The relevance of morphological plasticity in the simulation of competition between maize and Datura stramonium

The INTERCOM model was calibrated for Datura stramonium and maize with data from monoculture plots, and used to simulate the competition between the weed and the crop. The model adequately simulated the growth (leaf area index, above-ground biomass, plant height, yield) of both species in monoculture. In competition, the growth of D. stramonium and the competitive effect of D. stramonium on the growth of maize were clearly underestimated. Simulation of competition was improved when the observed morphological adaptations of D. stramonium under competitive light stress, as reflected in dry-matter partitioning, height growth and specific leaf area (SLA) were considered. Model analysis demonstrated that the observed changes in SLA and height growth were more relevant for the increased competitive ability of D. stramonium . The modification of the model in order to simulate the effect of shading by an overtopping species in the SLA of the shaded species improved model simulation of competition.     

Effects of nitrogen supply on competition between wheat and three annual weed species

Three pot experiments have investigated the effects of nitrogen (N) supply on interspecific competition between three weed species ( Phalaris minor Retz., Chenopodium album L, and Sinapis arvensis L.) and spring wheat ( Triticum aestivum L.) cv, Alexandria. The treatments tested included monocultures of each species and a mix-ture containing them in equal proportions that were combined factorially with two levels of N supply (20, 120 kg N ha^-1). Low N supply decreased net photosynthetic rate (P_n), leaf N percentage, plant dry weight and N uptake of both wheat and weed species and gram dry weight of wheat. The effects of low N on P_n and dry weight of weeds were greater than the effects on wheat. In most cases the decrease in P_n at low N was due to non-stomatal factors. The relative competitive abilities of wheat and weeds were influenced by N supply. At high N, S.arvensis was more competitive than wheat, whereas P. minor was less competitive than wheat. C. album was more competitive than wheat at both N levels. The rank order of competitive ability of the weed species was C. album > P. minor > S. arvensis . The effects of interspecific competition on P_n were smaller than the effects of N supply and were not associated with corresponding effects on leaf N percentage and plant dry weight of both wheat and weed species and grain dry weight of wheat.     

Modelling the effects of weeds on crop production

In most quantitative studies on interplant competition, static regression models are used to describe experimental data. However, the generality of these models is limited. More mechanistic models for interplant competition, which simulate growth and production of species in mixtures on the basis of the underlying physiological processes, have been developed in the past decade. Recently, simulation models for competition between species for light and water were improved and a detailed version was developed for sugarbeet and fat hen (Chenopodium album L.). The model was validated with data sets of five field experiments, in which the effect of fat hen on sugarbeet production was analysed. About 98% of the variation in yield loss between the experiments (which ranged from –6 to 96%) could be explained with the model. Further analysis with the model showed that the period between crop and weed emergence was the main factor causing differences in yield loss between the experiments. Sensitivity analysis showed a strong interaction between the effect of the variables weed density and the period between crop and weed emergence on yield reduction. Different quantitative approaches to crop-weed competition are discussed in view of their practical applicability. Simulations of experiments, where both the weed density and the period between crop and weed emergence were varied over a wide range, showed a close relation between relative leaf cover of the weeds shortly after crop emergence and yield loss. This relation indicates that relative leaf cover of the weeds accounts for both the effect of weed density and the period between crop and weed emergence. This relation has the potential to be developed into a powerful tool for weed-control advisory systems.     

The relative importance of root and shoot competition between water-seeded rice and Echinochloa phyllopogon

The relationship between root and shoot growth were determined for water-seeded rice and Echinochloa phyllopogon in greenhouse experiments. When grown in monoculture at four nitrogen rates (0, 60, 120, 180 kg N ha⁻¹), root dry weight was highly correlated with canopy structure for both species. Echinochloa phyllopogon showed a significantly stronger response to the nitrogen rate than rice. When rice and E. phyllopogon were grown in competition, with roots either separated or allowed to mingle, root competition contributed more than shoot competition to reductions in the growth of the target species. The results suggest that root competition may be the primary mechanism determining competitive outcomes between water-seeded rice and E. phyllopogon . The importance of root competition and the relationship between root and shoot growth demonstrated in this study suggest that researchers should not rely solely on correlations between shoot traits and competitive ability as evidence that competition is primarily for light. Our results also suggest the importance of considering the whole plant when assessing rice cultivars for competitive ability.     

Ecophysiological characteristics of four maize hybrids and Abutilon theophrasti

Combining empirical research with simulation modelling may improve our understanding of the dynamics of crop:weed competition and for testing hypotheses on the importance of specific traits for enhancing crop performance in mixtures. Two field experiments were conducted to quantify and compare estimates of traits important for radiation interception and utilization in four maize hybrids and Abutilon theophrasti grown in monoculture. Early leaf area growth rate did not vary among maize hybrids within a year, but varied among years. The response of CO₂ assimilation rate to absorbed radiation and leaf nitrogen content did not differ among hybrids. Abutilon theophrasti and two old maize hybrids partitioned more new biomass to stem relative to reproductive organs than newer hybrids. Old hybrids had greater specific leaf area during the period of most rapid growth, grew taller, and leaf area was distributed higher in their canopy. Extinction coefficients for diffuse radiation did not differ among hybrids or between years. Results suggest that these four maize hybrids may differ in their ability to intercept incident radiation, which may influence their ability to compete for light.     

Annual weed competition in wheat crops: the effect of weed density and applied nitrogen

Experiments investigating the effect of weed density on the yjeld of a wheat crop at three levels of applied nitrogen were conducted in north-western Victoria. Australia, during 1970. There were five sites, each infested with a pure stand of one of the following annual broad-leaved weed species: Lithospermum arvense, Brassica tournifortii. Lamium amplt'xicaule, Amsinckia hispida and Fumaria parviflora. At the three-leaf stage of crop growth, the weed populations were systematically thinned with a specially developed spray boon) (which is described) to give a range of weed densities in competition) with the crop. The relalionship between dry matter production and population density for all but one weed species was curvilinear, but the degree of curvature was small and competition in the wheat crop was linear for four of the five weed species. There were large differences in the competitive ability of individual weed species and these have been described by regression equations. Applied nitrogen increased wheat yields at all sites but weed competition was not affected. The use of these grain yield-weed density relationships in predicting crop losses from weed competition is discussed.     

The influence of nitrogen supply on the ability of wheat and potato to suppress Stellaria media growth and reproduction

Summary This paper tests the hypothesis that increased soil nitrogen supply reduces the growth of late-emerging weeds in wheat and potato by enhancing canopy leaf area development and thereby reducing the availability of light for weed growth. Two series of experiments were conducted: one in spring wheat (1997, 1999) with sown Stellaria media and one in potato (1998, 1999) with naturally emerged weeds, including S. media . For each crop, two cultivars were grown at three levels of nitrogen supply. In wheat, as in a monoculture of S. media , total dry weight and seed number of the weed increased with soil nitrogen supply, whereas in potato the opposite was found. Increased soil nitrogen supply increased the nitrogen uptake of S. media in wheat, despite the reduced light availability, indicating that S. media in wheat was limited by nitrogen. In potato, on the other hand, growth of S. media was limited by light availability, which decreased with increased soil nitrogen supply . We conclude that the differences in response of S. media in wheat and potato to additional nitrogen supply are attributable to the dual influence of soil nitrogen supply on light and nitrogen availability, which are mediated by the crops.     

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.     

Evaluation of selected commercial oilseed rape cultivars for early vigour,weed suppression and yield in southern New South Wales

The potential of oilseed rape to suppress weed growth while maintaining optimal yield and quality is not well understood under field conditions in Australia. This study, conducted in Condobolin and Wagga Wagga, New South Wales (NSW), during 2015 and 2016, examined a diverse range of commercial oilseed rape cultivars for their inherent ability to suppress weeds and maintain yields when in competition with natural weed infestations, with and without pre-emergent herbicide treatment. Cultivar differences were observed in oilseed rape canopy architecture and yield; however, early-season biomass, light interception, leaf area index and visual vigour ratings exhibited both year and location interactions. Cultivars with the highest biomass, light interception, leaf area indices and visual vigour were typically also the most weed-suppressive, in particular GT-50 and Hyola 600RR. Although crop and weed biomass accumulation differed significantly among cultivars for both location and year, weed biomass was inversely related to cultivar biomass in both years and locations. Hybrid Hyola and GT-50 cultivars exhibited up to 50% less weed biomass while maintaining consistently high levels of dry crop biomass. In addition, pre-emergent herbicide applications reduced weed infestation and contributed to higher crop yield in both locations and years. Given the consistent aboveground competitive ability of certain oilseed rape cultivars, our study demonstrated that diverse cultivar-dependent competitive traits such as early growth vigour, biomass production, absorption of photosynthetically active radiation and production and retention of crop residue significantly impacted weed establishment and total weed biomass. Our findings suggest that cultivar selection offers potential as a tool for maintaining suitable grain yield in the presence of weeds while potentially delaying the development of herbicide resistance through efficacious weed suppression.     

Common ragweed (Ambrosia artemisiifolia) dry matter allocation and partitioning under different nitrogen and density levels

Dry matter partitioning and allocation is a major determinant of plant growth and its competitiveness. Common ragweed (Ambrosia artemisiifolia) is a troublesome agronomic weed species and it is also a major health concern in Europe and many other countries because of its rapid spreading and production of allergenic pollen. A field experiment was conducted in 2009 to determine the effect of nitrogen fertilizer levels (0, 100, and 200 kg ha^?1) and plant density levels (1.3, 6.6, and 13.2 plants per m²) on the leaf, stem dry matter partitioning, and dry matter allocation of ragweed. With an increasing density, the stem partitioning coefficient increased, whereas the leaf partitioning coefficient decreased. The addition of nitrogen had a limited effect on the leaf and stem dry matter partitioning. The root dry matter production decreased with an increasing density and was not influenced by the addition of nitrogen. Under intraspecific competition, ragweed exhibited a stronger above‐ground competition intensity than below the ground, which resulted in a greater root : shoot ratio, compared to the low‐density stands. The level of nitrogen influenced the vertical leaf, stem, and total dry matter distribution, with a greater allocation to the top stratum of the plants, thus increasing their competition for light. The biomass allocation of ragweed to the roots in response to the nitrogen supply exhibited a low plasticity, compared to the shoots, which displayed a high plasticity. The results of this study suggested that, under intraspecific conditions and with an increasing nitrogen supply, ragweed would be more competitive in above‐ground resource acquisition.     

Improved management of Avena ludoviciana and Phalaris paradoxa with more densely sown wheat and less herbicide

Summary The effectiveness of crop competition for better weed control and reducing herbicide rates was determined for Avena ludoviciana and Phalaris paradoxa . Four experiments, previously broadcast with seeds of the two weeds in separate plots, were sown with three wheat densities, and emerged weeds were treated with four herbicide doses (0–100% of recommended rate). The measured crop and weed traits were first analysed across experiments for treatment effects. Grain yield and weed seed production data were then analysed using cubic smoothing splines to model the response surfaces. Although herbicide rate for both weeds and crop density for P. paradoxa had significant linear effects on yield, there was a significant non-linearity of the response surface. Similarly, herbicide rate and crop density had significant linear effects on weed seed production, and there was significant non-linearity of the response surface that differed for the weed species. Maximum crop yield and reduction in seed production of P. paradoxa was achieved with approximately 80 wheat plants m⁻² and weeds treated with 100% herbicide rate. For A. ludoviciana , this was 130 wheat plants m⁻² applied with 75% herbicide rate. Alternatively, these benefits were achieved by increasing crop density to 150 plants m⁻² applied with 50% herbicide rate. At high crop density, application of the 100% herbicide rate tended to reduce yield, particularly with the A. ludoviciana herbicide, and this impacted adversely on the suppression of weed seed production. Thus, more competitive wheat crops have the potential for improving weed control and reducing herbicide rates.     

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.     

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.     

Influence de la densité et de la période de concurrence de Solanum nigrum L. sur la tomate de semis direct, en relation avec le désherbage

Effects of density and period of competition by Solanum nigrum L. on direct seeded tomatoes in relation to weed control The effects of density and period of competition from Solanum nigrum L. were measured in direct seeded tomatoes given weed control treatments currently used in south-east France. S. nigrum emerging after a diquat treatment at the 2–3 leaf stage of the crop and thinned to low densities (<12.8 plants ha^?1) at the 5–6 leaf stage of the crop caused significant yield loss if left to compete with the crop until harvest. Yield reduction was smaller if the same weed densities were present only until the onset of flowering. The regression curves of yield on weed density differed as annual climatic variations affected sowing date and plant growth; a comparison between years was made using the relation ‘crop yield × weed biomass/crop biomass’. Significant interactions between weed density and period of competition were found with yield of both green and red fruit. For late sown crops with low densities of S. nigrum two weed control treatments at the 5–6 leaf stage and at the onset of flowering were sufficient to prevent yield loss.     

Evaluation of three empirical models depicting Ambrosia artemisiifolia competition in white bean

The performance of three empirical models describing white bean yield loss (Y_L) from common ragweed competition was compared using field experiments from Staffa and Woodstock, both in Ontario, Canada, in 1991 and 1992. One model was based upon both weed density and relative time of emergence. The other two models described yield loss as a function of weed leaf area relative to the crop. The model based on both weed density and relative time of emergence best described the data sets. The predicted maximum yield loss (A) and the parameter for relative time of weed emergence (C) varied across locations and years whereas the yield loss at low weed density (I) was relatively more consistent across locations and years. Use of thermal time (base temperature=10^oC) rather than calendar days did not change the overall fit of the model, but reduced the value of the parameter for the relative time of weed emergence (C). The two parameter leaf area model accounting for maximum yield loss (m) gave a better fit to the data compared with the one parameter model. The relative damage coefficient (q) varied with time of leaf area assessment, location and year. Values of q calculated from relative leaf area growth rates of the crop and weed were similar to observed values. The relationship between q and accumulated thermal time was linear but varied with location and year. As management tools, models based upon relative leaf area have advantages over models based on density and relative time of emergence since the level of weed infestation needs only to be assessed once, whereas density and emergence time require frequent observations. The ability to assess accurately and quickly both the crop and weed leaf area, however, may limit the practical application of models based on leaf area. The inability of empirical models to account for year–to–year variation in environmental conditions was observed.     

Physiological mechanisms for the rapid growth of Pennisetum clandestinum in Mediterranean climates

Pennisetum clandestinum Hochst ex Chiov. (kikuyugrass) is a C₄ grass that has become an invasive weed in temperate climates. We examined ecophysiological mechanisms that have allowed it to become a successful weed in these locations by comparing P. clandestinum and two other common turigrass species, Festuca arundinacea Schreb. (tall fescue cv. Mojave), a C₃ cool-season grass, and Stenotaphrum secundatum (Walt.) Kuntze (St. Augustinegrass), a C₄ warm-season grass, grown in a warm or cool growth regime. We measured rates of photosynthesis over a range of leaf temperatures and also measured growth rate parameters of these species. At leaf temperatures between 25^C and 40^C P. clandestinum maintained the highest rates of photosynthesis in both temperature regimes. Under warm temperatures, this species rapidly increased biomass and leaf area to a greater extent than either of the other two grasses. Theoretical whole plant photosynthesis (mean leaf area × mean photosynthetic rate) was higher for P. clandestinum than for the other two species in both growth regimes and over most leaf temperatures. Our results suggest that P. clandestinum is a successful weed in Mediterranean climates as a result of its capacity to photosynthesize over the full range of temperatures found in those climates, its rapid growth during warm weather and its apparent tolerance to moderately cool temperatures.     

Prediction of the competitive effects of weeds on crop yields based on the relative leaf area of weeds

For implementation of simple yield loss models into threshold-based weed management systems, a thorough validation is needed over a great diversity of sites. Yield losses by competition wsth Sinapis alba L. (white mustard) as a model weed, were studied in 12 experiments in sugar beet (Beta vulgaris L.) and in 11 experiments in spring wheat (Triticum aestivum L.). Most data sets were heller described by a model based on the relative leaf area of the weed than by a hyperbolic model based on weed density. This leaf area model accounted for (part of) the effect of different emerging times of the S. alba whereas the density model did not. A parameter that allows the maximum yield loss to be smaller than 100% was mostly not needed to describe the effects of weed competition. The parameter that denotes the competitiveness of the weed species with respect to the crop decreased the later the relative leaf area of the mustard was determined. This decrease could be estimated from the differences in relative growth rate of the leaf area of crop and S. alba. However, the accuracy of this estimation was poor. The parameter value of the leaf area model varied considerably between sites and years. The results strongly suggest that the predictive ability of the leaf area model needs to be improved before it can be applied in weed management systems. Such improvement would require additional information about effects of abiotic factors on plant development and morphology and the definition of a time window for predictions with an acceptable level of error.     

The influence of the nematode Meloidogyne incognita on competition between Solanum nigrum and tomato

The effect of Meloidogyne incognita (Kofoid & White) Chitwood, 1949, on the competitive relationship between tomato (Lycopersicon esculentum Mill.) and black nightshade (Solanum nigrum L.) was investigated under glasshouse conditions. Two intraspecific competition treatments were set up for the crop and the weed, and five interspecific treatments where the emergence of S. nigrum plants was progressively delayed in relation to tomato. Nematodes reproduced in all inoculated plants, their multiplication rates being much higher in tomato than in S. nigrum plants. Under nematode-free conditions, intraspecific competition of tomato was more severe than the interspecific competition shown by the weed S. nigrum. Tomato was as tronger competition with S. nigrum than the weed was with itself. However, when infested by M. incognita, both species displayed a similar competitive ability. Tomato yield losses increased with prolonged weed competition but were greater under nematode-infested conditions.     

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.