Responses of rare and common segetal species to wheat competition and fertiliser type and dose

Fertiliser application is considered crucial for determining the crop–weed competition relationship by stimulating plant growth and modulating the competition for below‐ground (soil nutrients) and above‐ground resources (e.g. light and space). Segetal species, those preferentially inhabiting cereal fields, have suffered a strong decline, owing to agricultural intensification. Under continued pressure, particularly the rarest species may disappear. In this study, we aimed to assess the competitive ability of common and rare segetal species when growing with wheat and how this competitive relationship is modulated by the type (organic and mineral) and dose (low or high) of fertilisation in a pot experiment. The segetal species grew better at higher doses of fertilisers when growing alone. However, when growing with wheat, they were outcompeted by the cereal plants. Overall, flowering was not affected by the competition with wheat under the different fertiliser regimes. Only the onset of flowering of some segetal weeds tended to be delayed by crop competition. Some rare species were more affected by the competition with wheat, indicating that they may be disadvantaged in a community context. However, not all rare segetal species tested responded in the same way, which highlights that competitiveness alone is a poor predictor of the rarity of segetal species.     

Effects of weed harrowing and undersown clover on weed growth and spring cereal yield

Weed competition and nutrient scarcity often restrict organic cereal production, especially where the availability of livestock manure is limited. While harrowing of annual weeds and legume cover crops can be used, these methods are both executed in early spring and may hinder each other. Two cycles of a 2‐year crop rotation were carried out in south‐east Norway (60°42′N, 10°51′E, altitude 250 m) with weed harrowing and undersown cover crops (WHCC) at two fertiliser rates (40 and 100 kg nitrogen ha^?1). The effect of the WHCC treatments was measured by weed density and species, weed biomass, changes in weed seedbank and grain yield. The weed density depended on the interaction between WHCC, fertiliser and year. On average, pre‐emergence weed harrowing reduced weed density by 32% and weed biomass by 49%, while pre‐ and post‐emergence weed harrowing reduced weed density by 59% and weed biomass by 67% compared with the untreated control. Spergula arvensis became more abundant at low rather than at high fertiliser rates. On average, white clover cover crop sown after pre‐emergence weed harrowing resulted in the highest yields for both oat (+12.1%) and wheat (+16.4%) compared with the untreated control. Despite differences in weed population density and biomass among WHCC treatments within years, the weed biomass, weed density and seedbank increased for all WHCC treatments over the 4‐year period. More research is required into improving the efficacy of mechanical and cultural weed suppression methods that organic systems rely on.     

Avena sterilis and Lolium rigidum infestations hamper the recovery of diverse arable weed communities

Intensification of agricultural practices has severely reduced weed diversity in arable fields, which affects the delivery of ecosystem services. However, in parallel, some species have benefited from intensive farming and have vastly increased their abundance, as is the case for Lolium rigidum and Avena sterilis in cereal fields. These highly competitive species severely reduce yields but can also compete with other weed species, and, when less intensive practices are applied, they might limit the recovery of weed diversity and the success of arable species reintroductions. A gradient of infestation was established in a winter wheat field in Catalonia (north‐eastern Spain) by sowing seeds of both species at three different densities to test their effects on the abundance, diversity and composition of the natural weed community. The emergence of seeds and the survival and biomass of transplanted seedlings of two rare species, Agrostemma githago and Vaccaria hispanica, were also evaluated. Avena sterilis and L. rigidum infestations reduced the diversity, abundance and biomass and changed the composition of the natural weed community, even at low infestation densities. Moreover, infestations of both species affected the overall performance of A. githago and V. hispanica. This study reveals that A. sterilis and L. rigidum are highly competitive and that their infestations might hamper the recovery of diverse weed communities. Their densities should be considered when selecting suitable sites for promoting diversity and reintroducing rare species.     

Fertilisation and weed control effects on yield and weeds in durum wheat grown under rain-fed conditions in a Mediterranean climate

Summary A field study was undertaken to examine the effect of fertiliser type and weed control system on grain yield, yield components and weed population in durum wheat ( Triticum turgidum. var. durum) grown in a 4-year rotation [bare fallow–barley ( Hordeum vulgare )–vetch ( Vicia sativa )–wheat]. Fertilisation treatments were: no fertilisation, organic fertilisation with 2500 kg ha⁻¹ of compost (sheep manure and cereal straw) and chemical fertilisation with 100–60–60 kg ha⁻¹ of NPK. Weed-control treatments were: no control, herbicide, harrowing with long-flex spring tines and strip sowing with inter-row hoeing. Chemical fertilisation increased grain yield with respect to the other treatments, amongst which no significant differences were noted. Weed-control systems afforded no improvement in yield compared with controls; indeed, yields obtained using inter-row hoeing were lower. The weed population consisted of a large variety of species, of which the most important were Convolvulus arvensis and Polygonum aviculare . Application of weed-control systems reduced weed density. Herbicide was the most effective, reducing weed biomass by 80%, whilst harrowing and hoeing reduced weed biomass by 40% and 52% respectively. Inter-row hoeing does not appear to be a viable alternative to herbicides, when used as the sole weed control method in a non-diverse cropping system in Mediterranean climates. The lack of response to compost suggests a need for further long-term research.     

The effects of combining modified sowing methods with herbicide mixtures on weed interference in wheat crops

In India, wheat is prone to being heavily infested with weeds, which cause considerable yield losses. Agronomic practices, including sowing method, influence weed interference in wheat growth. A herbicide mixture is usually more effective than a single herbicide for controlling a broad-spectrum of weeds. It may prevent weed species shift and delay resistance development in weeds. This study attempted to develop an integrated approach, combining a modified sowing method with applying a herbicide mixture for efficient weed management in wheat. We found that flat bed missing-row sowing resulted in significant reductions in: (i) the three-year mean population densities of grassy, broad-leaved and total weeds at 40 days after sowing (d.a.s), (ii) total weed population density and biomass (dry weight) at 60 d.a.s., compared to a mainly, furrow-irrigated raised bed system. Applying a tank-mix of fenoxaprop-p-ethyl at 0.08 kg/ha and isoproturon at 0.4 kg/ha applied 30 d.a.s. resulted in significant reductions in total weed population density and biomass, leading to a significant improvement in wheat growth and yield compared to either a weedy check or a regime of fenoxaprop-p-ethyl applied at 0.10 kg/ha. However, a combination of the flat bed missing-row sowing and tank-mix application of fenoxaprop-p-ethyl 0.08 kg/ha and isoproturon 0.4 kg/ha 30 d.a.s. resulted in higher wheat yield through better management of weeds compared with other sowing/ weed control combinations.     

Investigation of threatened arable weeds and their conservation status in Turkey

Agronomic operations carried out in arable lands may cause changes in weed populations and also decrease the number of animal species that depend on these weed species. An emerging paradigm is the conservation and sustainability of weed species. In this study, risk status of arable weeds in Turkey was evaluated and it was determined that 112 of them (76 endemic and 36 non-endemic species) were at risk according to the IUCN Red List. Turkey is extremely diverse, with over 3000 endemic plant species out of nearly 12 000 recorded. Considering endemic weeds, they are classified in the following risk categories: critically endangered (four weed species), endangered (EN) (six), vulnerable (VU) (14), near threatened (seven), least concern (41) and data deficient (DD) (four species). Among the non-endemic weeds, three species are EN, 31 VU and two are DD. The families with the highest threatened weed species are Scrophulariaceae, Fabaceae, Asteraceae, Brassicaceae and Lamiaceae. The identification of weed species in arable habitats and their risk status is a useful tool for assessing and monitoring how the sustainability of weed populations is affected by farming practices. The results show the necessity of adopting new environment-friendly agricultural methods to conserve the high number of endemic weed species under threat in Turkey.     

The effect of sowing date, stale seedbed, row width and mechanical weed control on weeds and yields of organic winter wheat

Three field experiments were carried out in organically grown winter wheat in Denmark. The treatments were sowing time (normal or late sowing) and false seedbed, row width (12 and 24 cm) and weed control method [untreated; mechanical weed control (weed harrowing at 12 cm supplemented with inter‐row hoeing at 24 cm); and herbicide weed control]. Weed biomass in midsummer was greatest on plots sown at the normal sowing time (compared with delayed sowing) and was reduced by mechanical or chemical weed control (compared with untreated plots). Row width alone had no influence on weed biomass, but in the experiment with high weed pressure, the more intensive mechanical weed control used at a row width of 24 cm reduced weed biomass. Normal sowing time tended to give higher yields, but this was only statistically significant in one of the three experiments. Wide rows gave a yield decrease in the experiment with low weed pressure. The effect of weed control on yield was dependent on the weed pressure. At low weed pressure, mechanical weed control caused a yield decrease compared with untreated or herbicide treated. At intermediate weed levels there were no differences, whereas at high weed pressure, mechanical weed control and herbicide treatment caused a yield increase compared with untreated. False seedbeds were shown to contribute to a decrease in the soil seed reserve.     

Increased density and spatial uniformity increase weed suppression by spring wheat

It has been hypothesized that increased crop density and spatial uniformity can increase weed suppression and thereby play a role in weed management. Field experiments were performed over 2 years to investigate the effects of the density and spatial arrangement of spring wheat (Triticum aestivum) on weed biomass and wheat yield in weed-infested fields. We used three crop spatial patterns (normal rows, random and uniform) and three densities (204, 449 and 721 seeds m⁻²), plus a fourth density (1000 seeds m⁻²) in the random pattern. Increased crop density reduced weed biomass in all three patterns. Weed biomass was lower and crop biomass higher in wheat sown in the random and uniform patterns than in normal rows in both years. At 449 seeds m⁻², weed biomass was 38% lower in the uniform and 27% lower in the random pattern than in rows. There was evidence of decreasing grain yield due to intraspecific competition only at 1000 seeds m⁻². The results not only confirm that increasing density and increasing crop spatial uniformity increase the suppression of weeds, but also suggest that a very high degree of spatial uniformity may not be necessary to achieve a major increase in weed suppression by cereal crops. Rows represent a very high degree of spatial aggregation. Decreasing this aggregation increased weed suppression almost as much as sowing the crop in a highly uniform spatial pattern. While the random pattern produced as much crop biomass and suppressed weeds almost as well as the uniform pattern, the uniform pattern gave the highest yield.     

The impact of landscape structure and sown grass margin strips on weed assemblages in arable crops and their boundaries

A paired-field study was made in southern England in arable fields, each with or without sown 6-m wide grass margin strips. Seven field pairs were located in each of small, intermediate and open landscapes, which were based on mean field size. Ground cover, plant species diversity and assemblages were assessed in crop centres, crop edges and non-crop field boundaries. The data were used to test for effects of sown grass margins, differences in field location, impacts on rare weed species and for landscape effects on weed assemblages. Significantly higher plant species diversity was found in boundaries protected by buffer strips. Annual weeds associated with field edges, notably Anisantha sterilis , were found at lower cover where perennial grass strips were present. Sown grass strips enhanced boundary plant diversity, particularly by increasing polycarpic species. Margin strips had a small influence on the weed flora of the crop edge, possibly reducing weed cover, but had no influence on floras of field centres. Field size and landscape context did not affect weed assemblages, which show marked field-to-field variability, though crop type was an important influence. Grass margins did not enhance rare arable weed species and may be a threat to them, if margins are sited where such species are known to occur in the seedbank. With this exception, grass strips are a positive influence on boundary flora diversity and reduce margin weeds in arable landscapes.     

Integrated management of Bromus diandrus in dryland cereal fields under no‐till

As herbicides have limited effect in controlling Bromus diandrus in no‐till dryland cereal fields, the integration of chemical and cultural methods needs to be investigated. A field study was carried out in Lleida (Spain) during 2008–09, 2009–10 and 2010–11 seasons, in a no‐till winter cereal field integrating delayed crop sowing with herbicides in a barley–wheat–wheat rotation. Three crop sowing dates were considered: D1, mid‐October; D2, mid‐November; and D3, early December, and the herbicides mesosulfuron‐methyl plus iodosulfuron‐methyl‐sodium were applied in wheat. Weed density, cumulative emergence and fecundity were estimated for each sowing date. In all three seasons, a significant reduction in the cumulative emergence of B. diandrus as compared to D1 was observed in D2 (82.0, 97.5 and 98.1%) and D3 (80.8, 98.7 and 97.2%). In addition, a significant decrease in weed density and seed rain was observed across all sowing dates and seasons. The herbicide used in wheat was more effective under delayed sowing, due to lower weed density and presence of less developed weed seedlings. After three seasons, the populations of B. diandrus were completely depleted in D2 and D3. This study demonstrates the possibility of eliminating brome infestations in dryland cereal fields in no‐till systems through the integration of cultural and chemical strategies.     

Introducing an abundance index for assessing weed flora in survey studies

This study was carried out to propose a simple abundance index (AI) of weed species, to describe the ranking of weeds in weed communities, and to evaluate weed flora. Based on a 4 year weed survey program conducted in Tehran province, the Thomas methodology for determining the relative abundance (RA) in weed communities was applied and a new, improved method (called the AI method) was introduced. Sixty-one irrigated wheat fields were selected using a stratified sampling procedure. The boundaries of each county in the province formed the strata. For the Thomas method, the data were summarized for each county using five quantitative measures for determining the RA of each species. For the AI method, three quantitative measures were used, including frequency, uniformity, and mean field density. In the Thomas method, the mean field density had a significant role in determining the RA of a species, while the frequency and uniformity had a less significant role. However, for determining the AI, the frequency and uniformity had a higher value than the mean field density. Higher values for these two parameters for a particular species indicates that this species was more compatible with the soil and climate conditions in which it was growing, while higher values for the mean field density for a particular species indicates that the species had a higher competitive or reproductive ability than other species. In the Thomas method, the weed species with a low mean density were less abundant and not a serious weed for agroecosysytems but, in the AI method, an invasive weed could be detected before the colonization stage.     

Influence of sowing density and spatial pattern of spring wheat (Triticum aestivum) on the suppression of different weed species

To better understand the potential for improving weed management in cereal crops with increased crop density and spatial uniformity, we conducted field experiments over two years with spring wheat ( Triticum aestivum ) and four weed species: lambsquarters ( Chenopodium album ) , Italian ryegrass ( Lolium multiflorum ), white mustard ( Sinapis alba ), and chickweed ( Stellaria media ). The crops were sown at three densities (204, 449, and 721 seeds m⁻²) and in two spatial patterns (normal rows and a highly uniform pattern), and the weeds were sown in a random pattern at a high density. In most cases, the sown weeds dominated the weed community but, in other cases, naturally occurring weeds were also important. There were strong and significant effects regarding the weed species sown, the crop density, and the spatial distribution on the weed biomass in both years. The weed biomass decreased with increased crop density in 29 out of 30 cases. On average, the weed biomass was lower and the grain yield was higher in the uniform compared to the row pattern in both 2001 and 2002. Despite the differences in weed biomass, the responses of L. multiflorum , S. media , and C. album populations to crop density and spatial uniformity were very similar, as were their effects on the grain yield. Sinapis alba was by far the strongest competitor and it responded somewhat differently. Our results suggest that a combination of increased crop density and a more uniform spatial pattern can contribute to a reduction in weed biomass and yield loss, but the effects are smaller if the weeds are taller than the crop when crop–weed competition becomes intense.     

Weed community characteristics and crop performance: a neighbourhood approach

The presence of weeds in crop fields often causes yield reductions. However, the effects of weed diversity have not been fully examined. This study tested the hypothesis that, holding density constant, increased weed species richness would decrease the effects of competition on spring wheat target plant performance. Measurements were taken from circular neighbourhoods (16.5 cm radius) with a single spring wheat plant surrounded by combinations of Setaria viridis , Avena fatua , Kochia scoparia , Thlaspi arvense and spring wheat plants, representing all combinations of neighbour species at four density levels. Using regression models, we found that species richness had no significant direct effect on spring wheat biomass, yield, or relative growth rate and that there were no significant neighbour species interaction terms. For weedy target plants, the presence of negative interaction terms suggested that increasing species richness had negative effects on growth of individual weed species. Additional analysis suggested that increased species richness may limit competitive ability of dominant weeds. Although we found no evidence of a direct effect of weed species richness on crop performance, increased weed species richness had no negative effect on spring wheat performance. Further, species richness of the weed community appears to influence weedy plant performance, which may offer a future opportunity to influence crop performance.     

Response of weed flora to combinations of reduced tillage, biocide application and fertilization practices in a 3-year crop rotation

Weed flora in a crop rotation of soybean, sugar beet, and spring wheat were studied under combinations of conventional and reduced tillage, biocide application, and chemical fertilization to investigate whether any combination of conservational practices offers the potential to increase weed community diversity while maintaining weed biomass at an acceptable level. Weed density increased under reduced biocide application. Weed density and size increased under manure compost application (with reduced chemical fertilization) because of weed seed introduction. Weed emergence from seeds dispersed in the previous year was greater under reduced tillage. Two-year individuals increased under a combination of reduced tillage, reduced biocide application and manure compost application. These increases in weed population density and size under conservational practices were yet not consistent; rather, they showed an annually fluctuating trend. In terms of weed diversity, species richness and Shannon's diversity index were higher under manure application because of the introduction of new species and probably as a result of suppression of weed growth and reproduction, which can reduce the chance of dominance by certain species. When combined with manure application, reduced tillage may offer the potential for increasing diversity by reducing the competitiveness of dominant species and facilitating establishment of new or minor species.     

Predicting the risk of weed infestation in winter oilseed rape crops

Chemical weed control before crop and weed emergence is a systematic practice in winter oilseed rape crops in France. It would be profitable both for farmers and the environment to predict the level of weed infestation early on in the growing season and to control weeds only when necessary using post‐emergence weed control. The objective of this paper was to develop and evaluate simple models to predict weed biomass in oilseed rape crops. The model input variables were related to weed population characteristics and farmers’ practices. The models can be used to classify oilseed rape plots into two categories: plots with a level of weed infestation above a threshold or those with level of weed infestation below a threshold. A data set including 3 years of experiments, conducted across several regions in France, was used to estimate the parameters and to evaluate the models. High values of sensitivity and specificity were obtained when weed biomass was predicted as a function of sowing date, type of soil tillage, soil mineral nitrogen, crop density, weed density at emergence, and main characteristics of the most abundant weed species. Model performance strongly decreased when input variables related to the weed population were not taken into account. The best models correctly classified 90% of the plots with high weed infestation and 64% of the plots with low weed infestation.     

Increasing weed flora in Danish arable fields and its importance for biodiversity

A number of political initiatives have been implemented to meet growing concerns about negative side effects of the intensification of land‐use practice over the past 25 years. Declining species diversity and abundance and pollution of the environment have been reported worldwide. In order to assess the overall effect on wild flora of the changing management of arable land, national surveys are necessary. Surveys of the weed flora in Danish arable fields were conducted in 1987–89 and 2001–04. The frequency of common weed species in spring barley, spring rape, winter rye, winter wheat and grass leys was compared between time periods. Weed frequency increased overall during the interval. Species that are of particular importance as food sources for invertebrates and birds increased considerably in some of the crops. The frequency of some grass species increased remarkably, possibly because of increased areas of winter crops. Some perennial weeds also became more frequent, probably caused by less mechanical weed control and a reduction in the use of specific herbicides. The dominant weed species in 2001–04, with frequency higher than 10%, were largely the same as in 1987–89, but included three new species in 2001–04. Only a few species occurred less frequently in 2001–04 than in 1987–89 and only in some crops. As common weed species make up an important food reservoir for wildlife, new trends in the management of arable land in Denmark seem to have led to more biodiversity.     

Effects of density and sowing pattern on weed suppression and grain yield in three varieties of maize under high weed pressure

We tested the hypothesis that improved weed suppression by maize can be achieved through increased crop density and spatial uniformity. Field experiments on three varieties of maize sown at three densities (5, 7 and 10.5 seeds m^?2) and in two spatial patterns (grid pattern and rows) under very high weed pressure from Brachiaria brizantha were performed in 2012 and 2013. We measured weed biomass 1 month after sowing and at harvest, and grain yield at harvest. Density, variety and sowing pattern all had strong and significant effects on both weed biomass and yield. On average, weed biomass was reduced (by 72% in the first year and 58% in the second year), and grain yield was increased (by 48% and 44%) at the highest density in the grid pattern compared with standard sowing practices (medium density, row pattern). There was a significant density × variety interaction, which is evidence for genetic differences in the response of the varieties to density in characteristics that influence weed suppression. The variety that suppressed weeds best at high density had the lowest variation in the angle of insertion of the oldest living leaf at harvest (leaf 6), supporting the hypothesis that reduced phenotypic plasticity may be advantageous for weed suppression under high density and spatial uniformity. Increased density and uniformity can contribute to weed management in maize in many cases, potentially reducing the need for herbicides or mechanical weed control.     

Competition between weeds and spring wheat for 15N-labelled nitrogen applied in pig slurry

The changes in weed:crop competition as affected by the interaction of slurry distribution pattern with crop sowing pattern was examined, using the parameters dry matter (DM) production, nitrogen (N) uptake and recovery of applied N in spring wheat and weeds. Pig slurry enriched with isotopic nitrogen (¹⁵N) was applied either fully incorporated or in concentrated bands. The two slurry distribution patterns were combined with either crop density or distance between the slurry band and seed row. Spring wheat and weeds were sampled separately seven times during the season. A high and fast crop recovery of applied N coincident with a low recovery in weeds may be obtained by at least three management elements: sowing the crop with a high seed density, sowing the crop row close to the band-applied slurry N, and early sowing. The effects on crop DM, N-uptake and ¹⁵N recovery of a high crop density and a narrow distance between the slurry band and crop row appear clearly at the beginning of the spring wheat elongation phase. For weeds the effects were most pronounced for ¹⁵N recovery, and using DM as the only response variable may cause insignificant results. However, methods for N application may be part of a strategy controlling weeds, but to obtain satisfactory weed control the N management needs to be supplemented with chemical or mechanical weed management.     

Yield and competitive ability in potato cultivars characterised by different developmental timing

Potato is very susceptible to weed interference during the early growth stages due to slow emergence, and again at the end of the growing cycle when branches collapse and the canopy opens. Weed control usually is performed through a combination of physical and chemical methods. A growing concern for the environment and human health has encouraged the development of non‐chemical weed control. We evaluated the effects of an integrated weed management strategy consisting of physical and cultural methods on naturally emerging weeds over two field seasons in central Italy. One harrowing plus one hilling operation were conducted during the early crop stages, and the competitive abilities against late emerging weeds of six different cultivars of potato, characterised by differences in developmental timing and growth habit, were evaluated. Two measures of competition were evaluated, the competitive balance index (Cb) and the relative total biomass of crop and weed. Higher competitive ability (Cb) was associated with traits such as fast early above‐ground biomass production, height and final above‐ground biomass. Medium late maturity cultivars showed higher Cb than earlier ones, but this was associated with lower yield, providing evidence for a trade‐off between competitive ability and yield. The trade‐off was in part biased by the lack of adaptation of the medium late cultivars to hot weather conditions, so we concluded that cultivars characterised by different developmental time need to be screened and tested for local systems.     

Effects of long‐term fertilization on the weed growth and community composition in a double‐rice ecosystem during the fallow period

The vegetation cover during the non‐cropping season could have important implications for the maintenance and recovery of soil fertility, as well as for biodiversity conservation in croplands. In this study, five fertilization regimes (control: non‐fertilization; N: inorganic N fertilization; P: inorganic P fertilization; NPK: balanced fertilization with inorganic N, P and K; NPKM: balanced NPK plus farmyard manure) were conducted from 1981 in a double‐rice (Oryza sativa L.)‐cropping system in subtropical China. The effects of long‐term fertilization were investigated on the weed growth, diversity and community structure during the fallow period. The results showed that, relative to the control, both inorganic fertilization alone (N, P and NPK) and NPKM in the rice‐growing season significantly increased the weed density and biomass during the fallow period in the paddy field. There was no significant difference in the weed species richness (the number of species) among the treatments. Compared with the control, fertilization tended to reduce the weed diversity (Shannon's H′) and evenness (Shannon's E), especially in the N treatment. Long‐term fertilization resulted in a significant shift in weed community's composition during the fallow period. The weed community's structure was affected by soil nutrients in the order P > N > K.