Simulation of damage in winter wheat caused by the grain aphid Sitobion avenae. 2. Construction and evaluation of a simulation model

To evaluate the relative importance of various components of damage caused by grain aphid (Sitobion avenae F.) populations in winter wheat, a simulation model of crop growth and development is combined with a model of aphid injury. The model applies to the time interval from flowering to ripeness which constitutes the main period of grain aphid immigration and development in winter wheat in the Netherlands. The crop model describes crop growth and development as a function of the prevailing weather and the available amount of soil nitrogen and consists of sink-source relations and distribution functions for carbohydrates and nitrogen. Injury byS. avenae affects crop growth both directly and indirectly. Direct effects on growth are due to aphid feeding. Indirect effects are caused by the aphid excretion product honeydew which affects leaf net carbon dioxide assimilation. Alternative hypotheses on the nature of the direct effects are formulated. Inputs to the model are average daily temperature, daily global radiation, the amount of nitrogen in the soil and the density of the aphid population. The major output is grain weight.The accuracy of the model is assessed by visual and statistical comparison to field data. The accuracy of both crop and damage model is satisfactory except for the final part of the growing season. Then, insufficient information on processes involved in leaf death and the termination of phloem transport to the grains results in overestimation of the rate of grain filling.The consequences of the lack of detailed information on the relation between environmental factors and the effect of honeydew on leaf carbon dioxide assimilation are assessed in a sensitivity analysis.     

The use of incidence counts for estimation of cereal aphid populations. 3. Population development and the incidencedensity relation

The relation between the population density of the cereal aphid,Sitobion avenae, and the proportion of wheat tillers infested is examined. Three of the six models considered require the assumption that the aphids' spatial configuration can be described by a single statistical distribution; as this is not true forS. avenae these models are unsuitable. When the other three models were applied to field sample data, only that of Nachman (1981) yielded a regression equation that remained constant throughout the development of the aphid population.Samenvatting Om in de praktijk gebruik te kunnen maken van incidentie (de fractie bezette halmen) voor het schatten van graanluispopulaties moet dezelfde relatie tussen dichtheid en incidentie tijdens het hele groeiseizoen gelden. Uit analyses van veldgegevens voor de graanluisSitobion avenae blijkt dat wel het geval te zijn, zodat bepalingen van incidentie kunnen worden gebruikt om de populatiedichtheid betrouwbaar te schatten.     

Simulation of damage in winter wheat caused by the grain aphid Sitobion avenae. 1. Quantification of the effects of honeydew on gas exchange of leaves and aphid populations of different size on crop growth

Damage in winter wheat caused by the grain aphidSitobion avenae F., was studied in the laboratory and at field level.Honeydew substitute solution was applied to flag leaves of winter wheat plants. One and fifteen days after application, the rates of net carbon dioxide assimilation and transpiration were measured. The rate of dark respiration was significantly higher one day after application of honeydew substitute as compared to the control. Other parameters describing the carbon dioxide — light response were not significantly affected. Fifteen days after honeydew substitute application, the two experimental years yielded different results. Under hot and dry conditions, the rate of dark respiration was higher while the late of carbon dioxide assimilation at high irradiance, the mesophyll conductance and the leaf nitrogen content were lower than for the untreated control. In addition, chlorotic symptoms were visible. At moderate temperature and relative humidity, the initial light use efficiency was significantly lower fifteen days after treatment with honeydew substitute whereas other parameters were not significantly affected.In field experiments at two locations natural aphid populations consisting of mainlyS. avenae were treated weekly with an aphicide from crop development stages DC 71, 73, 75 and 77, respectively, to create infestations of different size and duration. In the unsprayed controls densities reached 15.8 and 44.4 aphids per tiller, amounting to 182 and 544 aphid-days, respectively. Crop samples were analysed weekly, starting at flowering. At the smaller natural infestation, no significant effect on any yield component nor on yield was found at any treatment. The larger infestation caused a significant reduction of leaf area index, leaf weight, amount of water-soluble carbohydrates and grain weight on various sample dates when the infestation was not sprayed before DC 75. No effects were found on tiller and grain density nor on nitrogen content of plant parts at harvest.Samenvatting Schade in wintertarwe veroorzaakt door de grote graanluisSitobion avenae F., werd bestudeerd in laboratarium-en veldproeven.De vlagbladeren van wintertarweplanten werden behandeld met honingdauw-substituut-oplossing. Eén en vijftien dagen na behandeling werden de netto fotosynthesesnelheid en de transpiratie van de behandelde bladeren gemeten. Na één dag werd een verhoging van de donkerademhaling gemeten in vergelijking met de controle. Vijtien dagen na behandeling werden verschillende resultaten gevonden in de twee experimentele jaren. Onder warme en droge omstandigheden in 1986 was de donkerademhaling oger terwijl de fotosynthese bij verzadigende lichtintensiteiten, de geleidbaarheid van het mesofyl en het stikstofgehalte van de bladeren lager waren dan in de controle. Ook werden chlorotische vlekken waargenomen. Bij gematigde temperaturen en relatieve vochtigheid in 1987 werd een verlaging van de initiële efficiëntie van lichtbenutting gemeten.In twee veldexperimenten werd een natuurlijke aantasting van voornamelijkS. avenae op verschillende tijdstippen chemisch bestreden. Op deze wijze werden aantastingen van verschillende omvang en duur verkregen. Vanaf bloei van het gewas werden wekelijks gewasmonsters geanalyseerd. Een natuurlijke aantasting vanaf DC 71 met een bladluisindex van 182 en een piekdichtheid van 15,8 bladluizen per halm, leidde niet tot significante reduktie van de gewasproduktie. Een aantasting beginnend in DC 71 met een bladluisindex van 544 en een piekdichtheid van 44,4 leidde tot significante verlaging van de bladoppervlakteïndex, de blad biomassa, de hoeveelheid water-oplosbare koolhydraten en het korrelgewicht. Geen effekten werden waargenomen op halm-en korreldichtheid of op het stikstofgehalte van plantedelen bij de oogst.     

Modelling the effects of sub-lethal doses of herbicide and nitrogen fertilizer on crop–weed competition

The effects of sub‐lethal dose of herbicide and nitrogen fertilizer on crop–weed competition were investigated. Biomass increases of winter wheat and a model weed, Brassica napus, at no‐herbicide treatment with increasing nitrogen were successfully described by the inverse quadratic model and the linear model respectively. Increases in weed competitivity (β₀) of the rectangular hyperbola and parameter B in the dose–response curve for weed biomass, with increasing nitrogen were also successfully described by the exponential model. New models were developed by incorporating inverse quadratic and exponential models into the combined rectangular hyperbola with the standard dose–response curve for winter wheat biomass yield and the combined standard dose—response model with the rectangular hyperbola for weed biomass, to describe the complex effects of herbicide and nitrogen on crop–weed competition. The models developed were used to predict crop yield and weed biomass and to estimate the herbicide doses required to restrict crop yield loss caused by weeds and weed biomass production to an acceptable level at a range of nitrogen levels. The model for crop yield was further modified to estimate the herbicide dose and nitrogen level to achieve a target crop biomass yield. For the target crop biomass yield of 1200 g m^?2 with an infestation of 100 B. napus plants m^?2, the model recommended various options for nitrogen and herbicide combinations: 140 and 2.9, 180 and 0.9 and 360 kg ha^?1 and 1.7 g a.i. ha^?1 of nitrogen and metsulfuron‐methyl respectively.     

基于同化叶面积指数和条件植被温度指数的冬小麦单产估测

应用数据同化方法将遥感信息与作物生长模型融合,是估测区域作物产量的重要方法之一。以2008—2014年越冬后的冬小麦为研究对象,选择与作物长势、产量及水分胁迫信息密切相关的叶面积指数(LAI)和条件植被温度指数(VTCI),采用粒子滤波算法对CERES-Wheat模型模拟和遥感数据观测的LAI和VTCI实施同化,分别基于观测LAI和VTCI、同化LAI和VTCI构建冬小麦单产估测模型。结果表明,同化LAI变化趋势更加符合关中平原冬小麦的实际生长状况,同化VTCI能更好地反映冬小麦的水分胁迫程度。应用观测LAI和VTCI构建的估产模型决定系数为0.402,而单独应用LAI或VTCI单变量构建的估产模型决定系数分别为0.279和0.339,说明应用LAI和VTCI双变量构建的估产模型的精度优于单独应用LAI或VTCI单变量的精度。相比于观测LAI和VTCI构建的估产模型,基于同化LAI和VTCI构建的估产模型的决定系数从0.402提高到0.547。表明基于同化LAI和VTCI构建的估产模型的精度明显提高。     

甘蓝蚜危害油菜产量损失的研究

作者用一种衡量蚜虫危害的新尺度——“累计蚜量(头/天)”,对油菜不同生育阶段的单株损失及补偿作用分别作了研究。结果表明:油菜苗期、苔花期、荚果期都存在着一个造成损失的临界危害量(C)。苗期 C 为360～440(头/天),苔花期 C 为13500～16500(头/天),荚果期 C 为2700～3300(头/天)。危害量与损失率的关系符合 logistic 曲线。根据研究结果提出“有害株”的概念,既有蚜虫而又影响产量的植株。并结合蚜虫消长规律及累计蚜量的计算原理推导出确定“有害株”蚜量标准的公式B=C/(1/2)T(1+Q~n)+ITQ(1-Q~(n-1))/(1-Q)经比较,认为“危害株率”是蚜虫防治指标理想的尺度,它不仅明确可靠,而且简单易行。     

Reverse modelling to estimate yield losses caused by crop diseases

Yield loss analysis is critical to inform tactical and strategic decisions in crop health management, and requires quantification of three elements: the levels of injury caused by disease or pest, the actual (injured) yield, and the attainable (uninjured) yield. Reverse modelling allows reconstruction of an object or a process from limited information combined with a mathematical model. This approach is applied to estimate yield losses caused by diseases in winter wheat using a process‐based simulation model (WHEATPEST), in combination with field data generated by a network of experiments across France, where multiple disease injuries and actual yields, but not attainable yields, were measured. The analysis covers 70 [year × region × variety × crop management] combinations encompassing five years (2004–2008), four French regions, two winter wheat varieties (one high‐yielding and one hardy variety), and two levels of crop management corresponding to two levels of chemical intensification. The analysis involved three main successive modelling steps where actual yield, attainable yield and yield losses associated with individual diseases were simulated. Overall, simulated yield losses to combined diseases ranged from 0 to 4.2 t ha^?1, and averaged 0.80 t ha^?1. Septoria tritici blotch caused the highest mean yield loss of 0.66 t ha^?1. The results highlight the contribution of varietal improvement to agricultural sustainability and performances. Reverse modelling can be applied to other crops and diseases or pests, in order to estimate individual and combined disease yield losses.     

兼抗麦长管蚜和大麦黄矮病毒的小麦种质田间鉴定筛选

为鉴定筛选兼抗麦长管蚜和大麦黄矮病毒(Barley yellow dwarf virus, BYDV)的小麦种质,采用自然感蚜/感病系数法,对36个外引和远缘杂交选育的小麦种质材料进行了2年的田间鉴定,并分析了感虫性与感病性的相关关系。结果表明,2年中均兼抗麦长管蚜和BYDV的种质仅有KOKIPPCAS、KOK、Amigo-3和PI137739共4个材料,占总鉴定材料的11.11%;对二者均敏感的有98-10-35q-9、186Tm39、Tam200e12-14a、Tam200(27)7、小偃22、西农1376和小偃6号共7个材料,占19.44%。其它材料仅抗虫或仅抗病,或仅在一年中表现抗病或抗虫,如材料98-10-30和98-10-35a8抗麦长管蚜,但对BYDV敏感;材料Tam200(13)G和PIG23(2)C感蚜,但对BYDV有抑制作用。BYDV发生普遍率(发病株率)和严重度(病情指数)与有蚜株率显著相关,严重度还与感蚜指数显著相关,但感病植株的病级均值与有蚜株率无显著相关性。表明自然界长期的进化和选择使许多抗病虫基因得以保存下来,但较多抗性基因只在抗病或抗虫的某一方面表现有效,需给予更多关注。     

Crop damage and yield loss caused by two species of rodents in irrigated fields in northern Nigeria

We investigated the damage and yield loss caused by two rodent species, Arvicanthis niloticus and Mastomys (Praomys) natalensis, in cereal crops in fields near Kano, northern Nigeria from August, 1990 to April, 1992. Using the random cluster technique, we selected 100 clusters of 30 hills in each rice and wheat field, and assessed damage fortnightly, using the Cut Tiller Count method. In premature crops, damage increased from tillering stages to the dough stages. Differences in the percentages of damage (yield loss) in mature rice (4.8% in 1990 and 12.6% in 1991) and in wheat (30.0% in 1991 and 21.7% in 1992) were statistically significant between years. Severe yield losses in wheat, compared to low and moderate levels in rice (grown for the most part during the rains), were probably a result of greater consumption of the wheat, grown entirely by irrigation during the dry season when other rodent food sources were less abundant. Results of ANOVA showed significant effects of developmental stage of the crop, year of cultivation, and their interactions on the magnitude of crop damage.     

Identification of barley yellow dwarf virus and cereal aphid Infestations In winter wheat by aerial photography

In 1976 distinctive patterns were identified on aerial photographs of winter wheat crops in East Anglia. The patterns consisted of numerous dark foci which were found to be associated with sooty moulds (mostly Cladosporium spp.) in the crops. Both green and mature crops were photographed in 1977 and the patterns were matched with crop inspection data. These suggested that barley yellow dwarf virus (BYDV) was the cause of the patterns and that sooty moulds secondarily invaded the infected plants. A different type of focal pattern appeared to be associated with damage caused by aphid feeding. All crops showing the pattern associated with BYDV were sown before 13 October but there was no relationship with sowing date for the crops damaged directly by aphids. Yield losses within foci were estimated by sampling and the total crop areas affected were calculated using an image analysis computer: yield losses were estimated to range from 10% (BYDV) to 2% (aphid feeding). The epidemiology of BYDV is discussed and reasons are suggested for the development and appearance of the foci.     

小麦禾谷缢管蚜发生为害与生态环境研究

八十年代以来,我区麦蚜频繁成灾,减产2—3成的麦田较为普遍,研究证明:成灾的主要原因是优势种群已由麦长管蚜Sitobion avenae(Fabr.)转变为禾谷缢管蚜Rhopalosiphumpadi(L.)。田间调查弄清了禾谷缢管蚜在不同海拔高度的早、中、晚播麦田的不同生育阶段,以及在植株各部位的分布。穗期禾谷缢管蚜发生轻重与3月中旬平均温度呈正相关,与3月下旬雨量呈负相关。由此建立了预测禾谷缢管蚜发生程度的多元回归方程式: Y=0.792 0.205X_1-0.015X_2用这个预测式回测8年田间实测值与预测值极近。     

Een geval van noodrijpheid bij blauwmaanzaad

Summary Premature dying of seed poppies, of unknown cause, was observed in The Netherlands, in 1959, at the end of the vegetative growth period.In a spraying experiment with systox the untreated plants died prematurely and showed a 50% decrease in yield based mainly on the weight of harvested seed. This disorder may be due to sugar beet yellows virus or to aphid damage alone. One spray before flowering reduced the disease and increased the yield.The fungusPyrenophora calvescens (Fr.) Sacc., which also is supposed to cause a premature ripeness of seed poppies, was present to the same degree in both the untreated and the sprayed plots.     

Simulation modelling of yield losses caused by wheat stem rust

Stem rust, or black rust, of wheat, caused by Puccinia graminis f. sp. tritici, has recently re-emerged in several parts of the world, with epidemics occurring in eastern Africa, as well as northern and southern Europe. Damage mechanisms from disease dynamically affect the physiology of the crop as it grows and develops, and as the epidemic progresses, leading to yield losses in the stem rust-diseased wheat stand. Process-based agrophysiological models that include disease-induced damage mechanisms can help to better understand the physiological processes leading to yield losses, and to inform strategic decisions such as breeding strategies. Such models have not been developed for wheat stem rust so far. Two damage mechanisms for stem rust, light stealing and assimilate diversion, were incorporated in the agrophysiological simulation model WHEATPEST. The model, tested from experimental field data retrieved from the literature, provides a satisfactory representation of the system, although consistently underestimates relative yield losses by about 6.9%, resulting in relative yield losses between 17% and 56%. Analyses highlight the importance of the diversion of assimilates toward the pathogen in the magnitude of yield loss. Considering only the reduction of green leaf area would underestimate damage from stem rust by at least threefold. The analysis also shows the importance of the dynamic interplay between disease and crop growth, especially the dynamics of leaf area, on yield loss. Directions to consider additional damage mechanisms are proposed, and perspectives for future research, especially in relation to plant breeding strategies under climate change, are offered.     

Aphidius colemani Vier. (Hymenoptera, Braconidae, Aphidiinae) detected in cereal fields in Germany

Studies conducted in the 2000 cropping season at two different localities, Flaeming and Magdeburger Boerde in Germany, have provided new information on cereal aphid (Sitobion avenae (F.), Metopolophium dirhodum (Walker), and Rhopalosiphum padi (L.)) parasitoids in winter wheat; their species composition, relative abundance, hosts, and location effects. The average aphid population density was higher at Magdeburger Boerde and lower at Flaeming. Among the aphid species, Sitobion avenae was more abundant at Flaeming and Metopolophium dirhodum at Magdeburger Boerde. In total, eight species of primary parasitoids were recorded: Aphidius colemani Viereck, Aphidius rhopalosiphi DeStefani Perez, Aphidius uzbekistanicus Luzhetzki, Aphidius ervi Haliday, Aphidius picipes (Nees), Ephedrus plagiator (Nees), Praon gallicumStarý, and Praon volucre (Haliday). The predominant parasitoid species were Aphidius colemani and Aphidius rhopalosiphi on Metopolophium dirhodum and Aphidius uzbekistanicus on Sitobion avenae. A low number of hyperparasitoids were also recorded. Aphidius colemani was recorded for the first time in the open winter wheat fields in Germany, although it has been used as a biocontrol agent in glasshouses in many European countries and overseas. An analysis of the aforementioned information shows that Aphidius colemani detected as a parasitoid of cereal aphids in Germany is likely a result of an accidental escape of parasitoids from a glasshouse, as well as their successful overwintering and establishment in the area. This study provides baseline information essential for assessing future changes in aphid parasitoid species guild and dynamics in cereal fields in Germany.     

Impact of the Common Vole,Microtus arvalis (Pallas) on Winter Wheat and Alfalfa Crops1)

Impact of the common vole, Microtus arvalis (Pallas), upon the winter wheat and alfalfa crops was determined. Net-bottomed coops were used for estimating the effect of vole grazing. Exposure of winter wheat to vole grazing at the stages of earing and flowering was the most harmful. Losses were up to 91% of the potential crop yield, while the proportion of the best grain size fraction (2.8 mm) dropped to a minimum level. Coefficients reflecting the influence of vole grazing on the crop were determined. The coefficient TOTAL is an index of the total impact of consumption. The impact, I_t, at time t equals the consumption, Ct, at this time multiplied by the value of TOTAL. For alfalfa, these coefficients range from 1.28 to 11.73. The results obtained in this study have made it possible to determine the effective rodent pressure on crop yield.     

Crop rotation effects on Cyperus rotundus and C. esculentus population dynamics in southern California vegetable production

Control options for Cyperus rotundus and Cyperus esculentus (purple and yellow nutsedge) were evaluated within three cropping systems in the low desert of southern California: (1) standard vegetable crop rotation (weed‐free, uncontrolled nutsedge and cultivation) with spring cantaloupe (Cucumis melo) – summer fallow – winter broccoli (Brassica oleracea), (2) cover crop rotation (halosulfuron and smother crop) with spring wheat (Triticum aestivum)/corn (Zea mays) – summer sudangrass (Sorghum sudanense) – Winter Fallow, (3) rotation with solarization (non‐solarization and solarization) with spring wheat – summer fallow/solarization – winter broccoli. After two growing seasons, broccoli was planted without Cyperus control, to study the effect on yield. Cyperus rotundus tubers increased from 0.66 tubers per m² to 1260 tubers per m² in the uncontrolled treatment over two seasons. Cultivation during the growing season reduced C. rotundus tubers by 93% compared with the uncontrolled plots. Cover crop rotation did not reduce the number of C. rotundus tubers significantly, despite the dense sudangrass canopy shading the soil during most of the summer. Cyperus rotundus was effectively controlled by the solarization treatment. All methods controlled C. esculentus, especially when there were no crops growing in the summer. When broccoli was grown after two years of various management strategies, the cultivation treatment showed a 44% yield reduction compared with the weed‐free control, while the solarization treatment increased broccoli yield by 64% compared with the non‐solarization treatment. Rotations that included sudangrass had low broccoli yield when either C. rotundus or C. esculentus were present.     

入侵杂草虉草在云南的发生、危害和防除状况调查

采用倒置"W"九点取样法和5级目测法,调查了云南冬季作物田杂草虉草的发生和危害状况。结果表明,截至2017年在云南有7个地州33个区县发现有虉草危害,其中保山地区危害最为严重,占全省受害面积的92%,其次是大理白族自治州。虉草在冬季作物田大麦、小麦、油菜和蚕豆田均有发生。危害的虉草有两个种,分别是小籽虉草Phalaris minor和奇异虉草P.paradoxa。其中小籽虉草的发生较为普遍,奇异虉草为零星发生。虉草造成的危害指数在6%~50%之间,对农作物造成的产量损失在40~110kg/667m2之间。虉草的防除以化学防除为主,人工防除为辅,但目前尚缺乏经济有效的防除农田虉草的除草剂。     

Competition between winter wheat and Veronica hederifolia: influence of weed density and the amount and timing of nitrogen application

The effects at Veronica hederifolia. densities on the yield of winter wheat (Triticum aestivum L.) were studied in field trials conducted at the same location in 1992 and 1993. In 1992, nitrogen at 60 kg ha^-1 was applied at tillening followed by a further 80 kg ha^-1 at the first node of stem elongation. In 1993. four regimes of nitrogen applications were compared: a total of 140 kg ha^-1 supplied at three dales. 60 kg ha^-1 supplied at tillering, 60 kg ha^-1 Supplied al the first node of stem elongation and no nitrogen as the control. Competitive effects from V. hederifolia (ranging from 17 to 192 plants m^-22), were greatest in 1993, the year with the best early growth development of this weed. In 1993, yield losses, as described with a non-linear model changed for each regime of fertilization. In both years, V. hederifolia decreased wheat ear number per unit area for each nitrogen treatment. This is explained hy an increase in tiller mortality and a nitrogen deficiency in wheat at the stem elongation and flowering stages. Nevertheless, with late application of nitrogen, individual grain weight increased and the effect of V. hederifolia on wheat yield loss was lowest. The results are discussed in relation to the effects on all yield components, and show the importance of choosing the rieht nitrogen fertilization for specific yield targets of wheat production when infested by weeds.     

Role of stalk-anatomy and yield parameters in development of charcoal rot caused byMacrophomina phaseolina in winter sorghum

Charcoal rot caused byMacrophomina phaseolina (Tassi) Goid. damages stalk tissue and weakens stalk strength, leading to lodging of crop and loss in grain yield in sorghum. In an attempt to understand the pathogenicity byM. phaseolina in relation to stalk characters and plant outputs, the effects of morphological (stalk-thickness), anatomical (bundle-number, bundle-density, and vascular tissue) and physiological characters (water and soluble sugar content) of the stalks of winter sorghum genotypes on charcoal rot development were analyzed. Bundle-number and stalk-thickness had a significant influence on internal spread of charcoal rot. A thicker stalk facilitated the spread of lesions more than did a thinner one. A stalk with densely packed vascular bundles inhibited lesion advancement. Field studies with 24 winter sorghum genotypes demonstrated that most of the parameters that could improve grain and stover yield also increased the length of charcoal rot lesions. Lesion development in stalks showed a high degree of association with grain yield (R²=0.51) and 100-grain weight (R²=0.42). The rate of symptom development in stalks differed during dough and maturity stages. It was concluded that anatomical characters of sorghum stalk, especially bundle-number, along with yield parameters played an important role in determining the extent of stalk damage by charcoal rot in winter sorghum. http://www.phytoparasitica.org posting March 25, 2008.     

Predicting the growth and competitive effects of annual weeds in wheat

The growth and competitiveness of 12 annual weed species were studied in crops of winter wheat, in which weeds were sown to give a wide range of plant densities. Weed growth patterns were identified; early species which senesced in mid-summer were less competitive than those with a growth pattern similar to that of the crop. Most species had little effect on crop yield in 1987, and this was attributed to a high crop den sity. Crop yield-weed density relationships for all species in 1988 and for Galium aparine in 1987 were well described by a rectangular hyperbola. Species were listed in the following competitive order based on the percentage yield loss per weed m^?2: Avena fatua > Matricaria perforata > Galium aparine > Myosotis arvenis > Poa trivialis > Alopecurus myosuroides > Stellaria media > Papaver rhoeas > Lamiumpur-pureum > Veronica persica > Veronica hederi-folia > Viola arvensis. Prediction of yield loss is discussed. The assumptions inherent in using Crop Equivalents (based on relative weights of weed and crop plants), are challenged; with intense competition, weed biomass at harvest failed to replace lost crop biomass, and harvest index was reduced. It is concluded that a competi tive index, derived from yield density relation ships, and expressed as the percentage yield loss per weed m^?2, is more likely to reflect the com petitive ability of a species than an index obtained from plant weights in the growing crop.