An on-farm approach to investigate the impact of diversified crop rotations on weed species richness and composition in winter wheat

Weed species diversity may benefit from organic farming due to enhanced temporal diversification of crop species in a rotation and omission of herbicide applications. However, in intensively managed conventional systems, little evidence exists as to what extent diversified crop rotations contribute to higher weed species richness. Using an on-farm approach, the effect of crop rotation (organic, conventional diverse (CD) and conventional simple (CS) crop rotations) and weed control (with vs. without) on weed species richness, cover, community composition and crop biomass, was analysed in 24 winter wheat fields. Weed species with beneficial functions for invertebrates and birds were analysed separately. Weed species richness was higher in the organic crop rotation, but did not differ between CD and CS crop rotations. Weed control treatment reduced species richness in both conventional rotations, but not in the organic one. Redundancy analyses revealed that crop rotation intensity accounted for the largest part of the explained variation in weed species composition. Results from the study indicate that the maintenance of weed species richness and conservation of species with important ecological functions requires not only temporal diversification of crop species in the rotation, but also an adjustment of weed control strategies.     

Weed control in squash and tomato fields by soil solarization in the Jordan Valley

Black (BPE) and clear polyethylene mulches (CPE), 0.08 and 0.06 mm thick, respectively, were compared for their effectiveness for soil solarization over three seasons during 1986–1989 in weedy fields. Solarization for 6 weeks reduced weed growth and enhanced crop yields. However, further mulching with BPE after solarization with either BPE or CPE gave the best results. Not all weeds were sensitive to solarization. Some weed species were completely controlled; others were reduced to varying degrees; yet other weed species seemed to be enhanced by solarization. Solarization without further mulching was no better than farmer-practice in reducing weed growth or in increasing crop yield. Weeds required further removal after the middle of the growing season. Any soil disturbance after solarization reduced the weed control effect of solarization. Crops grew best in plots after solarization with BPE if they were planted in the same mulch after it was perforated.     

Effects of pre- and post-emergence weed harrowing on annual weeds in peas and spring cereals

To assess the effects of timing and frequency of weed harrowing on weed abundance and crop yield, different pre- and post-emergence weed harrowing sequences were applied to spring cereals and peas in field experiments performed during 2003 and 2004 in Sweden. Post-emergence harrowing was performed at crop growth stages 2–3 and 5–6 true leaves respectively. The best weed control was obtained by a combination of pre- and post-emergence harrowing, but these treatments also caused yield losses of 12–14% in spring cereals, while no yield losses were observed in peas. Pre-emergence weed harrowing treatments alone or combined with weed harrowing shortly after crop emergence proved to be most effective against the early emerging annual weed species Sinapis arvensis and Galeopsis spp. Post-emergence harrowing alone in peas had no effect on S. arvensis . The late emerging annual weed species Chenopodium album and Polygonum lapathifolium were most effectively controlled when pre-emergence weed harrowing was combined with one or two weed harrowing treatments after crop emergence.     

Effects of inter-row hoeing and fertilizer placement on weed growth and yield of winter wheat

Inter‐row hoeing is known to control tap‐rooted and erect weed species more effectively in winter wheat than weed harrowing. However, little is known about its effectiveness for use in the spring in winter wheat grown at wide row spacing (240 mm) under the influence of different placement of fertilizer. Two field experiments, one in 1999 and one in 2000, were conducted to study the influence of fertilizer placement, timing of inter‐row hoeing, and driving speed on the weeding effect on different weed species and crop growth. Placement of fertilizer below the soil surface improved crop growth and grain yield in both years compared with placement on the surface, but the more vigorous crop did not give any better suppression of the weeds surviving hoeing. Timing was not important in one experiment, whereas hoeing twice beginning in early April was more effective in the other experiment where weed growth over the winter had been vigorous. Driving speed had no influence on either the weeding effect or the yield, except for one case where increasing speed reduced the control of well‐developed weeds. Compared with unweeded reference treatments, inter‐row hoeing reduced total weed biomass by 60–70% and tap‐rooted and erect weed species in particular by 50–90%. Sowing at 240 mm row spacing yielded less than 120 mm (Danish standard), and inter‐row hoeing for winter wheat needs to be adapted to narrower row spacing to avoid such yield decreases.     

Modelling the effect of crop and weed on herbicide efficacy in wheat

Recommended field application rates of herbicides have to give effective weed control in every situation and are, thus, often higher than that required for specific fields. An understanding of the interaction between crop:weed competition and herbicide dose may, in many cases, allow herbicide application rates to be reduced, important both environmentally and economically. We have developed a model of the interaction between crop:weed competition and herbicide dose, using an empirical model of the relationship between crop yield and weed biomass (related to weed density), and an empirical model of the relationship between weed biomass and herbicide dose. The combined model predicts crop yield, given herbicide dose and weed biomass at an interim assessment date. These crop yield loss predictions may be used to quantify the herbicide dose required to restrict yield loss to a given percentage. Parameters of the model were estimated and the model tested, using results from experiments, which used cultivated oats ( Avena sativa ) or oilseed rape ( Brassica napus ) as model weeds in a crop of winter wheat ( Triticum aestivum ).For the crop:weed:herbicide combinations investigated there was little increase in crop yield for herbicide dose rates above 20% of recommended field rates, in broad agreement with the model predictions. There may still be potential for further reduction below this level on economic grounds; the model could be used to estimate the `break-even' herbicide dose.     

Effect of stale seedbed preparations and subsequent weed control in lettuce (cv. Iceboll) on weed densities

The effects of stale seedbed preparations and several weed control methods on the emergence of weeds in lettuce were studied. The specific goal was to evaluate the use of a stale seedbed in combination with chemical or mechanical weed control methods in the field. Depending on location and year, stale seedbed preparations followed by weed control prior to planting reduced the amount of weeds during crop growth by 43–83%. Control of the emerged seedlings after a stale seedbed preparation was more effective with glyphosate than with a rotary harrow. Covering the rotary harrow during control to prevent light reaching the soil improved its effect on the weed density during crop growth in two of 3 years. Radiation with far red light (FR) did not reduce the number of emerging weeds in this study. Mechanical control by finger weeder, torsion weeder and hoe was applied without stale seedbed preparations. These measures reduced the weed densities by 88–99%, compared with the untreated control and were more effective than chemical weed control with carbetamide and chlorpropham. The results show that the stale seedbed technique in combination with mechanical control of emerging weeds can reduce the weed population during crop growth as effectively as chemical control. The technique may therefore help reduce the use of herbicides in lettuce crops in the future.     

Rückgang von Ackerunkräutern in Baden-Württemberg – ein Vergleich von vegetationskundlichen Erhebungen in den Jahren 1948/49, 1975–1978 und 2011 im Raum Mehrstetten – Empfehlungen für Landwirtschaft und Naturschutz

In the period from the beginning of grain cultivation in Central Europe until the middle of the last century, the number of arable weed species has steadily increased due to diverse and extensive cropping systems. Since 1950, crop production systems have been intensified, arable land has been used for development (construction), chemical and mechanical weed control has improved and many crops have disappeared. These factors all have contributed to a strong decrease of weed species diversity. Based on detailed vegetation assessments this pattern was confirmed in the Mehrstetten area (Reutlingen County). Sampling data from 2011 were compared to data available for the same sites from 1948/1949 and 1975–1978. In the period covered, weed diversity decreased from by 64 % (97 species). Abundance of weed species was significantly higher in the field margins as compared to the center of the fields. Vegetation data obtained in 2011 no longer allowed for the identification of plant species communities since no species distinctly characterizing certain communities were recorded. There was no shift in the Ellenberg, Zeigerwerte von Pflanzen in Mitteleuropa, 1991, indicator values. None of the 23 endangered species still present in 1948/49 was recorded in 2011. Based on survey results, we assume that decrease in weed species diversity was caused by intensive cropping practices associated with the use of mineral fertilizer and herbicides as well as other effective methods of weed control.     

Image-based weed recognition and control: Can it select for crop mimicry?

As highly adaptable plants, weeds have evolved numerous mechanisms to evade control in agroecosystems. For example, reliance on herbicides has resulted in widespread evolution of resistance in many species. Minimising weed adaptation is a major driver for integrated weed management strategies. Crop mimicry is a notable example of weed adaptation, where weed species evolve to avoid control by mimicking aspects of the crop phenotype. Visual selection by hand weeding has been documented to select for crop mimics that are difficult to distinguish from the crop at the vegetative stage. With recent advancements in weed recognition technologies, image-based weed recognition for in-crop, site-specific weed control is on the cusp of becoming widely adopted. Whilst the control methods used in site-specific weed control will be varied (e.g., spot spraying or lasers), they will share weed recognition technology. Visual selection via image-based deep learning represents a selection pressure for weeds that can evade detection by mimicking crops. This mimicry may reduce weed recognition accuracy and thus weed control efficacy over time and result in difficult to manage mimetic weed phenotypes. Therefore, it is timely to explore the potential for selection of crop mimics by image-based weed recognition algorithms.     

Effects of rye cover crop residue and herbicides on weed control in narrow and wide row soybean planting systems

Α three‐year, non‐irrigated field study was conducted in 1998, 1999, and 2000 at the Southern Weed Science Research Unit farm, Stoneville, MS to study the effects of rye cover crop residue, soybean planting systems, and herbicide application programs on the control, density and biomass of several weed species and soybean yield. The soybean planting systems comprised 19 cm rows with high plant density, 57 cm rows with medium plant density, and 95 cm rows with low plant density. The herbicide programs evaluated were pre‐emergence, postemergence, pre‐emergence followed by postemergence, and no herbicide. Flumetsulam and metolachlor were applied pre‐emergence, and acifluorfen, bentazon, and clethodim were applied postemergence. The presence or absence of rye cover crop residue and a soybean planting system did not affect weed control of the species evaluated (browntop millet, barnyard grass, broadleaf signal grass, pitted morningglory, yellow nutsedge, Palmer amaranth and hyssop spurge), when herbicides were applied, regardless of the application program. In addition, rye cover crop residue was not an effective weed management tool when no herbicide was applied, because density and biomass of most weeds evaluated were higher than a no cover crop residue system. Among soybean planting systems, narrow with high plant density soybeans reduced density of grasses, broadleaf weeds and yellow nutsedge by 24–83% and total weed biomass by 38%, compared to wide with low plant density soybeans. Although weed pressure was reduced by narrow with high plant density soybeans, herbicide applications had the most impact on weed control, weed density and biomass. All herbicide programs controlled all weed species 81–100% at two weeks after postemergence herbicide applications, in comparison to no‐herbicide. Density of grasses and all broadleaf weeds as well as total weed biomass was lower with the pre‐emergence followed by postemergence program than these programs alone. Soybean yields were higher in the pre‐emergence followed by postemergence, and postemergence only programs than the pre‐emergence alone program. Planting crops in narrow rows is one cultural method of reducing weed pressure. However, even with the use of this cultural practice, prevalent weed pressure often requires management with herbicides.     

Robotic weeders can improve weed control options for specialty crops

Specialty crop herbicides are not a priority for the agrochemical industry, and many of these crops do not have access to effective herbicides. High‐value fruit and vegetable crops represent small markets and high potential liability in the case of herbicide‐induced crop damage. Meanwhile, conventional and organic specialty crop producers are experiencing labor shortages and higher manual weeding costs. Robotic weeders are promising new weed control tools for specialty crops, because they are cheaper to develop and, with fewer environmental and human health risks, are less regulated than herbicides. Now is the time for greater investment in robotic weeders as new herbicides are expensive to develop and few in number, organic crops need better weed control technology and governments are demanding reduced use of pesticides. Public funding of fundamental research on robotic weeder technology can help improve weed and crop recognition, weed control actuators, and expansion of weed science curricula to train students in this technology. Robotic weeders can expand the array of tools available to specialty crop growers. However, the development of robotic weeders will require a broader recognition that these tools are a viable path to create new weed control tools for specialty crops. © 2019 Society of Chemical Industry     

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     

Factors influencing the weed flora in the Gezira Scheme, Sudan

Agriculture plays a major role in the economic and social development of Sudan. The most important agricultural area in Sudan is the irrigated Gezira Agricultural Scheme (GAS), which takes water from the Sennar Dam on the Blue Nile. This study investigated weed communities in relation to geographical location and crop rotation in 2000 and 2001. Five agricultural units were sampled from the central sector of the GAS; within each unit we surveyed weed communities in different phases of the crop rotation: cotton, groundnuts, sorghum, and a fallow. The distribution of weed species showed some similarities in the 2 years sampled, with most species being found in all agricultural units but their relative proportions differed. Broad-leaved weeds were dominant, suggesting that herbicides selected for weed control in the Gezira scheme need to be effective against this group of species. Agricultural unit and crop type affected weed communities in both years. The weed flora was largest in cotton, intermediate in groundnuts and sorghum, and smallest in the fallow. Although most weeds were found in all agricultural units; however, their relative abundances differed greatly, either as a result of differences in management between the units, or some other spatial effect. As such, weed control programmes need to be tailored at the agricultural unit scale.     

发展化学除草重视综合治理

我国农田杂草有２５０多种，全国农田受草害面积４３００多万ｈｍ２，平均受草害减产１３．４％，每年减产粮食１７５０万ｔ，皮棉２５．５万ｔ和大豆５０万ｔ。传统农业生产采用机械作业及人力等除草。随着农村经济的发展，化学除草面积迅速扩大，全国农田化学除草面积从１９７５年的１７０万ｈｍ２增加到１９９５年的４１３３万ｈｍ２。但是，长期化学除草也带来了除草剂土壤残留对后茬作物药害、农田杂草种群更替和产生抗药性等新问题。必须重视农田杂草综合治理，通过采用各种有效的农业技术措施，为农作物保持良好的生态条件，结合化学除草才是最有效的防除杂草方法     

Competition between annual weeds and vining peas grown at a range of population densities: effects on the weeds

Linear regression of dry weight of weeds against crop density, together with the use of diversity indices and principal component analysis were used to derive information about changes in the behaviour of annual weeds over the growing season and in response to a wide range of crop densities in vining peas Pisum sativum L. Using linear regression it was possible to quantify reductions in weed dry weights per unit increase in crop plant density The ‘weed losse’ factor was acceptably consistent between experiments. Indices examining richness and evenness showed that numbers of weed species declined with increasing crop density and as the season progressed, but although species evenness became less at successive sampling dates the presence of a pea crop, whatever its density, did not radically alter the composition of the weed flora. Principal component analysis demonstrated that although there was competition within the weed flora, the crop did not replace the dominant weed species on high density plots, but reduced growth of all weed species alike.     

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.     

Weed communities respond to changes in the diversity of crop sequence composition and double cropping

Agricultural intensification, besides increasing land productivity, also affects weed communities. We studied weed shifts in cropping sequences differing in the identity and number of crops grown. We also evaluated whether dissimilar weed communities in different cropping systems converge towards more similar communities, when the same sequence is cropped during 2 years. In three locations in the Rolling Pampa, Argentina, field experiments were conducted including five cropping systems in the first year (winter cereal/soyabean, field pea/soyabean, and field pea/maize double crops, and maize and soyabean as single crops), while the same sequence was grown in the following 2 years (wheat/soyabean double crop and maize). Changes in weed community composition and structure were analysed through multivariate analyses and frequency–species ranking plots. Weed communities differed first among sites, while weed shifts within each site were mainly associated with growing season and crop type. Differences among crop sequences were higher in the first year, mostly related to specific crop grown, rather than to the number of crops in the sequences. Differences were reduced when the same sequence was grown during two consecutive seasons. Frequency of highly common weeds was negatively associated with the number of days with high crop cover. Our findings contribute to understand weed shifts in consecutive growing seasons, which may help readapting crop sequences to reduce the occurrence of abundant weed species.     

Effects of drought on weed emergence and growth vary with the seed burial depth and presence of a cover crop

In conservation agriculture, weed seed germination could decrease with the presence of a cover crop, surface weed seed location and temporal drought in summer just after seed shedding. This study simultaneously examined the effects of a cover crop, burial depth (seed location) and hydric stress on weed emergence and early growth. It was hypothesized that drought would reduce weed emergence and the initial growth of weed seeds and that this effect would be greater when the seeds were on the soil surface and in the presence of a cover crop. Four annual weed species were chosen that are frequently found (Anisantha sterilis, Vulpia myuros, Sonchus asper, Veronica persica) and not frequently found (Alopecurus myosuroides, Poa annua, Cyanus segetum, Capsella bursa‐pastoris) in fields that implement conservation agriculture. The unburied seeds had 26% lower emergence, on average, than the buried seeds (significant for six of the eight species), hydric stress reduced emergence by 20% (for seven of the eight species) and the presence of a cover crop reduced the level of emergence by 17% (for all species). The unburied seeds with hydric stress were emerging under the “most stressful” set of factors, with a 45% decrease in emergence, compared with the seeds emerging under the “least stressful” set of factors (buried seeds without hydric stress). All the weed growth measurements (height, dry matter content and number of leaves) decreased with the presence of a cover crop. The species that are found frequently in the fields that implement conservation agriculture, compared with the species that are not frequently found in conservation agriculture fields, had higher rates of germination and a higher tolerance of hydric stress when their seeds were unburied.     

Increased crop diversity reduces the functional space available for weeds

There is a drive to improve the sustainability of agricultural systems including the biodiversity component. Cultivar mixtures offer yield benefits from the same land area, but the mechanisms behind this overyielding have not been completely worked out. One potential mechanism is improved competition with weeds. We use an experimental approach of varying barley (Hordeum vulgare L.) genotypic and phenotypic diversity to test the hypothesis that increases in diversity have an impact on weed growth strategies and community assembly, or if responses are driven by barley performance. There was no effect of increasing barley mixture diversity on weed traits, either in terms of species means or the community-weighted mean. However, Functional Richness of the weed community decreased with increasing barley mixture diversity mainly as a result of reduced specific leaf area Functional Richness. This pattern was driven by a reduction in species richness of the weed community rather than by reduced variation within species. Whilst barley phenotype had different impacts on weed traits, there were no specific mixtures of phenotypes or genotypes that had consistent effects on community assembly or weed species responses. The competitive exclusion of weed species could have agronomic and environmental benefits, through better targeting or less frequent use of herbicides. Growing crop mixtures is one of many strategies available to improve agricultural sustainability and resilience, and one that has clear benefits.     

Experimental methods in plant competition research in crop-weed systems

The implications of recent studies on the inappropriatness of replacement series and additive methods in competition studies, and some possible alternatives, are discussed in the context of weed research, Replacement series are usually inadequate to assess competitive interactions and can be misleading. In particular they may be biased in favour of the larger species. Many of the criticisms of replacement series also apply to additive experiments. Response models relating yield per individual to the densities of the species in the mixture provide methodology for answering many questions about mixtures. This paper proposes a framework for using these models to: (i) measure the effect of weed species on yield per individual and yield per unit area for the crop and weed species; (ii) develop methods of biological control of weeds both within a seaons and over seasons, based on the interference between crop and weed species and the population biology of the weed species; (iii) establish a cost-benefit analysis of certain of the biological weed-control programmes. The inclusion of the effect of relative emergence time and management practices in response models is considered as well as experimental design for crop-weed experiments.     

Prevalence of herbicide‐resistant weed species in Malaysian rice fields: A review

The management of weeds in Malaysian rice fields is very much herbicide‐based. The heavy reliance on herbicide for weed control by many rice‐growers arguably eventually has led to the development and evolution of herbicide‐resistant biotypes in Malaysian rice fields over the years. The continuous use of synthetic auxin (phenoxy group) herbicides and acetohydroxyacid synthase‐inhibiting herbicides to control rice weeds was consequential in leading to the emergence and prevalence of resistant weed biotypes. This review discusses the history and confirmed cases and incidence of herbicide‐resistant weeds in Malaysian rice fields. It also reviews the Clearfield Production System and its impact on the evolution of herbicide resistance among rice weed species and biotypes. This review also emphasizes the strategies and management options for herbicide‐resistant rice field weeds within the framework of herbicide‐based integrated weed management. These include the use of optimum tillage practices, certified clean seeds, increased crop competition through high seeding rates, crop rotation, the application of multiple modes of action of herbicides in annual rotations, tank mixtures and sequential applications to enable a broad spectrum of weed control, increase the selective control of noxious weed species in a field and help to delay the resistance evolution by reducing the selection pressure that is forced on those weed populations by a specific herbicidal mode of action.