Weed Research
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List of Contributors xv
Preface xix
1 Weed Science Research: Past, Present and Future Perspectives 1
Robert J. Froud?]Williams
Introduction 1
Factors Influencing the Weed Flora 2
Succession 2
Clean Seed 3
Rotation 3
Fallow 4
Cultivation 5
Straw Burning 5
Soil Amelioration, Drainage and Fertiliser Use 5
Nitrogen 6
Herbicides 6
Consequences of Changing Practices 9
Changing Weed Floras 9
Episodic Decline 13
Weed Spatial Distribution 13
History of Weed Science in the UK and Origins of the Weed Research Organisation 14
Origins of the European Weed Research Society 17
Weed Research (Journal): Origin of Papers and Discipline 18
Changing Attitudes to Weeds 18
Set?]Aside and Agri?]Environment 19
Weeds, Climate and Invasive Aliens 20
Future Directions (Quo Vadis?) 21
Environmental Weed Management 21
Evolutionary Genetics and the Role of Molecular Ecology 22
Is there a Need for a Change of Emphasis? 22
Conclusion 23
Acknowledgements 24
References 24
2 Descriptive and Mechanistic Models of Crop-Weed Competition 33
Lammert Bastiaans and Jonathan Storkey
Introduction 33
Descriptive Models for Yield Loss Due to Weed Competition 34
The Hyperbolic Yield Loss-Weed Density Curve 34
Accounting for Differences in Relative Time of Emergence 36
Other Factors Influencing Parameter i 39
Management Aimed at Modifying Crop-Weed Competitive Relations 40
A Quantitative Characterisation of Differences in Weed?]Suppressive Ability of Crop Cultivars 45
Mechanistic Models for Crop-Weed Competition 46
Structure and Function of Process?]Based Models for Crop-Weed Competition 46
A First Application: Ideotyping of More Weed?]Suppressive Cultivars 50
A Second Application: Predicting the Impact of Climate Change on Weed Distribution 51
Conclusion 55
References 56
3 Approaches and Objectives of Arable Weed Species Mapping: Where Next? 61
Hansjörg Krähmer and Paolo Bàrberi
Weed Species Mapping: Why? 61
Scientific Literature: State of the Art 62
Mapping Herbicide?]resistant Biotypes 63
Mapping Invasive Species 63
Weed Species Mapping: Who? 65
Weed Species Mapping: Where and What? 66
Maps of Weeds in European Arable Crops 66
Field?]Level Mapping 71
Weed Species Mapping: How? 72
Geo?]Referencing 72
Timing of Assessment 74
Sampling Parameters 74
Documentation and Maps 74
What to Conclude from Weed Mapping Data? 75
Weed Mapping: Where to Go? 76
Acknowledgements 80
References 80
4 Seed Biology and Population Dynamics 85
Kirsten S. Tørresen, Laila M. Karlsson and Jose Luis Gonzalez?]Andujar
Introduction 85
Seed Biology 86
Seed Production and Dispersal 86
Seed?]Bank 88
Germination and Dormancy 90
Germination 90
Dormancy 91
Sprouting from Vegetative Plant Parts 96
Predicting Seedling Emergence 97
Empirical Models 97
Mechanistic Models 97
Challenges in Predicting Emergence 98
Importance for Weed Control 99
Population Dynamics 100
Dynamics in Time and Space 100
Modelling 100
Non?]Spatial Models 101
Spatial Models 103
Practical Applications in Weed Science 103
Evaluation of Management Systems 103
Decision Support Systems 104
Challenges in Modelling Population Dynamics 104
Future Prospects 104
Conclusion 105
Acknowledgements 106
References 106
5 Weeds and Biodiversity 115
Bärbel Gerowitt, Paolo Bàrberi, Henri Darmency, Sandrine Petit, Jonathan Storkey and Paula Westerman
Introduction 115
Arable Weeds in the Context of Biodiversity 116
Functional Biodiversity 116
Agronomic Services and Disservices Associated with Weeds 117
Genetic Diversity in Weeds 117
How to Measure Genetic Diversity 119
At Which Scale Can Genetic Diversity Be Described? 120
Why Is It Important to Understand Weed Genetic Diversity? 121
Rare Weed Species as Objects of Conservation 122
Drivers of Arable Weed Declines 123
The Rare Weed Trait Syndrome 124
Conserving Rare Weed Communities 124
Weeds in Food Chains of Arable Systems 124
Factors Influencing Seed?]Based Food Webs in Agroecosystems 126
Weed Seed Production 126
Within?]Season Temporal Variability 126
Between?]Season Temporal Variability 126
Spatial Variability 127
Seed Morphology and Chemistry 127
Weed Diversity 127
Current Status of Seed?]Based Food Webs on Farms and Management Options 127
Diversity of Weeds and Arable Management 129
Site Conditions of Arable Fields Filter for Weed Communities 129
Methods to Identify and Separate the Influence of Arable Site and Arable Management on Weed Diversity 130
Arable Management Determines Weed Diversity 131
Weed Diversity Versus Weed Abundance 131
Diversity in Weeds Facilitates Management Options 132
Diversity of Weeds in a Landscape Context 133
The Landscape Context of Weeds 133
Conducting Landscape?]Scale Weed Studies 134
Landscape Effects on Weed Biodiversity: Empirical Evidence 135
Biodiversity of Weeds and Public Interest 136
Field Margin Programmes 136
Encouraging Weed Diversity in Farming 136
Conclusions and Perspectives 137
References 138
6 Optimising Herbicide Performance 149
Per Kudsk
Introduction 149
Herbicide Classification 150
Optimising Herbicide Performance: How to Study It 151
Biotic Factors 154
Weed Flora 154
Weed Growth Stage 156
Crop Competition 157
Abiotic Factors 158
Soil Texture 158
Climatic Conditions 159
Light 159
Temperature 160
Humidity 161
Precipitation 162
Soil moisture 163
Wind 164
Concluding Remarks 164
Application Technique 165
Adjuvants 166
Mixtures with Other Herbicides 168
Concluding Remarks and Future Challenges 170
References 172
7 Herbicide Resistance in Weeds 181
Stephen Moss
Historical Perspective 181
What Is Herbicide Resistance? 182
The Worldwide Occurrence of Resistant Weeds 183
Herbicide Mode of Action and Risk of Resistance 185
Resistance Mechanisms 188
Target?]Site Resistance 188
PSII (Triazines) 189
ALS Inhibitors 190
ACCase Inhibitors 190
Other Herbicide Classes 191
Non?]Target?]Site Resistance 191
Reduced Herbicide Uptake 193
Reduced Herbicide Translocation 193
Enhanced Herbicide Metabolism 194
Evolution of Herbicide Resistance 194
Initial Frequency of the Resistance Trait and Size of Weed Population 195
Genetic Basis of Resistance 197
Selection Pressure 199
Frequency of Herbicide Use 199
Persistence of the Herbicide and Pattern of Weed Emergence 199
Intrinsic Activity of the Herbicide and Degree of Resistance Conferred by the Resistance Mechanism(s) 200
Specificity of the Herbicide: Number of Species the Herbicide Controls 201
Seed Bank in the Soil 201
Resistance Risk 201
Prevention and Management of Herbicide Resistance 203
Detection of Resistance in the Field 203
Integrated Weed Management 203
Non?]Chemical Control Methods 204
Herbicidal Control 204
Alternative Herbicides 204
Mixtures, Sequences and Rotations 205
Managing Resistance in Alopecurus Myosuroides (Black?]grass): A Case Study 205
Farmer Psychology: An Under?]Recognised Component of Resistance Management 206
Conclusion 209
References 209
8 Weed Biological Control 215
Richard H. Shaw and Paul E. Hatcher
Introduction 215
Definitions of Weed Biocontrol 217
Biocontrol of Weeds in European Extensive Agriculture 218
Cirsium Arvense 219
Rumex Species 221
Biocontrol of Weeds in Intensive Agriculture 222
Biocontrol of Non?]native Weeds 224
Ambrosia 228
In Summary 230
Combining Biocontrol with Other Weed Control Techniques 230
Combining with Other Non?]Chemical Control Methods 231
Combination with Herbicides 232
Arthropod Biocontrol Agents 232
Fungal Biocontrol Agents 233
Legislation, Responsibilities and Drivers 234
Arthropods 234
Fungi 235
Conclusion 235
References 236
9 Non?]Chemical Weed Management 245
Bo Melander, Matt Liebman, Adam S. Davis, Eric R. Gallandt, Paolo Bàrberi, Anna?]Camilla Moonen, Jesper Rasmussen, Rommie van der Weide and Francesco Vidotto
Introduction 245
Preventive and Cultural Weed Control 246
Objectives, Principles and Practices 247
Objective 1: Reduce Weed Density 247
Objective 2: Reduce Damage Per Surviving Weed 248
Objective 3: Prevent Undesirable Shifts in Weed Community Composition 249
Current Adoption and Challenges 250
Cover Crops and Mulches 250
Mechanisms of Cover Crop-Weed Interactions 251
Challenges for Research 252
Mechanical Weed Control 253
How It Works 256
Shortcomings 257
Challenges for Research 258
Thermal Weed Control 259
Thermal Weed Control in Practice 262
Challenges for Research 263
Conclusion 263
References 264
10 Invasive Plants 271
Christian Bohren
Introduction 271
Why Do Invasive Plants Symbolise such a Threat? 271
Invasive Weeds and Human Health 271
Ambrosia 272
Giant Hogweed 273
Weedy Crops, Super Weeds and Mimetic Weeds 274
Invasive Aquatic Weeds 275
Human Intervention 276
Human Curiosity 276
Reasons for Increased Occurrence of Invasive Weeds 276
Responsibility 277
Scientific Prioritisation 278
Popular Prioritisation 278
Implementation 279
Facts Concerning Plant Invasion 280
The Early Beginnings 280
Changing Land Use and Fishery 281
Rapid Adaptation 282
Weeds, Invasives and Climate Change 282
What Makes Plant Invaders so Successful? 283
Can We Predict Plant Invasions? 284
What Has Been Done so Far? 285
Databases 285
European Initiative 285
European Food Safety Agency (EFSA) 288
Euphresco 288
SMARTER 288
Role of the EWRS Invasive Plants Working Group 289
Mission 289
Working Group Activities 289
Ponta Delgada, Azores, Portugal, 2006 289
Osijek, Croatia, 2008 290
Ascona, Switzerland, 2011 290
Montpellier, France, 2014 290
EPPO Trabzon 291
NEOBIOTA 291
Aquatic Weeds 291
Definitions and Plant Lists 291
Definitions 291
Weed 292
Invasive Plant 292
Plant Invader 294
Invasion Trajectory 294
Invasive Species Lists 294
Control Strategies for Invasive Weeds 294
Biological Control Versus Conventional Control 294
Learning to Control Invasions 298
Social and Economic Aspects 300
Anthriscus 300
Japanese Knotweed 300
Bracken 301
Ambrosia 302
Strategies 302
Prevention 302
Early Detection 302
Rapid Response 302
Pest Risk Assessment 303
Species?]Specific Control 303
Conclusion 303
References 306
11 Parasitic Weeds 313
Maurizio Vurro, Alejandro Pérez?] de?]Luque and Hanan Eizenberg
Introduction 313
Classification 315
Orobanchaceae (Broomrape Family) 315
Cuscuta 315
Life?]Cycle 316
Broomrapes 316
Dodder 317
Distribution at the European Level, Host Range and Yield Losses 318
Management Strategies 325
Management and Control 325
Biological Control 325
Natural Products 328
Strigolactones and Other Germination Stimulants 329
Nanotechnological Approaches 332
Genetic Resistance 334
Defensive Mechanisms 335
Novel Genetic Approaches 337
Chemical Control of Broomrapes 337
Herbicide?]Resistant Crops for Broomrape Control 340
Developing Models for Optimising Chemical Control of Root Parasitic Weeds 341
Precision Agriculture 342
Conclusion 346
References 346
12 Weed Management Systems in Vegetables 355
Francesco Tei and Euro Pannacci
Introduction 355
Weed Flora 357
Weed-Vegetable Crop Interactions 358
Integrated Weed Management 365
Preventive Measures 366
Cultural Methods 366
Crop Rotation 366
Cover?]crops 367
Stale Seed?]Bed Preparation 368
Cultivar Selection 368
Planting Method, Planting Pattern, Row Spacing and Crop Density 368
Physical Weed Control 368
Non?]Living Mulches 369
Solarisation 369
Flaming 369
Steaming 370
Mechanical Weed Control 370
Hand?]Weeding 371
Biological Weed Control 371
Chemical Weed Control 371
Conclusions and Perspectives 377
References 380
13 Perennial Weeds 389
Paul E. Hatcher
Introduction 389
Perennating Structures 390
Fragmentation, Nutrient Reserves and Regrowth 391
Dormancy of Vegetative Structures 392
Grassland Perennials 392
Perennials in Organic Arable Systems 394
Perennials of Southern European Agriculture 396
Cyperus Species 397
Sorghum Halepense 398
Bracken 399
Conclusion: Perennial Weeds in the Future 401
Climate Change 401
Reduced Tillage 402
References 403
Index 000
1
Weed Science Research: Past, Present and Future Perspectives
Robert J. Froud-Williams
'Russets', Harwell, Oxon, UK
Introduction
Plants popularly referred to as weeds have been described by Sir E.J. Russell (1958) as 'The ancient enemy'. In his text on agricultural botany, Sir John Percival (1936) made the observation that the idea of uselessness was always present in the mind when weeds are being spoken of, while, in the editor's preface to Weeds and Aliens by Sir Edward Salisbury (1961), weeds are likened to criminals - when not engaged in their nefarious activities both may have admirable qualities: 'an aggressive weed in one environment may be a charming wild flower in another'. Our relationship with weeds certainly is as old as agriculture itself and the concept of weediness was recognised from biblical abstracts, for example the gospel according to St Matthew (Ch. 13 v. 7, the parable of the sower): 'Other seed fell among thorns, which grew up and choked them'. Yet weed science as a discipline is less than one hundred years old, albeit Fitzherbert (1523) in his Complete Boke of Husbandry recognised the injurious effect of weeds on crop production: 'Weeds that doth moche harme' included kedlokes, coceledrake, darnolde, gouldes, dodder, haudoddes, mathe, dogfennel, ter, thystles, dockes and nettylles'. These are recognised today as corncockle, charlock, darnel, corn marigold, dodder, cornflower, mayweed, stinking mayweed, fumitory, thistles, docks and nettles, several of which are now greatly diminished in abundance.
A major development in weed removal from within crops was achieved with the development of the seed drill by Jethro Tull c. 1701. Initially, the objective of this invention was to enable cereals to be sown in rows, whereby a horse-drawn hoe could be used to pulverise the soil in the inter-row. Tull conjectured that such 'pulverisation' would release nutrients beneficial to the crop, but coincidentally enabled weed removal, whereby 'horse-hoeing husbandry' became standard practice, reducing weed competition and the necessity of fallow, a serendipitous discovery.
Despite the efficacy of technological advances in weed control, weeds still exert great potential to reduce crop yields. Weeds are considered the major cause of yield loss in five crops (wheat, rice, maize, potato and soybean and a close second in cotton) (Oerke, 2006). Estimated potential losses due to weeds in the absence of herbicides were 23, 37, 40, 30, 37 and 36% for the six crops respectively, while weed control reduced these losses to 7.7, 10.2, 10.5, 8.3, 7.5 and 8.6%, albeit with considerable regional variation (Oerke, 2006). Efficacy of crop protection practices varied between geographic regions, but whereas efficacy of disease and pest control was only 32 and 39% respectively, efficacy of weed control was almost 75%. The greater efficacy of weed control was attributed to the ability to employ both physical and chemical methods. Possible reasons for the apparent mismatch between weed control efficacy and actual yield losses were ascribed to changing cultural practices such as monoculture, multiple cropping, reduced rotation and tillage and the introduction of more vulnerable crop cultivars dependent on increased fertilisation.
Weeds have a major impact on human activities for not only do they adversely affect economic crop yield indirectly through interspecific competition (see Bastiaans & Storkey, Chapter 2) directly as a result of parasitism (see Vurro et al., Chapter 11) and allelopathy, but also they affect human health and the well-being of livestock through physical and chemical toxicity. Additionally they may negatively impact environmental quality and functionality, such as that posed by alien invasive species including aquatic weeds (see Bohren, Chapter 10).
The objective of this preliminary chapter is one of scene setting. It seeks to associate 'man's' controversy with weeds as a consequence of their detrimental as well as beneficial relationships. Our changing perception of weeds is examined in terms of a shift in emphasis from that of pragmatic weed destruction to one of management and rational justification for their suppression.
Agronomic practices greatly influence weed population dynamics and these are outlined with particular attention to the UK weed floras. The history of weed science is explored as a discipline, together with a brief history of weed control technology including the discovery and development of synthetic herbicides. The origins of the Weed Research Organization (WRO) are discussed, together with the subsequent formation of the European Weed Research Society.
Weed science as a discipline originated at Rothamsted in England, the first agricultural research institute to be established in the world, with the pioneering work of Winifred Brenchley on the classic long-term continuous winter wheat experiment, Broadbalk, where she investigated the impact of various agronomic factors such as manuring, liming and fallow on the arable weed flora.
Factors Influencing the Weed Flora
Succession
The British flora is not an event, but a process that is continuing both with respect to accretions and diminutions (Salisbury, 1961). Vegetation is never static and weed populations are probably subject to greatest fluctuation as their habitat is continually disturbed. Two types of change within plant communities may be recognised: fluctuating and successional. Arable plant communities are subject to fluctuations as a consequence of direct intervention. Weeds are fugitives of ecological succession; were it not for the activities of man they would be doomed to local extinction and relegated to naturally disturbed habitats such as dune and scree. Weeds have been described as the pioneers of secondary succession, of which the weedy arable field is a special case (Bunting, 1960).
Successional change is less likely within ephemeral communities, although potentially capable in systems of prolonged monoculture and non-tillage. Two types of successional change may be recognised - autogenic and allogenic. Autogenic succession occurs in response to changes within the habitat, as species better adapted to a changing habitat oust previous inhabitants. A classic example of autogenic succession is Broadbalk Wilderness, whereby climax vegetation was achieved 30 years after the abandonment of an arable crop (Brenchley & Adam, 1915). Allogenic succession occurs in response to modified environmental factors such as fertiliser and herbicide input.
Prior to the advent of selective herbicides in 1945, weeds were kept in check by a combination of rotation, cultivation and clean seed, the three tenets of good husbandry. Previously, weed control was strategic, but the availability of herbicides enabled a tactical approach. However, the realisation that some weed species are of beneficial value to the arable ecosystem rendered the pragmatic destruction of weeds other than those that were most intransigent less acceptable; maximisation of yield was not necessarily synonymous with maximisation of profit.
Clean Seed
The use of clean seed as a consequence of the development of threshing machinery was greatly assisted by improvements in seed screening and legislation such as the 1920 Seeds Act designed to reduce the number of impurities. Regular inspection by the Official Seed Testing Station (OSTS) provides testament to the merits of seed certification. Early casualties of improved sanitation were the mimetic weeds such as Agrostemma githago L. (corncockle)*, a formerly characteristic weed of cereals which could be separated by seed screening. Prior to 1930 it was a frequent grain contaminant, as witnessed by records of the OSTS; the last authenticated record of its occurrence was documented in 1968 (Tonkin, 1968). A further factor contributing to its demise was the fact that its seeds are of short persistency in soil and require continual replenishment for survival. A survey of cereal seed drills in 1973 indicated considerable contamination by weed seeds including wild oats (Avena spp.) and couch grass Elymus repens (L.) Gould) as well as Galium aparine L. (cleavers) and Polygonum spp. (Tonkin & Phillipson, 1973). EU legislation designed to reduce the incidence of weed seed impurities in crop seed has certainly reduced this as a source of infestation, with, for example, only a single wild oat seed permitted per 500-g sample, provided that the next 500-g sample is entirely free of contamination.
Rotation
The season of sowing is the greatest determinant of weed occurrence (Brenchley & Warington, 1930). Hence, in the 1960s when spring barley predominated, spring-germinating species were prolific, the most significant of which was Avena fatua L., but also a diverse array of broad-leaved species, the periodicity of which is predominantly or entirely in the spring. The shift to autumn cropping in the 1980s disadvantaged spring-germinating species as a consequence of crop competition. Avena fatua exhibits a bimodal pattern of germination such that it was not necessarily disadvantaged, but it is possible that the related Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne., which is entirely autumnal in germination periodicity, may have supplanted it as the dominance...
