Competition of sorghum cultivars and densities with Japanese millet (Echinochloa esculenta)

Field studies were conducted at two locations in southern Queensland, Australia during the 2003–2004 and 2004–2005 growing seasons to determine the differential competitiveness of sorghum (Sorghum bicolor L. Moench) cultivars and crop densities against weeds and the sorghum yield loss due to weeds. Weed competition was investigated by growing sorghum in the presence or absence of a model grass weed, Japanese millet (Echinochloa esculenta). The correlation analyses showed that the early growth traits (height, shoot biomass, and daily growth rate of the shoot biomass) of sorghum adversely affected the height, biomass, and seed production of millet, as measured at maturity. “MR Goldrush” and “Bonus MR” were the most competitive cultivars, resulting in reduced weed biomass, weed density, and weed seed production. The density of sorghum also had a significant effect on the crop's ability to compete with millet. When compared to the density of 4.5 plants per m², sorghum that was planted at 7.5 plants per m² suppressed the density, biomass, and seed production of millet by 22%, 27% and 38%, respectively. Millet caused a significant yield loss in comparison with the weed‐free plots. The combined weed‐suppressive effects of the competitive cultivars, such as MR Goldrush, and high crop densities minimized the yield losses from the weeds. These results indicate that sorghum competition against grass weeds can be improved by choosing competitive cultivars and by using a high crop density of >7.5 plants per m². These non‐chemical options should be included in an integrated weed management program for better weed management, particularly where the control options are limited by the evolution of herbicide resistance.     

Diversity in weed seed production and the soil seed bank: Contrasting responses between two agroecosystems

This study was carried out to compare the diversity in seed production and the soil seed bank in a dryland and an irrigated agroecosystem in the dry tropics. Both agroecosystems showed a comparable number of species, but only 25% and 38% similarity during the winter and rainy cropping seasons, respectively. In the irrigated agroecosystem, the amount of seed production diversity was almost double in the winter season, compared to the rainy season. The weed seedbank diversity was low but was sensitive to cropping practices and seasons in both agroecosystems. A considerably smaller soil seedbank size in the irrigated agroecosystem (cf. dryland) was related to lowered weed seed production. The dryland agroecosystem showed a greater accumulation of the seeds of broad‐leaved weeds, whereas the irrigated agroecosystem accumulated more seeds of the grasses or sedges. About three‐fourths of the seeds during the winter season were accounted for by Anagallis arvensis and Chenopodium album in the dryland agroecosystem and by C. album and Melilotus indica in the irrigated agroecosystem. However, during the rainy season, Ammannia baccifera, Echinochloa colona and Cyperus rotundus dominated in both agroecosystems. The changes in the weed seed bank and its diversity are mainly attributed to differences in water management, which tends to reduce species diversity, especially at a lower depth, but leads to the dominance of some potentially noxious weeds (e.g. Phalaris minor and M. indica). Approximately double the soil seedbank size and a greater diversity at a lower depth might indicate an adaptive mechanism in the storage of weed seeds in the dryland agroecosystem.     

A simple model of crop loss by weed competition from early observations on relative leaf area of the weeds

A new simple empirical model for early prediction of crop losses by weed competition was introduced. This model relates yield loss to relative leaf area of the weeds shortly after crop emergence using the relative damage coefficient q as the single model parameter. The model is derived from the hyperbolic yield density relationship and therefore accounts for the effects of weed density. It is shown that the model also accounts for the effect of different relative times of weed emergence. A strong advantage of the approach is that it can be used when weeds emerge in separate flushes. The regression model described experimental data on sugar-beet – lambsquarters (Beta vulgaris L. –Chenopodium album L.) and maize-barnyard grass (Zea mays L. –Echinochloa crus-galli L.) competition precisely. The model describes a single relationship between crop yield loss and relative leaf area of the weeds over a wide range of weed densities and relative times of weed emergence. Possibilities for scientific and practical application of the model are discussed.     

Response of onion (Allium cepa L.) plants to fertilizers, weed competition duration, and planting times in the central Jordan Valley

The competitive ability of onion with weeds and the effects of weeds on onion crop growth and yield were investigated in three field experiments carried out in the central Jordan Valley during the 1997–1998 growing season. Fertilizer application to the onion plants in the presence of weeds significantly lowered the onion yield and reduced the bulb diameter, compared to the unfertilized, weed-infested onion crop. Weed competition and onion yield reduction increased with an increase in the fertilizer rate. All weed-free treatments with or without fertilizer gave a significantly higher onion yield than any of the fertilized, weedy plots. The highest yield of onion was obtained from the weed-free plots with a moderate application of fertilizer. Weed competition reduced the growth, bulb yield, and size of onion in the plots established from bulbs, seedlings, and seeds when compared to the relevant weed-free controls. The effect increased with competition duration and was more pronounced in direct-seeded onion than in the other two types of planting material. However, onion grown from bulbs or seedlings gave better growth, bulb weight, and number and was more competitive than the direct-seeded onion. The results showed that the minimum number of days of weed competition needed for a significant reduction in onion growth was 42 days when propagated from bulbs or seedlings and 21 days after direct-seeding. Onion grown from bulbs tolerated weed competition better and produced a higher bulb yield than that obtained from direct-seeded or transplanted onion, irrespective of the planting dates. December was found to be the best planting time for onion that was grown from bulbs and seedlings, while November was the best planting time for direct-seeded onion, provided the plots were weed-free throughout the growing season.     

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.     

Critical period of weed competition in three vegetable crops in relation to management practices

The critical period of weed competition was determined in three vegetable crops: early cabbage (Brassica oleracea var. capitata L.), pickling cucumbers (Cucumis sativus L.), and field-seeded processing tomatoes (Lycopersicon esculentum L.). There were significant interactions between weed-removal treatments, year, and row width. Cabbage yields were reduced if plots were not kept weed-free for at least 3 weeks after transplanting or if weeds which emerged with the crop were allowed to remain longer than 4–5 weeks, Cucumber yields were reduced if plots were not kept weed-free for up to 4 weeks after seeding or if plots remained weed-infested longer than 3–4 weeks. Higher crop population densities (narrower row widths) in cabbage and cucumbers resulted in smaller plants, earlier competition from weeds, and therefore a shorter period that the crop could remain weed-infested without suffering reduced yields. Yields of direct-seeded tomatoes were reduced if plots were not kept weed-free for up to 9 weeks after seeding or if weeds which emerged with the crop were allowed to remain longer than 5 weeks. In each crop the timing of the critical period of competition was verified by weed removal only during this interval. There was a true critical period in direct-seeded tomatoes, but not in cabbage or cucumbers where a single weeding was sufficient to prevent yield losses.     

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.     

Modelling herbicide dose and weed density effects on crop:weed competition

The effects of a range of herbicide doses on crop:weed competition were investigated by measuring crop yield and weed seed production. Weed competitivity of wheat was greater in cv. Spark than in cv. Avalon, and decreased with increasing herbicide dose, being well described by the standard dose–response curve. A combined model was then developed by incorporating the standard dose–response curve into the rectangular hyperbola competition model to describe the effects of plant density of a model weed, Brassica napus L., and a herbicide, metsulfuron‐methyl, on crop yield and weed seed production. The model developed in this study was used to describe crop yield and weed seed production, and to estimate the herbicide dose required to restrict crop yield loss caused by weeds and weed seed production to an acceptable level. At the acceptable yield loss of 5% and the weed density of 200 B. napus plants m^–2, the model recommends 0.9 g a.i. metsulfuron‐methyl ha^–1 in Avalon and 2.0 g a.i. in Spark.     

Consequences of narrow crop row spacing and delayed Echinochloa colona and Trianthema portulacastrum emergence for weed growth and crop yield loss in maize

Echinochloa colona and Trianthema portulacastrum are weeds of maize that cause significant yield losses in the Indo‐Gangetic Plains. Field experiments were conducted in 2009 and 2010 to determine the influence of row spacing (15, 25 and 35 cm) and emergence time of E. colona and T. portulacastrum (0, 15, 25, 35, 45 and 55 days after maize emergence; DAME) on weed growth and productivity of maize. A season‐long weed‐free treatment and a weedy control were also used to estimate maize yield and weed seed production. Crop row spacing as well as weed emergence time had a significant influence on plant height, shoot biomass and seed production of both weed species and grain yield of maize in both years. Delay in emergence of weeds resulted in less plant height, shoot biomass and seed production. However, increase in productivity of maize was observed by delay in weed emergence. Likewise, growth of both weed species was less in narrow row spacing (15 cm) of maize, as compared with wider rows (25 and 35 cm). Maximum seed production of both weeds was observed in weedy control plots, where there was no competition with maize crop and weeds were in rows 35 cm apart. Nevertheless, maximum plant height, shoot biomass and seed production of both weed species were observed in 35 cm rows, when weeds emerged simultaneously with maize. Both weed species produced only 3–5 seeds per plant, when they were emerged at 55 DAME in crop rows spaced at 15 cm. Infestation of both weeds at every stage of crop led to significant crop yield loss in maize. Our results suggested that narrow row spacing and delay in weed emergence led to reduced weed growth and seed production and enhanced maize grain yield and therefore could be significant constituents of integrated weed management strategies in maize.     

Predicting the competitive effects of weed and crop density on weed biomass, weed seed production and crop yield in wheat

Competition between winter-sown wheat and Viola arvensis Murray or Papaver rhoeas L. was studied in two experiments in two successive years. The effects of varying crop and weed density were modelled in terms of weed biomass over time, weed seed production and crop yield. Biomass model parameters, representing maximum weed biomass and intra- and interspecific competition, were obtained for different assessment dates, enabling biomass levels to be predicted during the two growing seasons. Weed biomass declined, and its maximum level was reached earlier, with increasing crop density. Intraspecific competition was higher in the absence than in the presence of crop, increasing with time and with weed density. Halving the wheat population increased June biomass of V. arvensis by 74% and of P. rhoeas by 63%. Crop yield losses with increasing weed density were greater with low than with medium and high crop populations. P. rhoeas was significantly more competitive than V. arvensis in both years. Weed biomass in 1989 responded more to reductions in crop density following the milder winter of 1988/89 than in the previous year; however crop yields were less affected in 1989 due to summer drought, restricting late weed growth and competition. Weed seed production was related to weed biomass; the progressive lowering of crop density increased seed production, and both species were very prolific in the absence of crop. By combining models, seed production could be derived for a given competitive effect on the crop. Threshold weed populations, based on low weed levels that are not economic to control, could then be equated with the accompanying weed seed production.     

Use of perennial cover crops to suppress weeds in Nicaraguan coffee orchards

Both uncontrolled weed growth and vegetation‐free orchard floors have been shown to affect coffee (Coflea arabica L.) negatively, but using cover crops as a solution has yielded conflicting results in different studies. In this study we tested the establishment success of three cover crop species under different management intensities and planting densities, as well as their long term weed‐controlling abilities and effects on weed community composition. Monthly manual weedings during the first 12 weeks after planting resulted in more rapid and extensive cover crop development compared with less intensive management. Transplanted Commelina diffusa Burm. f. grew most rapidly and controlled weeds by limiting light availability, but disappeared during the dry season and failed to establish at all on one of the farms. Arachis pintoi established and persisted for over 2 years, providing excellent weed control by outcompeting weeds for water and/or nutrient resources. Desmodium ovalifolium Wall required the longest time to establish and controlled weeds by an undetermined competitive mechanism. The sowing method of Desmodium led to intense intraspecific competition which probably decreased its effectiveness. Both Arachis and Desmodium led to lower relative abundances of grassy weeds and more perennial forbs, but total weed biomass was so low that these differences have no practical implications.     

Competition between safflower and weeds as influenced by crop genotype and sowing time

The effects of crop genotype and sowing time on competition between safflower (Carthamus tinctorius L.) and weeds were investigated in a 2-year field study. Each year, safflower was grown as a pure stand and in mixture with a natural weed infestation, mostly represented by Polygonum aviculare L., Fallopia convolvulus (L.) A. Löve and Chenopodium album L., in an additive design including weed stands grown without the crop. Grain yield reduction ranged from about 50% to 80% depending on crop genotype, and was higher under spring sowing (mid-March) than under winter sowing (mid-February). In general, those genotypes incurring the least yield reduction gave the greatest suppression of weed biomass. More competitive genotypes also tended to reduce the proportion of C. album in the weed biomass at harvest, particularly in 1994 (the wetter growing season). The competitive ability of the crop was mainly correlated with its biomass at early growth stages, but was not related to its grain-yielding ability in the absence of weeds. Results showed that more competitive crops may be obtained by sowing the most competitive genotypes early. Selecting for higher competitive ability in safflower does not seem to imply a reduction in grain-yielding ability.     

Influence of nitrogen on competition between cereals and their natural weed populations

The effects of nitrogen fertilizer on the growth and density of natural weed populations in spring barley (Hordeum vulgare L.) and winter wheat (Triticum aestivum L.) were investigated in the absence of herbicide. An increased level of applied nitrogen did not enhance: weed germination, tended to decrease the total weed biomass and had a differential effect upon the biomass of individual weed species in both wheat and barky. In competition with barley, Chenopodium album L. and Lamium spp. had lower nitrogen optima than the crop, while Urtica urens L. had a higher nitrogen optimum. In competition with wheat, Stellaria media (L.) Vill., Lamium spp. and Veronica spp. had lower nitrogen optima than the crop. The systematic changes in nitrogen effect with time were analysed by fitting orthogonal polynomials to the growth and density curves. The methodology could be recommended for other studies in which time or other systematic factors are included, as it supplies information which a traditional analysis of variance cannot provide. Since seed production is positively correlated with biomass, so nitrogen level affects seed production and, hence, the seed pool and future weed population, suggesting that fertilizer usage can be exploited in an integrated programme of crop: weed management. A trend towards lower N fertilizer application owing to concerns about the environment willfavour most of the weed species investigated in these experiments and change the composition of weed populations.     

WEED COMPETITION IN TRANSPLANTED SPRING CABBAGE

Summary. The presence of weeds during autumn and winter had no effect on the growth of transplanted spring cabbage provided they were removed before rapid growth of crop and weeds began in early spring. Weeds left beyond this time competed with the crop, resulting in smaller marketable heads. Increasingly severe competition affected internal head quality, reduced the numbers of plants producing heads and resulted in the death of a proportion of the crop plants. The main weed species responsible for crop loss was Stellaria media , which survived winter frosts and grew rapidly in early spring to fill all available ground space, dominating the weed flora and shading the crop foliage.
Application of propachlor at planting time failed to give sufficient control of S. media to avoid crop loss, although the onset of competition was delayed. Trifluralin gave excellent control of S. media and resistant species were kept in check by the crop.
Comparison of cropped and uncropped plots showed that the crop exerted considerable competitive pressure on the growth and development of weeds particularly where this had been reduced or delayed by herbicide treatment. The presence or absence of the crop did not, however, affect the relative proportions of the weed species, except on plots treated with trifluralin, where the crop suppressed Capsella bursa-pastoris.
Compétition des mauvaises herbes dans les choux de printemps repiqués     

The implications of spatially variable pre‐emergence herbicide efficacy for weed management

BACKGROUND

The efficacy of pre‐emergence herbicides within fields is spatially variable as a consequence of soil heterogeneity. We quantified the effect of soil organic matter on the efficacy of two pre‐emergence herbicides, flufenacet and pendimethalin, against Alopecurus myosuroides and investigated the implications of variation in organic matter for weed management using a crop–weed competition model.

RESULTS

Soil organic matter played a critical role in determining the level of control achieved. The high organic matter soil had more surviving weeds with higher biomass than the low organic matter soil. In the absence of competition, surviving plants recovered to produce the same amount of seed as if no herbicide had been applied. The competition model predicted that weeds surviving pre‐emergence herbicides could compensate for sublethal effects even when competing with the crop. The ED50 (median effective dose) was higher for weed seed production than seedling mortality or biomass. This difference was greatest on high organic matter soil.

CONCLUSION

These results show that the application rate of herbicides should be adjusted to account for within‐field variation in soil organic matter. The results from the modelling emphasised the importance of crop competition in limiting the capacity of weeds surviving pre‐emergence herbicides to compensate and replenish the seedbank. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

The effect of crop,cultivation and seed addition for birds on surface weed seed densities in arable crops during winter

Weed seeds present an agronomic threat, but are also an important food resource for wildlife in winter. Weed seed densities on the soil surface in winter were examined from 1999 to 2002 in 105 fields on three different farms in UK. The effect of the preceding crop, cultivation, position within the field and the application of seed for birds (bird seed) on surface seed abundance and species composition was tested. Six or fewer species comprised c. 80% of the weed seeds. By January of each study year, the densities of seeds important for farmland birds (key seeds) were 73% or 87% lower compared with early winter on two of the farms, but were stable on the third where seeds were incorporated through cultivation. At the edge and mid‐field, seed densities only exceeded 400 m^?2 in 17%, 10% and 12% of fields for total, key and dicotyledonous seeds respectively. The preceding crop only affected seed densities at one site; stubbles of winter barley had fewer seeds compared with winter wheat or spring barley. Seed densities varied between the edge and mid‐field, but trends were inconsistent between sites. The density of the larger seeds (Atriplex patula, Viola arvensis, Polygonum aviculare and Chenopodium album) were reduced in fields receiving bird seed. The objectives of weed control and conservation may not be mutually exclusive because seed return was most reduced where the ground remained uncultivated through the winter, yet this also provided the best foraging opportunities for surface feeding seed predators.     

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 seed bank as affected by tillage,residue, and fertilization management under sweet corn-wheat cropping sequence in Iran

Soil weed seed bank is an important factor determining above-ground floristic composition and weed density in agricultural systems. The quantitative and qualitative measures of weed seed bank can help growers to predict the extent to which they are facing weed problems. Along with tillage, crop residues can affect the fate of weeds in the upcoming crops. To investigate such effects, we compared the effects of tillage systems [conventional tillage (CT), reduced tillage (RT), and no tillage (NT)], wheat residue retention, and nitrogen (N) rates (0, 69, 138, and 207 kg N ha⁻¹) on depth-related characteristics of the weed seed bank under a sweet corn-wheat sequence during 2014–2015 growing seasons in Shiraz, Iran. Soil bank was not affected by tillage systems but tended to be slightly higher under RT. The highest (898 seeds m⁻²) and lowest (322 seeds m⁻²) weed population at 0–10 cm depth were found when 138 kg N ha⁻¹ in 2015 and 207 kg N ha⁻¹ in 2014 were applied. Species richness and diversity were higher under NT and RT practices at the top layer, but CT system was more diversified at deeper depths. They were higher when crop residues were retained as well. Barnyard grass (Echinochloa crus-galli [L.] Beauv), common lambsquarter (Chenopodium album L.), common purslane (Portulaca oleracea L.), field bindweed (Convolvulus arvensis L.), flixweed (Descoreinia sofia [L.] Webb. & Berth.), henbit (Lamium amplexicaule L.), pigweeds (Amaranthus spp.), and stinking goosefoot (Chenopodium vulvaria L.) were the most common weeds found in all tillage systems and soil depths. Grasses were relatively lower than broadleaves regardless of treatments. Weed seed bank was mostly affected by weather conditions than treatments in this short-term experiment.     

Effects of three management strategies on the seedbank, emergence and the need for hand weeding in an organic arable cropping system

The effects of three different weed management strategies on the required input of hand weeding in an arable organic farming system, the weed seedbank in the soil and the emerging weed seedling emergence were studied from 1996 to 2003. Strategies were based on population dynamic models and aimed for (1) control of weeds as carried out in standard organic farming practice, (2) control of all residual weeds that grow above the crop and (3) prevention of all weed seed return to the soil. Under all strategies, the size of the seedbank increased during the conversion from conventional to organic farming systems. The increase under strategy 3 was significantly smaller than the increase under the other strategies. From 1999 onwards, the weed densities in plots treated with strategy 3 became significantly lower than the weed densities in plots treated with the other strategies. The time needed for hand‐weeding required to prevent weed seed return, in addition to the time needed in standard organic farming practices, reduced during the course of the study. A management strategy aimed at the prevention of seed return (strategy 3) can reduce the size of the increase of the seedbank, which is usually observed after transition from conventional to organic farming. This study provides unique real‐world data that are essential for evaluating population dynamic models. The results may contribute to the development of weed management systems based on ‘no seed’ threshold strategies and to a further decrease in the dependence on herbicides.     

Enhanced weed‐crop competition effects on growth and seed production of herbicide‐resistant and herbicide‐susceptible annual sowthistle (Sonchus oleraceus)

Enhanced crop competition could aid in the management of annual sowthistle (Sonchus oleraceus L.), a dominant weed of Australian cropping systems. A two‐year pot study was conducted to evaluate the effect of wheat (Triticum aestivum L.) planting densities (0, 82, and 164 wheat plants/m²) on growth and seed production of glyphosate‐resistant (GR) and glyphosate‐susceptible (GS) biotypes of annual sowthistle. Without competition, both biotypes produced a similar number of leaves and biomass, but the GS biotype produced 80% more seeds (46,050 per plant) than the GR biotype. In competition with 164 wheat plants/m², the number of leaves in the GR and GS biotypes was reduced by 62 and 61%, respectively, in comparison with the no‐competition treatment, and similarly, weed biomass was reduced by 78 and 77%, respectively. Compared to no‐competition treatment, the seed production of GR and GS biotypes was reduced by 33 and 69%, respectively, when grown with 82 wheat plants/m², but increasing wheat density from 82 to 164 plants/m² reduced the number of seeds only in the GS biotype (81%). Both biotypes produced greater than 6,000 seeds per plant when grown in competition with 164 plants/m², suggesting that increased crop density should be integrated with other weed management strategies for efficient control of annual sowthistle.