable farmers in Wisconsin and other northcentral states. Our research objectives were to determine the effectiveness of Tartary buckwheat relative to buckwheat for weed suppression, both during the cover-cropping phase and after cover-crop termination during cabbage production, and quantify weed suppression, soil compaction, soil nitrogen availability, and cabbage yield in no-tillage (roller-crimped or sickle-bar mowed) and conventional-tillage (rototilled) systems. Across three site-years, we found that buckwheat emerged earlier and produced 64% more shoot dry biomass than Tartary buckwheat. Pretermination weed shoot biomass (predominantly Amaranthus and Setaria spp.) in Tartary buckwheat treatments was approximately twice that of buckwheat, and did not differ from weed shoot biomass in a control fallow treatment. Cabbage yield did not differ between cover crop species nor did yield differ between conventional-tillage cover cropped and control fallow treatments. However, weed biomass was greater, and cabbage yield was reduced, in no-tillage compared to conventional-tillage treatments. We also found evidence of greater soil compaction and less nitrate–nitrogen (NO3–N) availability in no-tillage than conventional-tillage treatments. These results suggest that Tartary buckwheat is not a suitable summer cover crop alternative to buckwheat for weed suppression prior to cabbage production.

Previous studies have demonstrated that emergence and growth of Powell amaranth is inhibited in soils where buckwheat has been grown and incorporated. The primary objectives of this research were to (1) evaluate the possible role of allelopathy in explaining that suppression; (2) distinguish between suppression caused by incorporation of fresh buckwheat residues from suppression caused by changes in soil during buckwheat growth; and (3) quantify the relative importance of buckwheat root vs. shoot tissues in suppression. When all buckwheat plant parts were removed from soil in which buckwheat was grown, Powell amaranth emergence was not suppressed, but growth was reduced 70% compared to bare soil. Addition of buckwheat shoots, but not roots to these soils reduced emergence by 80%, and contributed to additional reduction in growth. Addition of chemically activated carbon did not increase emergence or growth in buckwheat-amended soil. However, thermally activated carbon resulted in greater adsorption of phenolics than chemically activated carbon and alleviated suppression of Powell amaranth in buckwheat-amended, high organic-matter soils. However, suppression was not overcome on mineral soils. In addition to adsorbing phenolics, activated carbon changed the nitrogen (N) content and electrical conductivity of soil extracts. Aqueous shoot extracts of buckwheat stimulated Powell amaranth germination slightly, but inhibited radicle growth. Aqueous soil extracts from buckwheat-amended soil inhibited germination of Powell amaranth compared with extracts from unamended soil. Results suggest that emergence suppression of Powell amaranth by buckwheat residues might be due to allelopathic compounds concentrated in the shoot tissues. However, these inhibitory effects appear to depend on interactions of buckwheat residues with soils. In contrast, suppression of growth of Powell amaranth appears to be associated primarily with lower N availability in buckwheat-grown soils.

Laboratory and field experiments were conducted to assess the allelopathic potential of buckwheat. In the field, buckwheat demonstrated strong inhibitory activity by suppressing weeds. In laboratory studies, aqueous and organic solvent extracts of the aerial parts of common buckwheat inhibited the root and shoot growth of lettuce seedlings. The chloroform and ethyl acetate extracts showed maximum activity, and plants grown in the presence of the ethyl acetate extract showed severe root browning. The allelopathic constituents of the ethyl acetate phase were isolated and identified as gallic acid and (+)-catechin by nuclear magnetic resonance spectroscopy. Gallic acid and (+)-catechin were present in the upper part of buckwheat at concentrations of 0.02 and 0.01%, of fresh weight, respectively. Gallic acid was found to be selectively and strongly inhibitory to root and shoot growth of tested plants at 100 and 10 μg ml−1. (+)-Catechin, however, inhibited plant growth to a lesser extent. These results suggest that buckwheat may have allelopathic potential and that when used as a ground cover crop or green manure may produce inhibitors, which could suppress weeds.

The activity-density of Amara aenea (DeGeer) and Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) was monitored in an experiment that compared five management treatments representing a range of disturbance frequencies, crops, and aboveground biomass production. In 2004 and 2005, three treatments comprised of multiple summer cover crops were compared to bare fallow and soybean, the latter of which used mechanical cultivation to manage weeds. In 2005 weed seed predation was assessed from June to September in two of the treatments (bare fallow and oat–pea/rye–hairy vetch). Beetle activity-density varied with treatment, time of sampling, and year. In 2004 peak activity-density of A. aenea was highest in the mustard/buckwheat/canola, but there was no difference in H. pensylvanicus activity-density. In 2005 activity-density of H. pensylvanicus was higher in oat–pea/rye–hairy vetch than in soybean treatment. Seed predation rates were relatively consistent across treatments, averaging between 38 and 63%. In fallow and oat–pea/rye–hairy vetch, H. pensylvanicus activity-density accounted for 29 and 33% of the variation in seed predation, respectively. Our findings suggest cover crops have a positive effect on the activity-density of A. aenea and H. pensylvanicus and that disturbance negatively influences their activity-density in the absence of cover crops.

The Brassicaceae contain glucosinolates, which hydrolyze to form compounds toxic to plants, fungi, nematodes, and certain insects. Lower weed density and biomass in crops grown following incorporation of brassica cover crops suggest that they may contribute to weed management in agricultural systems. Field experiments were conducted to determine whether incorporated brassica cover crops, including canola, rapeseed, and yellow mustard, reduce subsequent weed and crop establishment; a companion paper describes separate but related field experiments that examined the influence of brassica cover crops on plant growth. Emergence rate and total emergence of sixteen weed and crop bioassay species were measured following brassica cover crops, fallow, or incorporated residues of other short-season cover crops including oat, crimson clover, and buckwheat. The bioassay species, representing a range of seed sizes, were chosen to determine whether larger seed size confers protection from residue-mediated effects on emergence. Averaged over bioassay species, brassica cover crops reduced emergence by 23 to 34% compared with fallow; emergence following brassicas was delayed by approximately 2 d. The effects of the incorporated brassica residues were similar to those of the other short-season cover crops, which reduced emergence of the bioassay species by 19 to 39% and delayed emergence by 2 d. Seed size was a poor predictor of a species' establishment. These results suggest that brassica residues are capable of delaying seedling emergence and reducing establishment, although the magnitude of their effects were comparable to other widely available cover crops.