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.     

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.     

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.     

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.     

Interference by the weed Parthenium hysterophorus L. with grain sorghum: Influence of weed density and duration of competition

Two sets of field experiments were conducted at two sites (lowland and intermediate altitude) for 2 years in eastern Ethiopia to determine the influence of parthenium weed ( Parthenium hysterophorus ) density and duration of competition on grain yield of sorghum. In the first set of experiments, target weed densities of 0, 3, 7, 13, 27, 53 and 100 plants m -2 were used. Yield loss was severely affected by parthenium weed density, peaking at 97% at the lowland site in 2000. At this site, even very low density (e.g. three plants m -2 ) resulted in a high yield loss (69%). Owing to differences between sites and years, however, it was not possible to specify meaningfully the threshold densities for weeding. In the critical weed-free period experiments, four parthenium weed-infested and four weed-free periods from emergence to harvesting of sorghum were maintained by hand hoeing. The critical periods for weed control, i.e. the period over which weeding had the greatest benefit on yield, were 19 - 69 and 40 - 57 days from emergence of sorghum in 1999 and 2000, respectively, at the intermediate altitude (assuming an acceptable loss of 10%). In the lowland, however, it ranged directly from emergence to 61 and 66 days, indicating more severe competition at this site. The substantial variation in yield and yield loss between sites and years illustrates the problems of attempting to give well-established, accurate recommendations for threshold densities and critical periods in small-scale, rain-fed agriculture.     

Relationships between crop yield and weed time of emergence/removal: modelling and parameter stability across environments

Seventeen Italian experiments relating to maize, soyabean and durum wheat were used to analyse the variability of duration of tolerated competition (DTC) and weed-free period (WFP) curves across reasonably homogeneous areas. The data sets were analysed by regression analysis using four models relating yield loss to weed density, time of emergence and removal. These models differ in the way they account for the effect of time of weed removal on potential competitiveness. A sigmoidal relationship between these two variables appears necessary. The model with the best overall performance was then used to test the stability of the parameters that give the shape of the curve in relation to time of emergence and removal. This was done by comparing a full model with shape parameters specific to each experiment and a reduced model with a common set of parameters for all the experiments referring to a specific crop. For all three crops, the residual sum of squares of the reduced model did not increase significantly, indicating that, across tested environments, the yield loss caused by mixed weed infestations can be expressed by a single set of parameters relating weed competitivity to time of emergence and removal. For a given area, it should therefore be possible to predict yield loss on the basis of a quite limited set of experiments, thus greatly simplifying the development and use of decision support systems (DSS).     

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.     

Critical period for weed control in sesame production

Planning effective weed control in cropping systems requires exact appraisal of the weed intensity and duration of their competition with the crops. This 2‐year study was carried out in order to determine the critical weed control period in sesame fields. Related and relative crop yields were monitored and analyzed using a four‐parametric log‐logistic model. We recorded data from weed‐free plots and compared these with data from different periods of weed interference. In both the study years, the longer period of weed interference decreased the relative yield of sesame, whereas the yield was increased with increasing duration of the weed‐free period. A 51–78.7% decline in sesame yield was noted if the weeds were allowed to compete with the crop from planting to harvest. In the first year, the duration of the critical period for weed control (CPWC) was 177–820 growing degree days (GDD), which corresponded to 14–64 days after crop emergence (DAE), and between 170 and 837 GDD (13–64 DAE) in the second year; this was based on a 5% acceptable yield loss. The results of this study clearly elaborated that maintaining weed‐free conditions is compulsory from as early as the second week after the emergence of sesame plants, and this should be maintained at least until the ninth week to avoid sesame yield losses by more than 5%. These findings show that growers can benefit from CPWC to improve weed control in sesame production, including the efficacy of a weed control program and its cost.     

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.     

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 weed interference on Zea mays: Growth analysis

In order to investigate the effect of weed competition on corn, growth trials were conducted in Shushtar, Iran, using a comparative growth analysis. In this study, two sets of treatments were imposed, based on the phenological stages of corn development, using a randomized complete block design with three replicates. The results showed that barnyardgrass and redroot pigweed were the most dominant weeds in these trials. Increasing the duration of weed interference reduced the corn leaf area index gradually. The reduction in the corn leaf area index led to a decline in the crop growth rate as a result of a reduction in the ability of corn to capture light and its photosynthetic ability. The total dry matter and relative growth rate of corn were decreased when the weeds were removed later as a result of a reduction in the leaf area index. According to these results, the leaf area index has a critical role in weed–crop competition.     

Elucidating the apparent maize tolerance to weed competition in long-term organically managed systems

In a long-term cropping systems trial comparing organically and conventionally managed systems, organic maize production sustained crop yields equal to conventional methods despite higher weed levels. In 2005 and 2006, an experiment nested within the trial was conducted to gain insight into this apparent crop tolerance to weed competition. Density of mixed weed species was experimentally manipulated to achieve a broad range of weed infestation levels. Under standard management conditions, all cropping systems produced equivalent maize yields, even though weedy plant biomass in the organic treatments was between fourfold and sevenfold greater than in the conventionally managed maize. Increased yield capacity, evidenced when plots were maintained weed-free, and enhanced crop competitiveness, were the main pillars of this apparent crop tolerance to weed competition in the organic systems. Increased soil resource availability and a faster relative crop growth rate in the organic systems probably contributed to these factors, which play an important role in buffering crop fitness during years of less than ideal weed control. Simultaneously, the experiment illustrated the poor efficacy of mechanical weed management in the organic systems, which is the main reason organic maize did not out-yield conventional maize under standard management conditions.     

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.     

Critical period of weed interference in chickpea

The critical period of weed interference in one variety of chickpea was determined in field experiments carried out at two sites, Tabriz 2002 and Kermanshah 2003, Iran. Chickpea culture was either kept free of weeds for 0, 12, 24, 36, 48 and 60 days after crop emergence (DAE) or weeds were allowed to grow for 0, 12, 24, 36, 48 and 60 DAE. In these experiments, chickpea yield increased with increasing duration of weed-free period and was reduced by increasing duration of weed-infested period. Unweeded conditions for the entire growing season caused 66.4% and 48.3% seed yield reduction when compared with the treatment that was weed-free throughout the growing season, at Tabriz 2002 and Kermanshah 2003, respectively. The results indicated that chickpea must be kept weed-free between the five-leaf and full flowering stages (24–48 DAE) and from the four-leaf to beginning of flowering stages (17–49 DAE) at the two sites, respectively, in order to prevent >10% seed yield loss. At both sites, reduction in seed yield, because of the increased weed interference period, was accompanied by simultaneous reduction in plant dry weight, number of branches, pods per plant and 100-seed weight. This was supported by significant and positive correlations between these traits and chickpea seed yield. There was no significant correlation between the number of seeds per pod and seed yield. A linear regression model was used to describe the relationship between weed dry weight and seed yield loss.     

Prediction of competition between oilseed rape and Stellaria media

Ten experiments have investigated competition between winter oilseed rape and Stellaria media (common chickweed). Yield losses caused by this weed were often high, but differed greatly between experiments, 5% yield loss being calculated to be caused by 1.4–328 plants m^?2. Predictions of yield loss based on relative weed dry weights [weed dry weights/(crop + weed dry weights)] in December were somewhat less variable than those based on weed density, 5% yield loss being caused by 1.4–10.6% relative weed dry weight. The variations in yield loss were related to variations in the competitiveness of the oilseed rape and the S. media, caused by weather differences between years and sites, and the long period between weed assessment and harvest (8–10 months). However, despite the lack of precise relationships, there were indications that the greater the crop dry weights in December, the lower the final yield loss. Delayed sowing of oilseed rape until late September did not clearly increase the competitive effects of the weed compared with late August/early September sowings. Weed competition was not clearly affected by reduced crop density (44–113 plants m^?2), because of the compensatory ability of the lowest density. The results of the experiments are discussed in relation to the prediction of yield loss and, thus, possible adjustment of weed control strategies to meet expected crop losses.     

Misinterpretations of results in weed research through inappropriate use of statistics

Examples of weed research experiments are described where inappropriate statistical analyses have commonly been used and, as a result, unjustifiable conclusions were reached. The examples are for herbicide performance trials, crop yield/weed density studies, time of onset of weed competition and ‘critical period’ studies. Erroneous conclusions were drawn because non-transformed data were used and multiple comparisons were made with methods such as Duncan's Multiple Range Test and the use of a Least Significant Difference. More thorough analyses, especially using graphs and regressions, are shown to lead to more reliable interpretations, which are quite different from those usually concluded. In particular, considerable treatment effects are revealed in cases where the original conclusions were that they were absent.     

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.     

Tolerance of competitive spring barley cultivars to weed harrowing

Two experiments were conducted in 14 spring barley cultivars to investigate if crop tolerance to post‐emergence weed harrowing is related to morphological traits that reflect competitiveness. The experiments were carried out in organically grown fields where low weed densities and biomass production were assumed to be without significant influence on crop growth. The experiments showed that different cultivars responded differently to post‐emergence weed harrowing in terms of yield reduction. Taller and higher yielding cultivars with high leaf area index (LAI) tended to be less tolerant to post‐emergence weed harrowing than shorter and lower yielding cultivars with low LAI. This conclusion, however, is only valid for 13 of 14 cultivars because one very tall cultivar was tolerant to harrowing. Although the tallest and highest yielding cultivars were damaged the most, they remained the highest yielding cultivars after weed harrowing. This study is the first attempt to relate competitiveness of cereal cultivars to tolerance to harrowing, and it is thought provoking that competitiveness and tolerance is found to be counterproductive.     

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.     

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

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