Weed flora in the Rift Valley sugarcane plantations of Ethiopia as influenced by soil types and agronomic practises

A survey was carried out on 72 sugarcane fields in Wonji-Shoa and Methara sugarcane plantations in the Rift Valley, Ethiopia, to determine the composition and distribution of weeds and to assess the influence of soil types and crop management practises on weed species composition and distribution. A total of 180 weed taxa belonging to 40 families was recorded. Poaceae, Asteraceae, Euphorbiaceae, Fabaceae, and Malvaceae were the most abundant families based on the number of species recorded. Annuals were more common (137 taxa) than perennials (43 taxa). The most frequent and problematic weed species in the plantations, irrespective of the soil types and crop types, were Cyperus spp., Sorghum spp., Euphorbia hirta , Ipomoea cordofana , Rhynchosia malacophylla , Datura stramonium , Launaea cornuta , Portulaca oleracea , and Amaranthus spp., with a frequency of > 65%. Perennial, rhizomatous Cyperus spp. were more dominant in ratoon crops than in plant crops. A partial canonical correspondence analysis (pCCA) showed that soil types, fertilizer application, and crop cycles were the major factors influencing the species distribution in the plantations among all the variables considered in the study. The first pCCA axis structured weed species associated with light-textured soil in contrast to weeds on heavy soil. D. stramonium , Tagetes minuta , and Galinsoga parviflora were mostly associated with light-textured soil, while Xanthium strumarium , Hibiscus trionum , Vigna fischeri , and Ipomoea eriocarpa were mostly associated with heavy-textured soils. The second axis distinguished those species associated with fields receiving fertilizers versus those that were not fertilized. Rottboellia cochinchinensis and Phyllanthus tenellus were mainly associated with fertilized fields while E. hirta and L. cornuta were common on unfertilized fields.     

Effects of human-mediated processes on weed species composition in internationally traded grain commodities

International trade is a major route by which non‐indigenous organisms are introduced into new habitats. Various kinds of weed seeds have been introduced through grain trade. The objectives of this study were to understand the factors that affect the initial assemblage of plant species introduced by the international grain trade and to extract their general attributes. We surveyed weed seed contamination of spring wheat imported from Canada to Japan and analysed the effects of the field abundance of each weed and of harvesting and cleaning on the quantity of weed seed included in the imported wheat. The field abundance was positively correlated with the weed seed quantity. Seeds of short weeds and seeds with a pappus were eliminated from the wheat by the harvesting or cleaning process. Many other crop plants contaminated the wheat. Because various transportation vehicles, temporary storage sites and port elevators are used commonly with all exported crops and it is difficult to remove all residues from them, other crops might be carried over into the wheat commodity. These relationships also apply to other grains.     

Seed production of Chenopodium album in competition with field vegetables

Total weed control within a crop is both difficult and expensive to achieve, so that some weeds will often remain to set seed. The seed production resulting from these weeds will ultimately affect the sustainability of the weed control strategy. If too much is allowed to return each season there could be a gradual, but significant, increase in the potential weed flora over a number of seasons. Field trials were carried out in 2000 and 2001 to quantify the potential magnitude of this weed seed return from Chenopodium album L., grown at two planting densities either in pure stands or in competition with one of two crops (cabbage or onion). Crop and weed weights and weed seed production were notably greater in 2001. Both dry weight and seed production of C. album were suppressed by increasing planting density or by the presence of crop, with cabbage having a more suppressive effect. Despite the plasticity in seed production, a linear relationship was demonstrated between log weed seed production and log weed biomass that was robust over a range of competitive situations with onion and cabbage, at different planting densities and in growing seasons. The study also demonstrated that the relationship could be combined with an existing simple competition model to allow the consequences of incomplete weed control to be assessed in terms of potential weed seed return.     

Population regulation of Galinsoga ciliata and G. parviflora

The population response of the two sympatric annual weeds, Galinsoga ciliata (Raf.) Blake and G. parviflora Cav., which occur abundantly in croplands and early secondary successional communities in the north-eastern hill region of India was studied in relation to sowing pattern (clump or scattered sowing), population density and soil moisture and texture. In a given sowing pattern, seed germination of both species declined with increase in sowing density and moisture stress. However, sowing of seeds in clumps and an increased proportion of sand in soil enhanced the germination, Fewer weeds died in the population raised from scattered sowing than from clumped distribution. Plants grown at high moisture level matured earlier, and fertility of the survivors was negatively correlated with both density and moisture stress in both species although G. ciliata was less affected. Seed output and dry matter yield per pot of both species increased with population density but the increase was not linearly related to the increase in plant numbers, and density-induced reduction in seed output was observed at high densities. Galinsoga ciliata appears to tolerate greater density-stress than G. parviflora. Asubstantial decrease in production of capitula, seeds and dry matter at low moisture level indicates the role played by soil moisture in regulating the population growth of the two weeds. An increased proportion of sand in soil resulted in significantly greater seed production in G. parviflora. Galinsoga ciliata also showed a similar trend although the differences were not statistically significant.     

Annual beet: Problems and prospects

Annual bolting sugar beets have contaminated fields in England and represent a potential hazard through their seed production capability (up to 150 viable seed/plant), which could make them an important weed in arable crops, and through their pollen production, which could lead to a degeneration in varieties of sugar beet being grown for seed nearby. Although control by rotation and herbicides in other crops are probably the easiest effective control measures, up to 100% reduction in seed production in the sugar beet crop resulted from bolter pulling, topping or treatment with a 100 g a.e./litre glyphosate solution (1 ml/plant) early in July. The glyphosate damaged other plants which it reached. Whichever control method is selected, mechanisation seems possible.     

Crop and density effects on weed beet growth and reproduction

Weed beet populations growing in each crop of the arable rotation could be a relay for the gene flow from adjacent transgenic herbicide‐resistant sugarbeet. In this study, weed beet growth and reproduction were assessed under several conditions which could be found in the rotation: various weed beet densities (ranging from 1 to 120 plants m^?2) and various crops (winter wheat, spring barley, spring pea, sugarbeet, maize, ryegrass). Measurements were carried out both on life‐cycle dynamics (bolting time, time to flowering onset, dynamics of flower opening) and on other quantitative data (survival rate, bolting rate and pollen, flower and seed production). Increasing weed beet density resulted in decreases in bolting rate and flower and seed production per plant. In cereals, weed beet establishment and reproduction were strongly reduced, compared with bare ground as a control situation. In pea, there was no effect on establishment, but the early harvest limited seed set. In the other crops, flower and seed production were reduced to a lesser extent. Parameters of the fitted equations on the bolting and flowering progress were modified by the weed beet density and by the crop. Our data may be used in a model predicting weed beet demographic evolution according to cropping system, and in assessing gene flow.     

Using a selectivity index to evaluate logarithmic spraying in grass seed crops

BACKGROUND: Grass seed crops are minor crops that cannot support the development of selective herbicides for grass weed control in grass seed crops. An option is to screen for selective herbicides with the use of logarithmic spraying technology. The aim of this paper is to assess selectivity of various herbicides in grass seed crops by using dose–response curves. RESULTS: Six grass species were subjected to logarithmic spraying with 11 herbicides and with Poa pratensis L. as a weed. The ratio between the doses that caused 10% of damage to the crop and 90% of damage to the weed was used as a selectivity index. Compounds with selectivity indices above 2 can be safely used in a crop. The two ACCase herbicides clodinafop‐propargyl and fenoxaprop‐P‐ethyl and a mixture of the two ALS herbicides mesosulfuron and iodosulfuron could be used selectively to control P. pratensis in Festuca rubra L., although the selectivity indices in no instances were greater than the desired 2.0. CONCLUSION: The logarithmic sprayer can be a rapid screening tool for identifying compounds with favourable selectivity indices. Good experimental design is needed to alleviate rates being systematically distributed and confounded with growth rate and soil fertility gradients. Copyright © 2009 Society of Chemical Industry     

Using phenology prediction in weed management: a review

The success of weed management based on ecological principles and weed biology will depend on a better understanding of the effect of environment on lift history strategies, growth, and competition of weeds; and crops, and particularly upon the ability to predict weed and crop phenology, This paper reviews the importance of phenotypic plasticity to weed and crop competition and other biological interactions. We also discuss the utility of phenological predictions in weed management and review current weed phenology models that are based on thermal time. By understanding the variables that drive plant phenotypic responses, new approaches and more long-term solutions for weed problems can be developed.     

Effects of soil and crop management practices and pedo‐hydrological conditions on the seedbank size of Galinsoga spp. in organic vegetable fields

Galinsoga quadriradiata and Galinsoga parviflora are very troublesome weeds in many organic vegetable crops in Europe. A very straightforward method to keep Galinsoga infestations under control is by targeting the Galinsoga seedbank. To identify cropping systems able to reduce the seedbank size in vegetable‐based cropping systems, the relationships between the seedbank size of Galinsoga species and prevailing soil/crop management practices and pedo‐hydrological conditions were investigated. Hereto, the seedbank of the 0–20 cm topsoil layer was sampled in 50 organic vegetable fields and analysed according to the seedling emergence method. Field history data were collected for the past 5 years, and physical, chemical and microbial soil quality was determined. Galinsoga quadriradiata was the most frequent and abundant Galinsoga species in the weed seedbank. The genus Galinsoga was present in 90% of the soil weed seedbanks of organic vegetable fields but displayed wide variation in abundance. Smallest Galinsoga seedbanks were found in fields that were predominantly tilled with non‐inversion implements or rotationally ploughed, and continuously cropped with competitive crops during the entire growing season (April 15‐November 15). Contrary to G. quadriradiata, seedbank size of G. parviflora was closely related to soil organic carbon content and sand fraction. Remarkably, soils with a low level of easily plant‐available phosphorus and concomitant high activity of arbuscular mycorrhizae had smaller G. quadriradiata seedbanks. To reduce Galinsoga infestations, fields should preferably be tilled without soil inversion, fertilised with organic amendments with low content of readily plant‐available phosphorus and cropped with competitive crops all season long.     

Integration of soil, crop and weed management in low-external-input farming systems

Greater adoption and refinement of low-external-input (LEI) farming systems have been proposed as ways to ameliorate economic, environmental and health problems associated with conventional farming systems. Organic soil amendments and crop diversification are basic components of LEI systems. Weed scientists can improve the use of these practices for weed management by improving knowledge of four relevant ecological mechanisms. First, multispecies crop rotations, intercrops and cover crops may reduce opportunities for weed growth and regeneration through resource competition and niche disruption. Secondly, weed species appear to be more susceptible to phytotoxic effects of crop residues and other organic soil amendments than crop species, possibly because of differences in seed mass. Thirdly, delayed patterns of N availability in LEI systems may favour large-seeded crops over small-seeded weeds. Finally, additions of organic materials can change the incidence and severity of soil-borne diseases affecting weeds and crops. Our research on LEI sweetcorn and potato production systems in central and northern Maine (USA) suggests that these mechanisms can reduce weed density and growth while maintaining crop yields. Low-external-input farming systems will advance most quickly through the application of interdisciplinary research focused on these and other ecological mechanisms.     

Weed management challenges in small-holder potato systems in Bhutan

Over the past four decades potato has emerged as the most important cash crop for Bhutanese mountain farmers. Farmers' feedback and field observations were used to describe weed populations, weed management and weed-production environment-yield interactions in small-holder potato production systems. Weed species across a range of locations in order of declining importance are: Persicaria runcinata , Galium aparine , Fagopyrum dibotrys , Digitaria ciliaris , Commelina maculata , Galinsoga parviflora and Persicaria nepalensis. The same species were common in the traditional buckwheat and maize production systems as well as the more recently established potato production systems. Limited associations between soil properties, management practices, weed species abundance and potato yield were observed. Species emerging as potential problem weeds, especially P. runcinata and F. dibotrys , are little known outside the Himalayan region. The increase in P. runcinata , reported by 42% of the respondents, was attributed to the use of metribuzin, cutting of rhizomes by plough or spade, high inputs of fertiliser and continuous cultivation of potato by 61%, 17%, 9%, 9% and 4% of respondents respectively. Metribuzin application reduced growth of P. runcinata but could not fully control it. Metribuzin is an effective and affordable weed management tool for small-holder potato farmers, but perennial weed species need further research, combining mechanical and chemical methods with cropping strategies.     

Paraquat and sustainable agriculture

Sustainable agriculture is essential for man's survival, especially given our rapidly increasing population. Expansion of agriculture into remaining areas of natural vegetation is undesirable, as this would reduce biodiversity on the planet. Maintaining or indeed improving crop yields on existing farmed land, whether on a smallholder scale or on larger farms, is thus necessary. One of the limiting factors is often weed control; biological control of weeds is generally of limited use and mechanical control is either often difficult with machinery or very laborious by hand. Thus the use of herbicides has become very important. Minimum cultivation can also be important, as it reduces the power required to work the soil, limits erosion and helps to maintain the organic matter content of the soil. This last aspect helps preserve both the structure of soil and its populations of organisms, and also sustains the Earth's soil as a massive sink for carbon, an important consideration in the light of global warming. The introduction of the bipyridinium herbicide paraquat in the early 1960s greatly facilitated weed control in many crops. Paraquat has the unusual property of being active only by direct spray onto plants and not by uptake from soil in which strong binding deactivates it. Together with its rapid action in light in killing green plant tissue, such properties allow paraquat to be used in many crops, including those grown by low-tillage methods. This paper reviews the ways in which agricultural systems have been and are being developed to make use of these properties, and provides a risk/benefit analysis of the world-wide use of paraquat over nearly 40 years.     

Improved management of Avena ludoviciana and Phalaris paradoxa with more densely sown wheat and less herbicide

Summary The effectiveness of crop competition for better weed control and reducing herbicide rates was determined for Avena ludoviciana and Phalaris paradoxa . Four experiments, previously broadcast with seeds of the two weeds in separate plots, were sown with three wheat densities, and emerged weeds were treated with four herbicide doses (0–100% of recommended rate). The measured crop and weed traits were first analysed across experiments for treatment effects. Grain yield and weed seed production data were then analysed using cubic smoothing splines to model the response surfaces. Although herbicide rate for both weeds and crop density for P. paradoxa had significant linear effects on yield, there was a significant non-linearity of the response surface. Similarly, herbicide rate and crop density had significant linear effects on weed seed production, and there was significant non-linearity of the response surface that differed for the weed species. Maximum crop yield and reduction in seed production of P. paradoxa was achieved with approximately 80 wheat plants m⁻² and weeds treated with 100% herbicide rate. For A. ludoviciana , this was 130 wheat plants m⁻² applied with 75% herbicide rate. Alternatively, these benefits were achieved by increasing crop density to 150 plants m⁻² applied with 50% herbicide rate. At high crop density, application of the 100% herbicide rate tended to reduce yield, particularly with the A. ludoviciana herbicide, and this impacted adversely on the suppression of weed seed production. Thus, more competitive wheat crops have the potential for improving weed control and reducing herbicide rates.     

Demography of Anoda cristata in wide- and narrow-row soyabean

Summary The annual weed Anoda cristata is recognized as a problem of increasing importance in soyabean crops. As seeds are the only source for the renewal of A. cristata populations, knowledge of its seedbank characteristics and seed production is useful in improving weed management decisions. There is a lack of information about the effect of the planting pattern of soyabean on weed population dynamics. The effects of 70-cm and 35-cm soyabean row spacings on A. cristata seedbank, seedling recruitment and mortality, plant biomass, seed production and seed shed were determined. Soyabean density was higher in 35-cm row spacings as an increase in planting rate in narrow-row soyabean is recommended for producers in Argentina. The different planting patterns modified A. cristata demography through changes in plant biomass and, subsequently, in seed production. Seedling emergence, plant mortality and the pattern and duration of seed shed were similar in both planting patterns and thus independent of crop spatial arrangement and density. Increments in A. cristata seedbank can be reduced by planting soyabean at narrower row spacings and by bringing forward soyabean harvest. The first practice reduces A. cristata biomass and consequently seed production, and the second practice decreases the amount of seeds contributing to the soil seedbank.     

Composition of floating weed seeds in lowland rice fields in China and the effects of irrigation frequency and previous crops

The diversity and composition of floating weed seed communities were surveyed in 27 sites across the main rice-growing regions in China with the aim of better understanding weed seed dispersal via irrigation water. Seed of 74 species, belonging to 20 families, were identified from floating matter on the water surface in lowland rice fields. Thirty-five species from three families: Poaceae (15), Asteraceae (11), and Polygonaceae (9), accounted for 47% of all species identified. Species with seed maturing in the summer accounted for 64% of the weed seed and their mean relative abundance was 0.74. Species richness, Shannon–Wiener index and Pielou evenness index were significantly different among the floating weed seed communities. The diversity of weed seed communities in the Yangtze river valley was higher than that in other sites, and some sites were dominated by only a few weed species, such as Beckmannia syzigachne , Alopecurus aequalis , A. japonicus , and Polypogon fugax. At all sites, the dominant weed seeds reflected the dominant weed species in the previous crop. The 27 sample sites of weed seed communities can be clustered into two groups on the basis of previous crop, either lowland rice or sites with previous crops of winter fallow, winter wheat or oilseed rape. Canonical correspondence analysis (CCA) revealed that irrigation frequency, previous crop, and latitude, but not soil type or longitude, significantly affected species composition. The numbers of floating weed seed species were high in lowland rice fields; composition was affected by previous crops and irrigation frequency. Filtering irrigation water and collecting and removing floating weed seeds from the water surface could be integrated into weed management practices to control weeds in lowland rice fields.     

Technical performance of some commercial glyphosate-resistant crops

Glyphosate-resistant (GR) crops have been sold commercially in the USA since 1996. The use of glyphosate alone or with conventional pre- and post-emergence herbicides with different modes of action gives growers many options for affordable, safe, easy, effective wide-spectrum weed control. Despite the overwhelming popularity of this technology, technical issues have surfaced from time to time as US growers adopt these crops for use on their farms. The types of concern raised by growers vary from year to year depending on the crop and the environment, but include perceptions of increased sensitivity to diseases, increased fruit abortion, reduced pollination efficiency, increased sensitivity to environmental stress, and differences in yield and agronomic characteristics between transgenic and sister conventional varieties. Although several glyphosate-resistant crops are commercially available, maize, soybean and cotton constitute the largest cultivated acreage and have likewise been associated with the highest number of technical concerns. Because glyphosate is rapidly translocated to and accumulates in metabolic sink tissues, reproductive tissues and roots are particularly vulnerable. Increased sensitivity to glyphosate in reproductive tissues has been documented in both glyphosate-resistant cotton and maize, and results in reduced pollen production and viability, or increased fruit abortion. Glyphosate treatments have the potential to affect relationships between the GR crop, plant pathogens, plant pests and symbiotic micro-organisms, although management practices can also have a large impact. Despite these potential technical concerns, this technology remains popular, and is a highly useful tool for weed control in modern crop production.     

Weed species composition of maize fields in Germany is influenced by site and crop sequence

During the last decade, maize has become the crop with the second largest acreage in Germany. Therefore, agricultural advisors and the plant protection sector are interested in an overview of the weed species composition in maize fields, their determining factors and trends. From 2001 to 2009, a weed survey was conducted in 1460 maize fields throughout Germany. Data on crop management and soil characteristics were collected via farmer questionnaires. Principal component analysis and redundancy analysis were used to analyse patterns in weed species composition. The late spring and summer germinating species Chenopodium spp., Echinochloa crus‐galli and Solanum nigrum occurred with high densities and frequencies, but their occurrence was determined by different factors. Other frequent weed species were those that typically accompany autumn‐sown crops. The variation in species composition was significantly related to environmental factors (9.1% explained variance), particularly geographical latitude and precipitation, and management factors (4.7% explained variance), particularly crop sequence. The relative importance of these factors seems universal, when compared with surveys in other crops and regions. The factor ‘year’ was of minor importance (0.9% explained variance). Over the 9‐year period, no changes in weed species composition could be determined. The results suggest that despite the limited impact of crop management on weed species composition, farmers can use crop sequence to suppress individual species. The survey furthermore sets a baseline against which future changes can be measured in a landscape of rapidly changing agricultural land use.     

Economic impacts of glyphosate-resistant crops

Glyphosate-resistant crops have been widely planted since their introduction in 1996. Growers have numerous choices for herbicide treatments and have chosen to plant glyphosate-resistant crops on the basis of economic factors. The economic effects of the widespread planting of glyphosate-resistant crops have included reductions in herbicide expenses, increases in seed costs, increased yield and changes in the relative profitability of crops that has resulted in changes in which crops are planted. In addition, non-pecuniary benefits have accrued as a result of the simplicity of weed management in the glyphosate-resistant crop systems.     

Glyphosate sustainability in South American cropping systems

South America represents about 12% of the global land area, and Brazil roughly corresponds to 47% of that. The major sustainable agricultural system in South America is based on a no-tillage cropping system, which is a worldwide adopted agricultural conservation system. Societal benefits of conservation systems in agriculture include greater use of conservation tillage, which reduces soil erosion and associated loading of pesticides, nutrients and sediments into the environment. However, overreliance on glyphosate and simpler cropping systems has resulted in the selection of tolerant weed species through weed shifts (WSs) and evolution of herbicide-resistant weed (HRW) biotypes to glyphosate. It is a challenge in South America to design herbicide- and non-herbicide-based strategies that effectively delay and/or manage evolution of HRWs and WSs to weeds tolerant to glyphosate in cropping systems based on recurrent glyphosate application, such as those used with glyphosate-resistant soybeans. The objectives of this paper are (i) to provide an overview of some factors that influence WSs and HRWs to glyphosate in South America, especially in Brazil, Argentina and Paraguay soybean cropped areas; (ii) to discuss the viability of using crop rotation and/or cover crops that might be integrated with forage crops in an economically and environmentally sustainable system; and (iii) to summarize the results of a survey of the perceptions of Brazilian farmers to problems with WSs and HRWs to glyphosate, and the level of adoption of good agricultural practices in order to prevent or manage it.     

Development of arable weed seedbanks during the 6 years after the change from conventional to organic farming

All of the arable land of a farm in southern Germany was investigated during an 8-year period, starting 2 years before and ending 6 years after the change to organic farming. The first 3 years after the conversion, the total seed number in the soil increased from 4050 to 17 320 m⁻². From the fourth to the sixth year, it dropped back to 10 220 m⁻². The number of seeds increased particularly at sites with a low crop cover and a high density of weed plants at the soil surface. The increase predominantly occurring on the more fertile soils may have been caused by the rotation commencing with less competitive crops. Crops which increased the seedbank by 30–40% were winter cereals, sunflowers and lupins. Potatoes and sown fallow caused no significant change and grass–clover mixtures even reduced the number of seeds by 39%. Among 44 species occurring frequently enough for statistical analysis, 31 increased and only 3 decreased. The change of management particularly increased summer annual, perennial and dicotyledonous weeds. This can be attributed to both operations which are characteristic of organic farming (e.g. replacing herbicide applications by mechanical weed control) and to general modifications of the management practice which may also occur in non-organic farming systems (increasing the percentage of broad leaved and spring sown crops in crop rotation). The present study confirms investigations into the aboveground vegetation that indicate that arable organic farming favours plant species diversity and provides evidence that the conversion need not encourage the dominance of a few noxious weeds.