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Search Results: 1 - 10 of 34411 matches for " Rene Van Acker "
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Direct versus Indirect Herbicide Effects: The Example of Preplant Incorporated Halosulfuron  [PDF]
Zhenyi Li, Rene Van Acker, Nader Soltani, Peter H. Sikkema
American Journal of Plant Sciences (AJPS) , 2017, DOI: 10.4236/ajps.2017.84044
Abstract: The potential indirect effects of soil applied herbicides have not been adequately explored. The objective of this study was to explore the potential and relative importance of the indirect effects of halosulfuron applied preplant incorporated (PPI) on several weed species common in white bean fields in Ontario. Halosulfuron applied PPI at 35 g ai ha-1 delayed common lambsquarters and wild mustard emergence and reduced their densities over 70%. It did not delay green foxtail emergence, but the density was decreased over 30%. Indirect herbicidal effects such as delayed emergence timing may be important for weed management in non-competitive crops such as white bean. Based on these results, the indirect effects of soil applied herbicides are real and are potentially important for providing additional prevention of yield loss in field crops. This may be particularly important for relatively non-competitive crops such as white bean and for reducing weed seed return to the seedbank.
Modelling Soil Water Retention for Weed Seed Germination Sensitivity to Water Potential
W. John Bullied,Paul R. Bullock,Rene C. Van Acker
Applied and Environmental Soil Science , 2012, DOI: 10.1155/2012/812561
Abstract: Soil water retention is important for the study of water availability to germinating weed seeds. Six soil water retention models (Campbell, Brooks-Corey, four- and five-parameter van Genuchten, Tani, and Russo) with residual soil water parameter derivations were evaluated to describe water retention for weed seed germination at minimum threshold soil water potential for three hillslope positions. The Campbell, Brooks-Corey, and four-parameter van Genuchten model with modified or estimated forms of the residual parameter had superior but similar data fit. The Campbell model underestimated water retention at a potential less than ?0.5?MPa for the upper hillslope that could result in underestimating seed germination. The Tani and Russo models overestimated water retention at a potential less than ?0.1?MPa for all hillslope positions. Model selection and residual parameter specification are important for weed seed germination by representing water retention at the level of minimum threshold water potential for germination. Weed seed germination models driven by the hydrothermal soil environment rely on the best-fitting soil water retention model to produce dynamic predictions of seed germination. 1. Introduction The soil water retention characteristic (SWRC) is a basic hydrophysical property of the soil that relates the water content of soil water to its energy state [1]. The water content-potential function is fundamental to the characterization of water holding capacity, water retention, and water flow in soil [2, 3]. The SWRC is necessary for modelling fluxes in soil water and is needed for germination studies where soil water is measured on a content basis. The timing of seed germination is a function of soil water potential [4]. As the soil dries, soil water potential is reduced, and it becomes increasingly difficult for seeds to imbibe water. At the minimum threshold (base) water potential, seeds do not imbibe sufficient water to initiate embryo growth and complete the germination process. The minimum threshold water potential at which germination ceases to occur in many agricultural weeds ranges from ?0.1 to ?1.5?MPa [5, 6]. Accurate representation of the SWRC over a wide range of water potential minimum thresholds is required for predictive modelling of seed germination. One of the greatest challenges in characterizing the SWRC for the shallow depth of the seedling recruitment zone (soil layer from which seeds germinate and emerge) across field topography is obtaining the parameters of the soil hydrological property. Determining the SWRC by direct
Modelling the Dynamics of Feral Alfalfa Populations and Its Management Implications
Muthukumar V. Bagavathiannan, Graham S. Begg, Robert H. Gulden, Rene C. Van Acker
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0039440
Abstract: Background Feral populations of cultivated crops can pose challenges to novel trait confinement within agricultural landscapes. Simulation models can be helpful in investigating the underlying dynamics of feral populations and determining suitable management options. Methodology/Principal Findings We developed a stage-structured matrix population model for roadside feral alfalfa populations occurring in southern Manitoba, Canada. The model accounted for the existence of density-dependence and recruitment subsidy in feral populations. We used the model to investigate the long-term dynamics of feral alfalfa populations, and to evaluate the effectiveness of simulated management strategies such as herbicide application and mowing in controlling feral alfalfa. Results suggest that alfalfa populations occurring in roadside habitats can be persistent and less likely to go extinct under current roadverge management scenarios. Management attempts focused on controlling adult plants alone can be counterproductive due to the presence of density-dependent effects. Targeted herbicide application, which can achieve complete control of seedlings, rosettes and established plants, will be an effective strategy, but the seedbank population may contribute to new recruits. In regions where roadside mowing is regularly practiced, devising a timely mowing strategy (early- to mid-August for southern Manitoba), one that can totally prevent seed production, will be a feasible option for managing feral alfalfa populations. Conclusions/Significance Feral alfalfa populations can be persistent in roadside habitats. Timely mowing or regular targeted herbicide application will be effective in managing feral alfalfa populations and limit feral-population-mediated gene flow in alfalfa. However, in the context of novel trait confinement, the extent to which feral alfalfa populations need to be managed will be dictated by the tolerance levels established by specific production systems for specific traits. The modelling framework outlined in this paper could be applied to other perennial herbaceous plants with similar life-history characteristics.
A propòsit del treball comunitari, la participació ciutadana i els moviments socials. Pensar la cooperació i el desenvolupament en clau educativa
Helen van Acker
Temps d'Educació , 2000,
Exploring the Relationships between Land-use System and Travel Behaviour Concepts: Some First Findings
Veronique Van Acker,Frank Witlox
Solstice : Electronic Journal of Geography and Mathematics , 2005,
Abstract: The aim of this exploration is to try to summarise what is commonly accepted when analysing the relationships between land use and transportation. The interaction between land use and transportation composes the land-use/transportation system. A large research body exists on the impact of land-use systems on travel behaviour (for reviews, e.g., Handy, 2002; Stead and Marshall, 2001; Crane, 2000; Wegener and Fürst, 1999). In order to obtain a clear overview, a three-fold distinction has been made based on type of variable included. Thus, three dimensions in travel behaviour research have been found: (i) a spatial dimension, (ii) a socio-economic dimension, and (iii) a behavioural dimension. Less is known about the reverse relationship, i.e., the impact of the transportation system on location decisions of households and firms (the land-use system). The greater part of this research utilizes “accessibility” as an intermediate concept to measure the influence of the transportation system on the land-use system. The presented literature review enabled us to detect some gaps in the knowledge on the land-use/transportation system. Understanding the interaction between land use and travel behaviour involves having (i) data on land-use patterns; on the socio-economic background of individuals; and on their attitudes, perceptions and preferences toward land use and travel; and (ii) a methodology dealing with potential multiple directions of causality. The first issue can be achieved by combining empirical, revealed, and stated-preference research. The second methodological question can be solved using structural equation modelling (SEM). This is a modelling technique which can handle a large number of independent and dependent variables, as well as multiple directions of causality. These characteristics of SEM seem useful in order to obtain an improved insight in the complex nature of travel behaviour. The link to the fulltext on a current website is given below. Should that link fail to work, please go to this persistent URL and download the associated file for this issue. http://deepblue.lib.umich.edu/handle/2027.42/58219While you are there, take a look around and see if there are other issues and documents that are related to your interests!
War as exit from exclusion?: the formation of Mayi-Mayi militias in Eastern Congo
K. Vlassenroot,F. Van Acker
Afrika Focus , 2001,
Abstract: In eastern Congo scores of young and marginalized people have been increasingly attracted to the mobilising efforts of new local actors. The inquiry into this phenomenon traces the emergence of the first militias to the end of the eighties. These first generation militias were a result of the growing willingness of marginalized youngsters and school drop-outs to form groups of under-aged combatants acting against eveiy representative of modern political authority and against their desperate feelings of exclusion, for which both their political and social environment were held responsible. As they had nothing more to lose than their marginalisation, rebellion became an option, both as a survival strategy and as a strategy of self-defence against a predatory political and social order. The shiftiness of their ideological basis and allies only further proved what these first militias were about: a search for alternatives to a situation of acute deprivation. This article reveals that the present RCD rebellion that rages through the eastern parts of the Democratic Republic of Congo has had an escalating effect on the proliferation of new militias. Before, the Kabila-led AFDL rebellion had already offered the already existing militias a new cause: that of an anti-Tutsi force fighting against foreign occupation. In resistance to the effects of state collapse and armed foreign interventions, rural and urban youth today have combined former traditions and newly developed patterns of mobility in an interpretation of customary and national defence. This has not meant that they link up with the traditional emanations of authority. Rather, a crisis in the social fabric has meant a shift in authority towards these combatants and the use of violence. In addition, shared feelings of antipathy towards the 'Tutsi-aggressors ' have facilitated the creation of links between these diverse local groupings and other, foreign, factions of armed militia roaming the local countryside. Consolidation, however, remains unlikely as this shared ideology does not run very deep and alliances continuously change. The question remains what the future impact of these militias might be on the local social order. On the one hand, for several years now in some remote areas these armed groupings have become the only representatives of any authority structure, even if this structure is based on violence. Contrary to other cases, these militias in South Kivu are still closely linked to the rural population and have not turned against them. On the other hand, the dynamic of the Inter-Congolese D
A comparison of Web-based and face-to-face Functional Measurement experiments
Frederik Van Acker,Peter Theuns
Psicológica , 2010,
Abstract: Information Integration Theory (IIT) is concerned with how people combine information into an overall judgment. A method is hereby presented to perform Functional Measurement (FM) experiments, the methodological counterpart of IIT, on the Web. In a comparison of Web-based FM experiments, face-to-face experiments, and computer-based experiments in the lab it is found that the computer-based method is less sensitive to experimental manipulations. However, different integration rules can be distinguished. The inability to monitor motivation in the unproctored setting can partly explain this effect. Consequently it is argued that Web-based experiments enable the researcher to test larger groups which enable more in-depth analysis of individual differences using single subjects analyses and clustering methods.
Understanding modal choices for leisure activities. Is it just objectively determined ? Het begrijpen van de vervoerswijzekeuze voor vrijetijdsactiviteiten. Wordt dit enkel be nvloed door objectief meetbare variabelen ?
Kobe Boussauw,Frank Witlox,Veronique VanAcker
Belgeo : Revue Belge de Géographie , 2012,
Abstract: Most studies on the link between the built environment and modal choice characterize and model this relationship by objectively measureable characteristics such as density and diversity. Recently, within the debate on residential self-selection, attention has also been paid to the importance of subjective influences such as the individual’s perception of the built environment and his/her residential attitudes and preferences. Expanding the analysis to also include both objective and subjective characteristics at other model levels (i.e., not only stage of life characteristics but also personal lifestyles ; not only car availability but also travel attitudes, not only modal choice but also mode specific attitudes) is the purpose of this paper. To this end, a modal choice model for active leisure activities is developed using data on personal lifestyles and attitudes, collected via an Internet survey, and estimated using a path model consisting of a set of simultaneous estimated equations between observed variables. While controlling for subjective lifestyles and attitudes, the effects of the built environment and car availability on modal choice can be determined correctly and thus insights in self-selection mechanisms can be gained. De meeste studies over de interactie tussen de bebouwde omgeving en vervoerswijzekeuze gebruiken objectief meetbare eigenschappen zoals dichtheid en diversiteit om deze relatie te modelleren en analyseren. Recent wordt tevens aandacht besteed aan het belang van subjectieve invloeden zoals de percepties en voorkeuren van het individu met betrekking tot de bebouwde omgeving. Dit gebeurde vooral binnen het debat over residenti le zelfselectie, maar zelfselectie kan ook op andere punten optreden. Daarom wordt in dit artikel de analyse uitgebreid zodat subjectieve kenmerken ook op andere niveaus van het model voorkomen (bijvoorbeeld, niet alleen levensfase maar ook leefstijl, niet alleen autobezit maar ook algemene mobiliteitsattitudes, niet alleen vervoerswijzekeuze maar ook attitudes tegenover specifieke vervoerswijzen). Daartoe is een model voor vervoerswijzekeuze voor actieve vrije-tijdsbesteding geschat. Dit model werd ontwikkeld op basis van gegevens uit een internet-enquête over persoonlijke leefstijlen en attitudes, en werd geanalyseerd met behulp van een padmodel dat bestaat uit een reeks van gelijktijdig geschatte vergelijkingen tussen de geobserveerde variabelen. De resultaten verduidelijken dat het effect van de bebouwde omgeving en autobeschikbaarheid op de vervoerswijzekeuze pas correct ge nterpreteerd kunnen worden
Embedding Graduate Skills in Capstone Courses
Liz van Acker,Janis Mary Bailey
Asian Social Science , 2011, DOI: 10.5539/ass.v7n4p69
Abstract: The purpose of this article is to explore the literature on capstone courses generally ? and capstones in undergraduate business degrees in particular ? to determine the role of these courses in embedding graduate skills, with specific emphasis on a study conducted by the authors and colleagues. The paper concludes by providing five core principles for embedding generic skills as a way of improving the design, teaching and assessment of capstone courses.
Partial Crosstalk Cancellation for Upstream VDSL
Cendrillon Raphael,Moonen Marc,Ginis George,Van Acker Katleen
EURASIP Journal on Advances in Signal Processing , 2004,
Abstract: Crosstalk is a major problem in modern DSL systems such as VDSL. Many crosstalk cancellation techniques have been proposed to help mitigate crosstalk, but whilst they lead to impressive performance gains, their complexity grows with the square of the number of lines within a binder. In binder groups which can carry up to hundreds of lines, this complexity is outside the scope of current implementation. In this paper, we investigate partial crosstalk cancellation for upstream VDSL. The majority of the detrimental effects of crosstalk are typically limited to a small subset of lines and tones. Furthermore, significant crosstalk is often only seen from neighbouring pairs within the binder configuration. We present a number of algorithms which exploit these properties to reduce the complexity of crosstalk cancellation. These algorithms are shown to achieve the majority of the performance gains of full crosstalk cancellation with significantly reduced run-time complexity.
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