Characterizing the spatial pattern of Abutilon theophrasti seedling patches

Spatial biology of weed populations is the study of weed patches and their relevant patch-level processes. In this context, a patch was defined as an area in which individuals are aggregated into discrete subdivided populations. Four Abutilon theophrasti seedling patches in two continuous maize production fields were surveyed using a contiguous grid of quadrats between 1995 and 1997. Surveyed area was dependent on patch size and ranged from 96 m² to 1134 m². Within each area, all seedlings were counted in each 1 m × 0.75 m quadrat in June, just before post-emergence weed control, and in mid-July after all weed control practices were completed. The spatial pattern observed in the seedling distribution maps was single or multiple focal points of high seedling density that decreased with distance from the focal point. Two-directional correlograms corroborated this visual observation, such that A. theophrasti seedling density in neighbouring quadrats was spatially autocorrelated, and correlation strength decreased with distance separating quadrats. Autocorrelation coefficients decreased at a greater rate across crop rows than parallel to crop rows. Visually, patch shape was elliptical and oriented in the direction of field traffic. Factors affecting patch-level processes of spatial aggregation, stability and edge dynamics were considered.     

Weed flora and soil seedbanks in fields dominated by Imperata cylindrica in the moist savannah of West Africa

Imperata cylindrica (L.) Raeuschel is a dominant and infamous grass weed in the savannah of West Africa. Research to reduce the weed to non-damaging levels is a priority activity at many agricultural institutions. The successful development and implementation of long-term I. cylindrica management strategies depend on the ability to predict changes in weed composition after I. cylindrica has been controlled effectively. The weed flora and soil seedbank were assessed from 329 fields dominated by this species in the fringes of the humid forest (HFF), coastal/derived savannah (CDS) and in the southern Guinea savannah (SGS) in 1996 and 1997. The objectives of the study were to correlate species composition of the weed flora with that of the soil seedbank and to determine the effect of management factors and soil properties on the composition of the weed flora. Species richness in the weed flora and in the weed seedbank was higher in the SGS than in the CDS and HFF. Mean weed density per field was generally higher in the HFF (156 ± 25.0 weeds m^–2) than in the CDS (108 ± 8.1 weeds m^–2) and in the SGS (92 ± 6.3 weeds m^–2). Weed composition varied with agroecological zone as well as with management factors and soil properties. Sørenson's index of similarity was low (mean=0.20) in all zones, indicating poor similarity between the weed flora above-ground and the soil seedbank.     

The influence of interspecific interference on the seed production of Stellaria media and Hordeum vulgare (volunteer barley)

The effects of the density and proportion of both volunteer barley ( Hordeum vulgare L.) and Stellaria media (L.) Vill. (common chickweed) on the seed yield of both species was investigated in linseed ( Linum usitatissimum L.) and autumn-sown field bean ( Vicia faba L.). A model was created to estimate these effects. It was a combination of two models. The first was a simple linear model relating weed seed number m² to weed dry weight m². The second was an inverse linear model relating weed dry weight per plant to weed density. A very good relationship existed between barley dry weight and number of seeds m² and between S. media dry weight and number of seed capsules m². This relationship was relatively consistent between experiments. The inverse linear model provided a good estimation of the relationship between weed density and weed dry weight per plant for both barley and S. media . Model variables, however, differed between experiments. Using the combined model it was estimated that, in the absence of other weed species and at a density of 800 plants m², S. media would produce between 4000 and 48 000 seeds m², whereas volunteer barley, at a density of 400 plants m², would produce between 2000 and 8000 seeds m². The presence of barley always reduced S. media seed yield and a barley population of 100 plants m² could reduce S. media seed yield by up to 79%. The presence of S. media reduced barley seed yield in only one of three experiments, in which a S. media density of 800 plants m² reduced barley seed yield by up to 68%. It was concluded that interspecific weed competition should not be ignored as a factor in models of weed population dynamics     

Effects of crop management practices on Echinochloa crus-galli and Chenopodium album seed production in a maize/soyabean rotation

Seed production of residual weed populations needs to be taken into account when estimating the long-term impact of low-input agronomic practices. The objective of this study was to measure the effects and interactions of crop, weed control, tillage practice and nutrient source on the seed production of the dominant residual weed species in a maize/soyabean rotation at two sites: Echinochloa crus-galli (L.) Beauv. on a Sainte-Rosalie clay and Chenopodium album L. on a Duravin clay loam. Seed production per unit area was estimated in each experimental unit. Weed seed production was greater under mechanical weed control compared with chemical weed control. In 1997, E. crus-galli seed production reached over 326 000 seeds m^–2 in mechanical weed control treatments, but averaged less than 500 seeds m^–2 in the chemical weed control treatments. Chenopodium album produced in the range of 766 000 and 73 000 seeds m^–2 in mechanical and chemical weed control treatments respectively. Very few or no weed seeds were produced in soyabean under chemical control. Tillage intensity and nutrient source did not affect seed production of either weed species, with the exception that E. crus-galli produced more seeds in chisel than in mouldboard plough tillage in soyabean. Weed control method had more impact on seed production than tillage intensity and nutrient source in a maize/soyabean rotation.     

Weed shoot morphology effects on competitiveness for light in direct-seeded rice

The effects of weed shoot morphology on competitiveness for light in rice ( Oryza sativa L.) have not been well described quantitatively and are difficult to study empirically. A rice:weed model was used to analyse the effects of weed leaf area densities (LAD; m² m^–3), leaf angles (as leaf light extinction coefficients, k _leaf) and maximum heights ( H _m, m) on growth and competition with rice. Weed morphologies were hypothetical but empirically based, as follows: LADs were skewed to the bottom or conical, k _leaf values varied from 0.2 (erectophile) to 0.8 (planophile), and H _m values were 0.5 H _R, 1 H _R and 1.25 H _R, where H _R was rice maximum height. Other parameters were equal to those of rice. Growth and competitiveness were evaluated using mature seed dry weights (g m^–2). Short weeds and weeds with conical LADs were weakly competitive, regardless of other traits. For other weed types, interference with rice was positively related to H _m, LAD skewness and more planophile leaves. All three traits were critical determinants of weed interference but no single morphological trait guaranteed competitiveness. All else being equal, weeds with highly skewed LADs produced the most seed dry weight. Planophile leaves were particularly beneficial for short weeds, giving over five times more seed dry weight than erectophile leaves.     

Real-time vision-based detection of Rumex obtusifolius in grassland

Rumex obtusifolius is a common grassland weed that is hard to control in a non-chemical way. The objective of our research was to automate the detection of R. obtusifolius as a step towards fully automated mechanical control of the weed. We have developed a vision-based system that uses textural analysis to detect R. obtusifolius against a grass background. Image sections are classified as grass or weed using 2-D Fourier analysis. We conducted two experiments. In the first (laboratory) experiment, we collected 28 images containing R. obtusifolius and 28 images containing only grass. Between 23 and 25 of 28 images were correctly classified (82–89%) as showing R. obtusifolius ; all grass images were correctly classified as such. In the second (field) experiment, a self-propelled platform was used to obtain five sequences of images of R. obtusifolius plants. We used the parameters that gave the best classification results in the first experiment. We found, after changing one of the algorithm's parameters in response to prevailing light conditions, that we were able to detect R. obtusifolius in each image of each sequence. The algorithm scans a ground area of 1.5 m² in 30 ms. We conclude that the algorithm developed is sufficiently fast and robust to eventually serve as a basis for a practical robot to detect and control R. obtusifolius in grassland.     

Response of winter wheat (Triticum aestivum L.) and weeds to tank mixtures of 2,4-D plus MCPA with clodinafop propargyl

Field and greenhouse experiments were conducted in 2004 and 2005 to study weed control and the response of winter wheat to tank mixtures of 2,4-D plus MCPA with clodinafop propargyl. The field experiments were conducted at Yazd and Oroumieh, Iran, with factorial combinations of 2,4-D plus MCPA at 0, 975, and 1300 g ai ha⁻¹ and with clodinafop propargyl at 0, 64, 80, 96, and 112 g ai ha⁻¹ in four replications. The greenhouse experiments further evaluated the effect of these tank mixtures on weed control, where each herbicide mixture was considered as one treatment and the experiment was established in a randomized complete block design with four replications. In the field experiments, the herbicides were applied at wheat tillering, while in the greenhouse experiments they were applied at the beginning of the tillering stage and at the four-leaf stage of the grass and broadleaf weeds, respectively. The results indicated antagonistic effects between 2,4-D plus MCPA and clodinafop propargyl. The best tank mixture with regard to weed control efficacy was 2,4-D plus MCPA at 975 g ai ha⁻¹ with clodinafop propargyl at 96 g ai ha⁻¹. The wheat grain yield was also increased by the tank mixture of clodinafop propargyl with 2,4-D plus MCPA. Generally, to inhibit clodinafop propargyl efficacy reduction due to tank-mixing with 2,4-D plus MCPA, it is recommended that the application dose of 64 g ai ha⁻¹ should be increased to 96 g ai ha⁻¹.     

Correlation between the soil seed bank and weed populations in maize fields

Annual weed populations establish every year from persistent seed banks in the soil. This 3 year study investigated the relationship between the number of weed seeds in the soil seed bank and the resultant populations of major broadleaf and grass weeds in 30 maize fields. After planting the crop, 1 m² areas were protected from the pre-emergence herbicide application. Soil samples were collected soon after spraying to a depth of 100 mm and the weed seeds therein were enumerated. The emerged weed seedlings in the field sampling areas were counted over the following 8 weeks. Up to 67 broadleaf species and five grass weeds were identified, although not all were found at every site and some were specific to a region or soil type. For the most abundant weeds in the field plots, on average 2.1–8.2% of the seeds of the broadleaf species and 6.2–11.9% of the seeds of the grass weeds in the soil seed bank emerged in any one year, depending on the species. Overall, the results showed a strong linear relationship between the seed numbers in the soil and the seedling numbers in the field for all the grasses and for most broadleaf weeds. For some species, like Trifolium repens , only a weak relationship was observed. In the case of Chenopodium album , which had the largest seed bank, there was evidence of asymptotic behavior, with seedling emergence leveling off at high seed numbers. An estimate of the soil seed bank combined with knowledge of the germination and behavior of specific weed species would thus have good potential for predicting future weed infestations in maize fields.     

Weed infestation and tea growth under various weed management methods in a young tea (Camellia sinensis[L.] Kuntze) plantation

A field experiment was conducted in the low country of Sri Lanka, during the period 1994–1995 to investigate the severity of weed infestation and tea growth in relation to weed management methods in newly established tea ( Camellia sinensis [L.] Kuntze). Manual weeding (hand and slash weeding) at various intervals was compared with various herbicides, with or without mulching. Weed control with herbicides was superior to that of hand weeding at 6-week intervals or more. Weed control with oxyfluorfen at 0.29 kg ai ha⁻¹ + paraquat at 0.17 kg ai ha⁻¹ or glyphosate at 0.99 kg ai ha⁻¹ + kaolin at 3.42 kg ha⁻¹ were superior. Plots unweeded for 12 weeks or more produced significantly greater ( P  < 0.05) weed biomass than plots unweeded for 6 weeks. Although the least weed dry weight ( P  < 0.05) and the greatest number of weed species were recorded with hand weeding at 2 week intervals, there was no particular benefit on tea growth when compared with hand weeding at 6 and 12 week intervals. Inter row mulching in chemically treated plots was more favorable for tea growth than no mulching, while living weed cover in unmulched slash weeded plots suppressed tea growth. A combination of mulching and herbicides, particularly oxyfluorfen and paraquat, followed by hand weeding at least every 6–8 weeks was considered the most appropriate weed management system for young tea.     

Establishment of, and preliminary impact studies on, the rust, Maravalia cryptostegiae, of the invasive alien weed, Cryptostegia grandiflora in Queensland, Australia

The rust fungus Maravalia cryptostegiae , from south-west Madagascar, was introduced into Australia in 1995 as a classical biological control agent against the highly invasive rubber-vine weed Cryptostegia grandiflora , a woody climber endemic to Madagascar. The rust was released at 69 sites between 1995 and 1997 and is now established throughout the plant's exotic range in Queensland, estimated at over 40 000 km². Dispersal was low in the first 3–4 months but was virtually linear thereafter, and the rust spread over 100 km within the first year; after 3 years it was recorded 550 km away from the nearest release site. Spraying both dry and aqueous inoculum of uredinioid teliospores from the ground using mist-blowers, as well as from the air by atomizing spore suspensions, resulted in rust-induced defoliation, producing an overall reduction in fecundity and biomass of the weed. In sites with low water tables, weed growth decreased markedly, with a reduction in plant volume from 9 m⁻³ to 1 m⁻³ over a 4-year period. Both rust- and drought-induced stress combined to cause up to 75% plant mortality at some sites, and at all monitored sites, seedling recruitment was virtually nil. Improved growth of indigenous grasses amongst rubber-vine thickets has increased fuel loads and created opportunities to use fire as a component of an integrated approach to the management of this economically and ecologically damaging weed.     

Control of weed barley species in winter wheat with sulfosulfuron at different rates and times of application

Field experiments were conducted at five locations in the major wheat production regions of Iran to evaluate the efficacy of sulfosulfuron in controlling weed barley species (including Hordeum spontaneum , Hordeum murinum , Hordeum distichon , and Hordeum vulgare ) in the 2004–2005 and 2005–2006 growing seasons. Sulfosulfuron was applied either postemergence (POST) or preplant-incorporated (PPI) at 0, 20.25, 30.75, 40.5, 51.0, 60.75 or 71.25 g ai ha⁻¹ to plots arranged in a randomized complete block design with four replications. Sulfosulfuron at the recommended rate (20.25 g ai ha⁻¹) failed to provide acceptable control of the weed barley species. However, the level of control increased with the application rate, particularly at rates >51.0 g ai ha⁻¹. Generally, PPI-applied sulfosulfuron resulted in markedly greater control levels than those of a POST application and complete control of H. murinum and H. vulgare was achieved with PPI-applied sulfosulfuron at all rates >20.25 and 30.75 g ai ha⁻¹, respectively. In most cases, the wheat yield increased with the application rate without any crop injury. The highest yield increase (186%) was obtained with a PPI application of 71.25 g ai ha⁻¹.     

Comparison of different weed management systems and their effects on yield of coconut plantations in Sri Lanka

The influence of five different weed management systems on nut yield of coconut were evaluated to determine an economical and effective method of controlling weeds in coconut plantations in the low country, dry zone in Sri Lanka. Treatments imposed included slashing and mulching around the palms with slash (T1), slashing and removing the slash (T2), application of glyphosate (N-(phosphonomethyl)-glycine) alone at 1.44 kg ai ha⁻¹ (T3), application of glyphosate alone at 2.88 kg ai ha⁻¹ (T4) and cover cropping with Pueraria phaseoloides (T5). All treatments were applied twice a year, except for the cover cropping treatment, T5. Based on a reduction in weed biomass, treatments T3, T4 and T5 were found to be significantly effective over other treatments. Coconut yield was increased significantly ( P  < 0.05) in glyphosate-applied plots at both tested rates. Control of weeds with the lower concentration of glyphosate (1.44 kg ai ha⁻¹) resulted in a 25% increase in nut yield over the uncontrolled weed plots. At this rate, it was found to be the most effective and economical method of controlling weeds in coconut plantations. Cover cropping with Pueraria phaseoloides was effective in controlling weeds in the long-term, but was not economical compared with the glyphosate application.     

Weed control in summer-sown soybeans with flumioxazin plus acetochlor and flumiclorac-pentyl plus clethodim

Field trials were conducted in Taigu, Shanxi province, China, to evaluate the efficacy of flumioxazin plus acetochlor and flumiclorac-pentyl plus clethodim applied to summer-sown soybeans at pre- and postemergence. It was demonstrated that tank-mixing flumioxazin at 50 g ai ha^-1 and acetochlor at 800 g ai ha^-1 created an effective soil-applied herbicide for weed control in soybean crops. The control efficacy was better than when the herbicides were applied individually, and no injury was caused to the soybeans. Flumiclorac-pentyl at 50 g ai ha^-1 plus clethodim at 70 g ai ha^-1 suppressed both broad-leaved weeds and grass weeds with an increased efficacy of more than 90%. Flumiclorac-pentyl applied alone or tank-mixed caused some injury to soybean seedlings, but the soybeans recovered 2–3 weeks after treatment and there was no reduction in the yield.     

Current status of biological control of paddy weeds in Vietnam

Rice is a staple food in Vietnam and accounts for > 7.7 × 10⁶ cultivated ha, which provide 35.5 × 10⁶ t of rice, of which 4.2 × 10⁶ t were exported in 2004. The enlargement of the cropping area and the enhancement of rice yield have rapidly increased the amount of agrochemicals, including herbicides, in crop production in Vietnam. From 1990–2003, the percentage of herbicides in total pesticides has increased ≈ 10-fold to 30.2%. In addition, the improper use of herbicides caused environmental hazards, unsafe agricultural products, and human health problems. Biological management integrated with traditional weed control techniques might help to reduce the dependence on synthetic herbicides and build eco-friendly, sustainable agricultural production in Vietnam. This paper reviews the efforts in establishing a strategy for biological management of weeds that was conducted in recent years by Vietnamese weed scientists. This has included cropping system management, water and soil management, integrated pest management, and utilization of plant allelopathy as major components of the strategy. Many plants with strong allelopathic potential can be a source for biological weed suppression and soil fertility improvement. The utilization of allelopathic properties in rice might also help to provide new rice cultivars with weed-suppressing characteristics.     

Effect of wind on the dispersal of oospores of Peronosclerospora sorghi from sorghum

The effect of wind on the dispersal of oospores of Peronosclerospora sorghi , cause of sorghum downy mildew (SDM) is described. The oospores are produced within the leaves of aging, systemically infected sorghum plants. These leaves typically undergo shredding, releasing oospores into the air. Oospores are produced in large numbers (6.1 × 10³ cm⁻² of systemically infected leaf) and an estimate of the settling velocity of single oospores (0.0437 m s⁻¹) of P. sorghi indicated their suitability for wind dispersal. In wind tunnel studies wind speeds as low as 2 m s⁻¹ dispersed up to 665 oospores per m³ air from a group of leaves previously exposed to wind and displaying symptoms of leaf shredding. The number of oospores dispersed increased exponentially with increasing wind speed. At 6 m s⁻¹, up to 12 890 oospores per m³ air were dispersed. Gusts increased oospore dispersal. A constant wind speed of 3 m s⁻¹ dispersed a mean of 416 oospores per m³. When gusts were applied the mean was 15 592 oospores per m³. In field experiments in Zimbabwe, oospores were sampled downwind from infected plants in the field and at a height of 3.8 m above ground level immediately downwind of an infected crop. These data indicate that wind could play a major role in the dispersal of oospores from infected plants in areas where SDM infects sorghum, perhaps dispersing oospores over long distances.     

Effects of winter wheat cultivars and seed rate on the biological characteristics of naturally occurring weed flora

Summary Differential competitive ability of six winter wheat cultivars and traits that confer such attributes were investigated for a range of seed rates in the presence or absence of weeds for a naturally occurring weed flora in two successive years in split-plot field experiments. Crop height and tillering capacity were considered suitable attributes for weed suppression, although competitiveness is a relative rather than an absolute characteristic. Maris Huntsman and Maris Widgeon were the most competitive cultivars whereas Fresco was the least competitive. Manipulation of seed rate was a more reliable factor than cultivar selection for enhancement of weed suppression, although competitiveness of cultivars Buster, Riband and Maris Widgeon was not enhanced by increased seed rate. Crop densities ranging between 125 and 270 plants m⁻² were found to offer adequate weed suppression. Linear relationships were observed between individual and total weed species dry weight and reproductive structures per unit area.     

Influence of developmental stage and time of assessment on hot water weed control

Summary The influence of plant developmental stage in hot water weed control was studied on the test weed Sinapis alba in field experiments. The dose was measured as thermal energy in the hot water (kJ m⁻²) and the response as reduction in plant weight. The energy dose for a 90% reduction in plant weight was 340 kJ m⁻² at the two-leaf stage, which is one-third of the energy required for the same reduction at the six-leaf stage. Treatment at an early stage saves energy, increases the driving speed and lowers the costs. Hard surface areas with naturally developed weeds were used to study the required treatment interval and the influence of time of assessment on the reduction in weed cover. The required treatment interval was 25 d on average, which is similar to that of flame weeding. A longer lasting effect requires a higher energy dose. A 50% higher energy dose was needed to obtain a 90% reduction in weed cover that lasted for 15 d instead of 7 d. After 3–4 weeks, hardly any reduction could be recorded because of regrowth of perennial weeds. However, hot water weed control has a potential on urban hard surfaces and railroad embankments, especially where the use of herbicides is restricted.     

Germinable soil seedbanks of central Queensland rangelands invaded by the exotic weed Parthenium hysterophorus L.

The germinable soil seedbank was determined at two sites in central Queensland on four separate occasions between February 1995 and October 1996. These sites were infested with parthenium weed ( Parthenium hysterophorus L.), a serious invasive exotic weed. During this period, the seedbank varied between 3282 and 5094 seeds m⁻² at the Clermont site, and between 20 599 and 44 639 seeds m⁻² at the Moolayember Creek site. Parthenium hysterophorus exhibited a very abundant and persistent seedbank, accounting for 47–73% of the seedbank at Clermont and 65–87% of the seedbank at Moolayember Creek. The species richness and species diversity of the seedbank, and the seed abundance of many species, was lower at Moolayember Creek during spring (the time of year when the most dense infestations of the weed originate). Parthenium hysterophorus seedlings also emerged more rapidly from the soil samples than did those of all other species. Hence, it seems that various aspects of the weed's seed ecology, including abundance and the persistence of its seedbank and the rapid emergence of its seedlings, are major factors contributing to its aggressiveness in semiarid rangeland communities in central Queensland. The domination by P . hysterophorus of the seedbanks of these sites suggests that the weed is having a substantial negative impact on the ecology of these plant communities. The diversity of these seedbanks was found to be lower in comparison with that observed in other grassland communities that were not dominated by an invasive weed species. Hence, the prolonged presence of P . hysterophorus may have substantially reduced the diversity of these seedbanks, thereby reducing the ability of some of the native species to regenerate in the future.     

Allelopathic potential of Trifolium resupinatum L. (Persian clover) and Trifolium alexandrium L. (Berseem clover)

This experiment was conducted in order to determine if Persian clover ( Trifolium resupinatum ) and Berseem clover ( Trifolium alexandrium ) contained water-soluble and methanol-soluble constituents that could affect the seedling growth of four test species; namely, Amaranthus retroflexus , Convolvulus arvensis (field bindweed), Secale cereale , and Sinapis arvensis (wild mustard) . The above-ground tissues of the Persian and Berseem clover plants were collected during the vegetative growth stage and oven-dried. Three concentrations of aqueous and methanolic extracts were used at full-strength (33.3 g L⁻¹), half-strength (16.7 g L⁻¹), and quarter-strength (8.3 g L⁻¹). Distilled water was used as the control. The weed seeds were placed in Petri dishes containing the legume extract or distilled water (control). The seedling growth of the weed species declined with the increasing concentration of the clover extracts. Wild mustard exhibited the highest sensitivity to both the legume extracts. Compared to the aqueous extract, the methanolic extract caused a greater decline in the seedling growth of the weeds. Berseem clover was the stronger inhibitor of the weed seedling growth, as compared to Persian clover. Therefore, the amounts of allelochemicals present might differ in these two clover species. Field bindweed showed the least sensitivity to both the legume extracts. Field bindweed showed more tolerance to the allelochemicals produced by the clover species.     

Weed seeds in long-term dryland tillage and cropping system plots

Successful crop production depends on effective weed control. Weed seedbanks were determined after 12 years of dryland cropping with winter wheat and grain sorghum under different tillage methods (no- and stubble mulch) and cropping sequences. Seeds of 12 species were detected. Amaranthus retroflexus was most abundant, but seed numbers were similar under all conditions. Portulaca oleracea , Panicum capillare , Setaria viridis and Sorghum halepense seed numbers differed because of some factors, with those for P. oleracea being greatest. For others, seed numbers were low (≤0.11 kg⁻¹ soil), except for Digitaria sanguinalis with 7.8 kg⁻¹ soil and Bromus japonicus with 1.3 kg⁻¹ soil. Most seeds were near the surface with both tillage methods, with enough present under most conditions to cause a problem if proper control measures were not used, especially with continuous cropping. When a weed problem occurs under conditions as in this study, the results indicate that it could be reduced by growing winter and summer crops in rotation, which permits controlling weeds with tillage and/or herbicides when a crop is not growing. Other possibilities for controlling weeds under conditions as in this study are to alternate between dicotyledonous and monocotyledonous crops, which would permit use of a wider range of herbicides, and to use selective in-crop herbicides.