Fundamentals of Weed Science

By Robert L Zimdahl

Fundamentals of Weed Science, Fifth Edition, provides the latest information on this constantly advancing area of study. Placing weed management in the largest context of weed research and science, the book presents the latest advances in the role, control and potential uses of weed plants. From the emergence and genetic foundation of weeds, to the latest means of control and environmental impact, the book uses an ecological framework to explore the role of responsible and effective weed control in agriculture. In addition, users will find discussions of related areas where research is needed for additional understanding.

Explored topics include the roles of culture, economics and politics in weed management, all areas that enable scientists and students to further understand the larger effects on society.

• Winner of a 2019 The William Holmes McGuffey Longevity Award (College) (Texty) from the Textbook Association of America
• Completely revised with 35% new content
• Contains expanded coverage of ethnobotany, the specific identity and role of invasive weed species, organic agriculture, and herbicide resistance in GM crops
• Includes an emphasis on herbicide resistance and molecular biology, both of which have come to dominate weed science research
• Covers all traditional aspects of weed science as well as current research
• Provides broad coverage, including relevant related subjects like weed ecology and weed population genetics

• Language: English
• Publisher: Academic Press
• Release date: Feb 7, 2018
• ISBN: 9780128111444

Robert L Zimdahl

Robert L. Zimdahl is a Professor of Weed Science at Colorado State University. He received his Ph.D. in Agronomy from Oregon State University. Among his many honors and awards, Dr. Zimdahl was elected a Fellow of the Weed Science Society of America in 1986 and currently serves as editor of that society’s journal, Weed Science. He has been a member of several international task forces and has authored a number of books and articles on the subject of weed science. He is the author of Fundamentals of Weed Science, and Six Chemicals that Changed Agriculture both from Elsevier.

Book preview

Fundamentals of Weed Science

Fifth Edition

Robert L. Zimdahl

Professor Emeritus, Department of Bioagricultural Sciences and Pest Management Colorado State University, Fort Collins, Colorado

icon

Winner of the 2019 William Holmes McGuffey Longevity Award

This award from the Textbook & Academic Authors Association (TAA) recognises textbooks which have maintained their excellence over time. First published in 1993, this long-standing legacy title has been empowering botany and agriculture students and researchers for more than 25 years.

Table of Contents

Cover image

Title page

Copyright

Dedication

Epigraph

Preface

Chapter 1. Introduction

Chapter 2. Weeds: The Beginning

1. The Beginning

2. Definition of the Word Weed

3. Characteristics of Weeds

4. Harmful Aspects of Weeds

5. Cost of Weeds

Things to Think About

Chapter 3. Weed Classification

1. Phylogenetic Relationships

2. A Note About Names

3. Classification Methods

Things to Think About

Chapter 4. Uses of Weeds - Ethnobotany

1. Food for Humans

2. Feed for Animals

3. Medical Uses

4. Agricultural Uses

5. Ornamental Uses

6. Insect or Disease Traps

7. Pollution Control

8. Other Uses

Things to Think About

Chapter 5. Weed Reproduction and Dispersal

1. Seed Production

2. Seed Dispersal

3. Seed Germination: Dormancy

4. Vegetative or Asexual Reproduction

Things to Think About

Chapter 6. Weed Ecology

1. Human Influences on Weed Ecology

2. The Weed–Crop Ecosystem

3. Environmental Interactions

4. Fundamental Ecological Concepts

5. Plant Competition

6. Plant Characteristics and Competitiveness

7. Relationship Between Weed Density and Crop Yield

8. Magnitude of Competitive Loss

9. Duration of Competition

10. Economic Analyses

11. Mathematical Models of Competition

Things to Think About

Chapter 7. Weed Population Genetics

1. Introduction

2. Genetic Diversity

3. Gene Expression and Phenotypic Diversity

4. Mating Systems

5. Evolution of Weed Populations

Things to Think About

Chapter 8. Invasive Plants

1. What Is an Invasive Species?

2. What Is the Extent of Plant Invasions?

3. Which Species Will Be Invasive?

4. Why Do Invasions Occur?

5. Consequences of Plant Invasions

6. Case Studies: Four Invasive Plants

7. Management of Invasive Plants

8. An Invasive Thought

Things to Think About

Chapter 9. Allelopathy

1. Allelopathic Chemistry

2. Production of Allelochemicals

3. Allelopathy and Weed-Crop Ecology

Things to Think About

Chapter 10. Methods of Weed Management

1. Prevention, Control, Eradication, and Management Defined

2. Preventive Techniques and Weed Laws

3. Nonchemical Methods of Weed Management

Things to Think About

Chapter 11. Weed Management in Organic Farming Systems

1. Introduction

2. What Is Organic Agriculture?

3. Feeding the World

4. Methods of Weed Management in Organic Agriculture

5. A Different View

Things to Think About

Chapter 12. Biological Weed Control

1. General

2. Methods of Application

3. Biological Control Agents

4. Integration of Techniques

Things to Think About

Chapter 13. Introduction to Chemical Weed Control

1. History of Chemical Weed Control

2. Advantages of Herbicides

3. Disadvantages of Herbicides

4. Classification of Herbicides

Things to Think About

Chapter 14. Herbicides and Plants

1. Factors Affecting Herbicide Performance

2. General

3. Foliar Active Herbicides

4. Physiology of Herbicides in Plants

Things to Think About

Chapter 15. Herbicides and Soil

1. Soil

2. Factors Affecting Soil-Applied Herbicides

3. Soil Persistence of Herbicides

Things to Think About

Chapter 16. Properties and Uses of Herbicides

1. Introduction

2. Inhibitors of Lipid Synthesis

3. Inhibitors of Amino Acid Synthesis

4. Seedling Growth Inhibition

5. Growth Regulators

6. Photosynthesis Inhibitors – Photosystem II (Table 16.8)

7. Photosynthesis Inhibitors – Photosystem I – Electron Diverters. WSSA Class 22(D)

8. Cell Membrane Disruptors

9. Inhibitors of Carotenoid Biosynthesis=Inhibitors of Pigment Production

10. Nitrogen Metabolism Inhibition

11. Inhibitors of Respiration

12. Unknown and Miscellaneous

13. Summary

Things to Think About

Chapter 17. Herbicide Formulation

1. Introduction

2. Types of Herbicide Formulations

3. Surfactants and Adjuvants

Things to Think About

Chapter 18. The Role and Future of Genetic Modification in Weed Science

1. Genomics/Genetic Modification

2. The Process

3. The Advent and Growth of Genetically Modified Crops

4. Actual and Potential Benefits

5. Concerns and Criticism of Genetically Modified Crops

6. Molecular Biology in Weed Management

7. Conclusions

Chapter 19. The Problem and Study of Herbicide Resistance

1. Definitions

2. The Growth and Extent of Herbicide Resistance

3. Mechanisms of Herbicide Resistance

4. Management Methods

5. Challenges

Things to Think About

Chapter 20. Herbicides and the Environment

1. Herbicide Performance

2. Ecological Change

3. Environmental Contamination

4. Energy Relationships

5. Herbicide Safety

Things to Think About

Chapter 21. Pesticide Legislation and Registration

1. The Principles of Pesticide Registration

2. Federal Laws

3. The Environmental Protection Agency

4. Procedural Summary

5. Tolerance Classes

6. The Procedure for Pesticide Registration

7. A Final Comment

Things to Think About

Chapter 22. Weed-Management Systems

1. Introduction

2. A Metaphor for Weed Management

3. The Logical Steps of Weed Management

4. Weed-Management Principles in Eight Systems

5. Weed-Management Decision Aid Modeling

6. Summary

Things to Think About

Chapter 23. Weed Science: The Future

1. Research Needs

2. Weed-Crop Competition and Weed Ecology

3. Allelopathy

4. Political Considerations

5. Conclusion

Things to Think About

Appendix 1. List of Crop and Other Nonweedy Plants Cited in Text, Alphabetized by Common Name

Appendix 2. Weeds Cited in Text Alphabetized by Common Name

Glossary of Terms Used in Weed Science

Index

Copyright

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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Dedication

This book is dedicated to the memory of Ann Osborn Zimdahl and Pamela Jeanne Zimdahl

Epigraph

How little we know of what there is to know.

I wish that I were going to live a long time.…

I'd like to be an old man and to really know

I wonder if you keep on learning or if there is

only a certain amount each man can understand.

I thought I knew about so many things that I

know nothing of. I wish there was more time.

Hemingway, E. 1940. For Whom the Bell Tolls Scribner. New York, NY. p.380.

Preface

About 50   years ago, Monsanto Company distributed a picture that hangs in my study. It shows four books ¹ with several weed seedlings emerging from each. Two (Ahlgren et al. and King) were textbooks and two (Muenscher and Fernald) were plant identification books. They were the beginning of a now greatly expanded literature of weed science.

Many, but not all of the early textbooks written for undergraduate weed science courses lacked an ecological-management perspective for the rapidly developing science of weeds and their control. This book does not ignore the history of weed science and the development of chemical weed control; rather, it portrays herbicides as one management technique among many. It is undeniable that in the agricultural production system of developed countries, herbicides dominate weed control and management. Whether this creates a sustainable agricultural system is an important question. It has been and will continue to be discussed. Contributing to the discussion is part of but not the primary purpose of this book.

Whether one lives in a developed or developing country and whether one is rich or poor, male or female, educated or not, we all live in a postindustrial, information-age society. We are fortunate to live in an era of scientific achievement and technological progress perhaps unequaled in human history, which has created the good life many of us enjoy and some of the problems we experience. The achievements include:

Waking up this morning to music from your cell phone,

Preparing breakfast in your microwave as you review the news on your computer, which gives you nearly instant access to information that is orders of magnitude greater than the resources of any of the world's libraries,

Medical advances that cure what used to kill or cripple,

Immunization to prevent childhood diseases,

Elimination of smallpox and possibly polio in the near future,

Vastly improved detection and control of some diseases,

Travel at speeds and convenience unknown to our grandparents, across oceans and mountains that were once formidable barriers,

and, finally, for many, abundant food.

The problems include climate change, global warming, pollution of all forms, social inequality, environmental degradation, and soil erosion. Many citizens of developed countries know and benefit from the achievements of science and technology and are concerned about the problems that science and technology have wrought.

Despite its many benefits, science is commonly regarded with suspicion, if not fear, by the public, which enjoys and benefits from science and its technology. Scientists, including weed scientists, eagerly accepted the credit when, after World War II, advances in societal development were widely regarded as contributions of science, and in fact were. The public regarded these advances, which included herbicides and other pesticides, as desirable and benign. Now science and its technology are held responsible for many societal, environmental, and ecological problems. Herbicides are no longer regarded as benign, but as threats to humans, other creatures, and the environment. They are commonly regarded as undesirable scientific creations. The public's attitude toward science and scientists is mingled awe and fear. The practice of science is constrained because although it claims to be an end in itself, it is publicly supported and tolerated because of its utility, its practical value. It is feared because of well-known undesirable consequences (see Endnote: Agricultural Examples). Weed science is typical, and because of its close identification with herbicides, it may be regarded with more fear than some other areas of agricultural science. A few glaring errors of the agricultural community (including weed science) are illustrated in comments made by James Davidson, Emeritus Vice President for Agriculture and Natural Resources, University of Florida. ²

With the publication of Rachel Carson's book entitled Silent Spring, we, in the agricultural community, loudly and in unison stated that pesticides did not contaminate the environment—we now admit that they do. When confronted with the presence of nitrates in groundwater, we responded that it was not possible for nitrates from commercial fertilizer to reach groundwater in excess of 10   parts per million under normal productive agricultural systems—we now admit they do. When questioned about the presence of pesticides in food and food quality, we reassured the public that if the pesticide was applied in compliance with the label, agricultural products would be free of pesticides—we now admit they're not.

When criticism of herbicides and other agricultural technology was offered, disturbingly often the agricultural community responded by questioning the accuracy of the science, attacking the credibility of the scientist(s), and warning of increasing, punitive costs if the critics, who were often labeled environmentalists (an epithet) were allowed to prevail.

This book will not pursue this discussion, but it is important that students of weed science know about the issues. The public's lack of understanding or its misunderstanding of what weed scientists do will not lessen the need for what is done, but it increases the responsibility of weed scientists to be clear about the problem of weeds and proposed solutions. The responsibility is not so much to educate the public about what we do as it is to engage in a conversation (a dialogue, not a monologue) with the public. It is using science not just to persuade and defend a position, but to explore and discover other views. It is an engagement in public scholarship in which original, peer-reviewed intellectual work is fully integrated with the social learning of the public (Jordan et al., 2002).

This book includes herbicides and their use as an important aspect of modern weed management and strives to place them in an ecological framework. (Common names of herbicides will be used throughout the text, except in some tables where they may be paired with one or more trade names.) Any book that purports to discuss current practice (and the art) of weed management would be of little consequence and limited value to students and others who wish to know about weed management if it omitted discussion of herbicides.

Many weed scientists believe agriculture to be a continuing struggle with weeds. That is, they believe that without good weed control, good, profitable agriculture is impossible and herbicides are an essential component of productive success. Weed science and other agricultural disciplines regard their role to be central to agriculture's success and continued progress. While not denying the importance of weed management to successful agriculture, its role in the larger ecological context is emphasized. The role of culture, economics, and politics in weed management is mentioned, but they are not strong themes.

This, the fifth edition, is not a complete revision of the earlier editions, but it has been changed in several significant ways, while striving to maintain an overall ecological framework.

Some references in the first edition have been omitted and 505 references have been added, 197 of which are work published after 2013. The literature review for this edition was completed in early 2017.

The chapters are arranged in a logical progression. Chapter 1 asks and answers the questions Why study weeds and Why are they important? The second chapter pursues discussion of the definition of weed begun in Chapter 1 and presents the characteristics and harmful aspects of weeds. It concludes with a discussion of what weeds cost. Chapter 3 classifies weeds in several ways, and Chapter 4, unique among weed science texts, discusses the fact that not all plants that are weedy in some environments are weeds (that is, undesirable) in all places. Many plants have uses known to many and are studied by ethnobotanists. Weed reproduction and dispersal and the important topics of seed germination and dormancy are presented in Chapter 5. Chapter 6 is important because it presents the fundamental ecological base of weed science, including plant competition and the interactions of weeds and other pests. The significance of weed-crop competition is included. Chapter 7, Population Biology of Weeds, was written by Dr. Michael Christophers, associate professor, Department of Plant Sciences, North Dakota State University, Fargo, North Dakota. It describes the role and increasing importance of population biology to weed science. Chapter 8, also unique among weed science textbooks, first appeared in the third edition. It is an extended discussion of the role and importance of invasive plant species. The coauthor is Dr. Cynthia S. Brown, professor, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, Colorado. It is followed by a discussion of allelopathy in Chapter 9, a subject included as a minor point in many weed science texts.

Chapter 10 begins consideration of methods of weed management. For many, this is the essence of weed science, the fundamental topic: how are weeds controlled? Weed problems are an inevitable part of production agriculture and any place where we grow plants. Those who wish to control them need to ask why the weed is there, as well as how to manage or control it (Zimdahl, 1999). Key concepts of prevention, control, and management are presented followed by presentation of mechanical, nonmechanical, and cultural control techniques. Chapter 11 discusses the challenges of weed management in organic cropping systems. Chapter 12 introduces important concepts related to biological control of weeds. Chapter 13 introduces herbicides and chemical control of weeds. This is not a how-to book. Herbicides are discussed in depth in this chapter and Chapter 16, but there are no recommendations about what herbicide to use in a crop. Chapters 14 and 15 are central to understanding interactions between herbicides and plants and herbicides and soil. Chapter 16, one of the longest and most difficult, classifies herbicides based on how they do what they do: their site of action and their chemical family. Herbicide formulation is covered in Chapter 17.

Chapter 18, new in this edition, deals with the role and future of genetic modification in weed science. Chapter 19 is an expansion of previously included information on the problem, study, and challenges of herbicide resistance among weeds. The influence of molecular biology on weed management is included. Chapter 20 returns to the ecological theme, but this time with information on the interaction between herbicides and the environment, including effects on water, humans, and global change. A central and intentionally unanswered question is how one balances and judges the potential harmful and beneficial aspects of herbicides. The chapter concludes with a discussion of herbicide safety. Chapter 21 is a brief presentation of the US legislative decisions required to address some of the questions raised in Chapter 19. Chapter 22 brings things together by discussing eight among many weed management systems, each of which is primarily conceptual, not completely prescriptive. A section on weed management decision aids completes the chapter. The last section, Chapter 23, presents a view of the future of weed science. It is meant to provoke thought and discussion. It is not an infallible prediction of what will be.

There is a strong, growing trend in weed science away from exclusive study of annual control techniques toward understanding weeds and the systems in which they occur. Control is important but understanding endures. Herbicides and weed control are important parts of the science and of this text, but it is hoped that understanding the principles of management and the biology and ecology of the weeds to be managed will be seen as the dominant themes. The primary objectives of the book are to introduce concepts fundamental to weed science and provide adequate citations so interested readers can pursue specific interests and learn more.

Study of weeds, weed management, and herbicides is a challenging, demanding task that requires diverse abilities. Weed science involves far more than answering the difficult question of what chemical will selectively kill weeds in a given crop. Weed science includes work on selecting methods to control weeds in a broad range of crops, on noncrop lands, in forests, and in water. Weed scientists justifiably claim repute as plant physiologists, ecologists, botanists, agronomists, organic and physical chemists, molecular biologists, and biochemists. However, lest the reader be intimidated by that list of disciplines, I hasten to add that this text will emphasize general principles (the fundamentals) of weed science and not attempt to include all applicable knowledge. It is tempting, and would not be much more difficult, to incorporate extensive, sophisticated knowledge developed by weed scientists. Although this knowledge is impressive and valuable, it is beyond the scope of an introductory text.

I have claimed, with only anecdotal data, that weeding crops consumes more human energy than anything else we do, and that much of it is done by women. I hope the book conveys some of the challenges of the world of weeds, their management, and the importance of weed problems to agriculture and society, and to meeting the demand to feed a growing world population. The aim has been to include most aspects of weed science without exhaustively exploring each. The book is designed for undergraduate weed science courses. I hope it is not too simple for sophisticated readers and that omissions of depth of coverage do not sacrifice accuracy and necessary detail. Readers should note that in nearly all cases I have used the units of measure used in the original reference rather than changing all to one measurement system.

Several colleagues provided helpful suggestions on this and earlier editions of the Fundamentals of Weed Science. I thank all of them, even though some comments were difficult to hear. The first edition had several errors of fact that have been corrected in subsequent editions. I thank the following colleagues for suggestions and critical review of portions of the manuscript included in this and earlier editions: Dr. Kenneth A. Barbarick, Dr. K. George Beck, Dr. Cynthia S. Brown, Dr. Sandra K. McDonald, Dr Philip Westra, Dr. Scott J. Nissen, and Mr. Steven Markovits of Colorado State University; Dr. William W. Donald US Department of Agriculture/Agricultural Research Service, University of Missouri; Dr. David L. Mortensen, The Pennsylvania State University; Dr. Robert F. Norris, University of California–Davis; Dr. Gregory L. Orr, Fort Collins, Colorado; Dr. Keith Parker, Syngenta Corp, Greensboro, North Carolina; Dr. Alan R. Putnam, Gallatin, Gateway, Montana; Dr. Albert E. Smith, Jr., University of Georgia; Dr. Malcolm D. Devine, vice president, Crop Development, Performance Plants, Saskatoon, Saskatchewan, Canada; and Dr. Steven Brunt, BASF Corp, Research Triangle Park, North Carolina. The book has been improved because of their efforts and the comments of anonymous reviewers and several colleagues on this and earlier editions.

My wife, Pamela J. Zimdahl (deceased 2012) encouraged my writing and offered comments and criticism when she thought they were appropriate. They usually were. Errors of interpretation or fact are solely my responsibility.

Robert L. Zimdahl

Fort Collins, Colorado, 2017

Endnote: Agricultural Examples

• The mid-1960s controversy over the real and suspected hazards of 2,4,5-trichlorophenoxyacetic acid, a component of Agent Orange used in Operation Ranch Hand, a vegetation control program during the Vietnam war. It was the first major public debate that challenged the intellectual foundation of weed science and its dependence on herbicides (see Chapter 2).

• On December 3, 1984, a poisonous cloud of methyl isocyanate, used in the manufacture of pesticides, escaped from Union Carbide's plant in Bhopal, India, killing 14,000 people and permanently injuring 30,000.

• Regarding pesticide poisoning: No one knows for sure, but it is estimated that between one and fivemillion cases of pesticide poisoning occur every year in the world, resulting in 20,000 deaths. Developing countries use 25% of pesticides but experience 99% of deaths (see Chapter 20).

• Debate within and outside the agricultural community over the risks and ultimate beneficiaries of genetic modification of crops has raised legitimate economic, social, and biological concern. A major concern about weed science is the widespread occurrence of herbicide resistance.

• Air and water pollution and animal suffering have resulted from confined animal feeding operations.

• Mad cow disease, swine flu, bird flu, meat recalls, and antibiotic resistance are all of concern or have been of major societal concern in the past.

• The ecological dead zone extending into the Gulf of Mexico from the Mississippi terminus.

Literature Cited

Jordan N, Gunsolus J, Becker R, White S. Public scholarship—linking weed science with public work. Weed Sci. 2002;50:547–554.

Zimdahl R.L. My view. Weed Sci. 1999;47(1).

---

¹  The books are: Ahlgren, G.H., Klingman, G.C., Wolf, D.E., 1951. Principles of Weed Control. J. Wiley & Sons, New York, NY. 368 pp.; Fernald, M.L., 1970. Gray's Manual of Botany. Eighth ed. American Book Co.; King, L.J., 1966. Weeds of the World: Biology and Control. Interscience Pub., Inc., New York, NY. 526 pp.; and Muenscher, W.C., 1935. Weeds. The Macmillan Co., New York, NY. 577 pp.

²  Davidson's comments were made several years ago and cited by Kirschenmann, F, 2010. Some things are priceless. Leopold Lett. 22 [1]:5.

Chapter 1

Introduction

Abstract

This chapter introduces the subject of weed science and the fundamentals of the subjects as they relate to students’ knowledge of agricultural, biological, and general science. The intention of the book is revealed: to introduce the fundamental concepts of weed science, show how they have changed with time, and conclude with some thoughts on the future of weed science.

Keywords

Climax vegetation; Fertilizers; Food production; Herbicide; Mechanization; Pest management; Population growth; Vegetation management

See them tumbling down,

Pledging their love to the ground

Lonely but free I’ll be found

Drifting along with the tumbling tumbleweeds.

Cares of the past are behind

Nowhere to go but I’ll find

Just where the trail will wind

Drifting along with the tumbling tumbleweeds.

I know when night has gone

That a new day starts at dawn.

I’ll keep rolling along

Deep in my heart is a song

Here on the range I belong

Drifting along with the tumbling tumbleweeds.

Composed in 1932 by Bob Nolan and recorded by the Sons of the Pioneers

Weeds have been the subject of songs and receive a lot of attention, but they have never been respected or well understood. The fact that many people earn a living and serve society by working to control and manage them is often greeted with amusement if not outright laughter. Even scientific colleagues who work in other esoteric disciplines find it hard to believe that another group of scientists could be concerned exclusively with what is perceived to be as mundane and ordinary as weeds.

Weeds have surely been with us since the advent of settled agriculture some 10,000   years ago. It has been suggested that the most common characteristic of the ancestors of our presently dominant crop plants is their weediness—their tendency to be successful, to thrive, in disturbed habitats, most notably those around human dwellings (Cox, 2006). Bailey (1906, p.199), to whom agricultural science owes so much, spoke of the Sisyphean battle against Russian thistle in the western United States:

What I have thus far stated is only a well-known truth in organic evolution, — that the distribution of an animal or plant upon the earth, and to a great extent the attributes of the organism itself, are the result of a struggle with other organisms. A plant which becomes a weed is only a victor in a battle with farm crops; and if the farmer is in command of the vanquished army, it speaks ill for his generalship when he is routed by a pigweed or a Russian thistle.

It is not surprising to me that students who enroll in a course about weeds often wonder why the course is recommended or, perhaps required, and what it is about. Students who enroll in chemistry or English have a reasonably good idea what the class will be about and how it fits in their curriculum. It is often not true for students of weed science. Of course, students from farms and ranches know about weeds and the problems they cause, but they do not always comprehend the complexities of weed management and the management skill required. Therefore, it is important to our pursuit that the nature of the subject be established and that the subject matter be related to students' knowledge of agricultural, biological, and general science. From the beginning, a textbook, the professor, and the student should strive to establish relationships between weed science, agriculture, and society. It is the intent of this book to introduce the fundamental concepts of weed science, show how they have changed with time, and conclude (Chapter 23) with some thoughts on the future of weed science. I risk thinking about the future in full recognition that prediction is very difficult, especially about the future. ¹ However, I hasten to add a thought from Saint-Exupéry (1950, p. 155)—As for the Future, your task is not to foresee it, but to enable it.

A brief review will lead to the conclusion that the story of agriculture is a story of the struggles that have ensued in consequence of the sudden overturning of established conditions, and the substitution therefor of a very imperfect and one-sided system of land occupancy (Bailey, 1906, p. 200)—what we know as modern agriculture. Agricultural history, a fascinating subject, is too large a task for this book and only small bits are included. Those interested in beginning a study of agricultural history and weed science are referred to Goodwin and Johnstone (1940) and Rasmussen (1975). The history of weed control was reviewed by Timmons (1970, republished in 2005), Appleby (2005), and Zimdahl (2010).

Formidable obstacles have been placed between humans and a continuing food supply. These include:

• Physical constraints: lack of appropriate technology, lack of good highways, inadequate or no infrastructure for transportation of supplies to farms or produce to markets. These are food distribution, not food production, problems.

• Economic constraints: lack of credit and operating funds. An overwhelming constraint to feeding people is poverty—no income or an income inadequate to purchase food.

• Environmental constraints: too much or too little water, short growing season, poor soil, highly eroded soil, uncontrolled weeds, insects, and plant diseases.

• Biological constraints: poor soil fertility, poorly adapted plant varieties, low or high soil pH, salinity, inadequate expensive fertilizer, inadequate inappropriate technology, and lack of applicable agricultural research, research scientists, and facilities.

• Political constraints: inadequate agricultural research funds and dysfunctional markets. There is no universal, or even a local, human right to food.

One of the most formidable environmental constraints has been pests. In many developing countries, between 40% and 50% of most crops are lost to insects, diseases, and inadequate storage before they reach the market. Surveys by the Food and Agriculture Organization (FAO) of the United Nations (1963, 1975) showed that in the 1960s and 1970s more than one-third of the potential annual world food harvest was destroyed by pests. In 1975, the $75   billion loss was equivalent to the value of the world's grain crop (about $65   billion) and the world's potato ² crop (about $10   billion). This means that insects, plant diseases, nematodes, and weeds deprived humans of food worth more than the entire world crop of wheat, rye, barley, oats, corn, millet, rice, and potatoes. The data reported herein were only up to harvest and do not include damage during storage—another large sum. Currently, less complete estimates show that in spite of the abundant research and pesticide use, losses due to pests of all kinds have increased since the first FAO estimates were made.

Estimates of potential and actual losses despite the current crop protection practices are given for wheat, rice, maize, potatoes, soybeans, and cotton for 2001–03 on a regional basis (19 regions) as well as for the global total. Among crops, the total global potential loss due to pests varied from about 50% in wheat to more than 80% in cotton. The responses estimated losses of 26%–29% for soybean, wheat, and cotton, and 31%, 37%, and 40% for maize, rice, and potatoes, respectively. Overall, weeds produced the highest potential loss (34%), with animal pests and pathogens being less important (18% and 16%) (Oerke, 2006; Pinstrup-Andersen, 2001). Another study reported that weeds produced the highest potential loss (30%), with animal pests and pathogens being less important (23% and 17%). The efficacy of control of pathogens and animal pests was 32% and 39%, respectively, compared to almost 74% for weed control. ³

Losses during production are important, but one must also acknowledge the continuing burden of food loss due to waste. Data from the Institution of Mechanical Engineers (IMechE) (Fox and Fimeche, 2013) show as much as 2   billion metric tons of food never make it to a plate. That strongly suggests that about half of all the food produced in the world is lost every year due to poor harvest practices, storage and transportation, and market and consumer waste. Fox and Fimeche estimate that 30%–50% (or 1.2–2   billion tons) of all food produced never reaches a human stomach. The waste does not reflect the fact that large amounts of land, energy, fertilizer, and water are used for no human benefit. IMechE blames the staggering loss on unnecessarily strict sell-by dates, buy-one-get-one-free marketing, and Western consumer demand for cosmetically perfect food, along with poor engineering and agricultural practices, inadequate infrastructure, and poor storage facilities.

Per capita food loss in Europe and North America is 280–300   kg/year. In sub-Saharan Africa and South/Southeast Asia it is 120–170   kg/year. Total per capita production of food for human consumption in Europe and North America is about 900   kg/year, whereas in sub-Saharan Africa and South/Southeast Asia it is 460   kg/year. Per capita food wasted by consumers in Europe and North America is 95–115   kg/year, but only 6–11   kg/year in sub-Saharan Africa and South/Southeast Asia. Food losses in industrialized countries are too high, but in developing countries more than 40% of the food losses occur at postharvest and processing levels, while in industrialized countries, more than 40% of the food losses occur at retail and consumer levels. Food waste by consumers in industrialized countries (222   million tons) is almost as high as the total net food production in sub-Saharan Africa (230   million tons). It is a bleak tale that challenges all of agriculture.

History is filled with examples of human conflicts with pests, from biblical to modern times. Examples include:

• The desert locust (Schistocerca gregaria Forskal), a pest since biblical times. They unexpectedly appear and can strip a field bare in an hour. They prefer grasses, but consume a wide range of crops.

• Short-horned grasshoppers and locusts—a large family found predominantly in warmer regions. As many as 500 species are agricultural pests. Locust swarms threaten primarily grass crops on about one-third of the world's land surface (Hill, 1994).

• Late blight [Phytophthora infestans (Mont.) D. By.] caused the Irish potato famine of 1845–49.

• The continuing worldwide presence of Colorado potato beetles (Leptinotarsa decemlineata Say).

• The 1970s epidemic of Southern corn leaf blight (Helminthosporium maydis Nisik and Miyake).

• Western corn rootworm (Diabrotica virgifera virgifera).

• The spread of the mountain pine beetle (Dendroctonus ponderosae) in British Columbia, Canada, and the western United States has killed millions of hectares of lodgepole pine forest and released an estimated 270million tons of carbon, converting the forest from a carbon sink to a large net carbon source.

• The Puccinia graminis tritici strain of wheat rust, discovered in Uganda in 1998, has spread across Africa, Asia, and the Middle East.

The battle has not ended. It has become more intense. In the industrialized (rich) world agriculture has evolved from many small farms to large industrial-scale, dominantly monocultural farms. The widespread use of synthetic fertilizer, chemical pesticides and the more recent introduction of genetic modification of plants have created enormous changes in the way agriculture is practiced. The most reliable estimates are from the United Nations population division, which shows the 2017 human population is 7.4   billion and is projected to be 9.7   billion by 2050 and possibly 11.2   billion by 2100. Today the world must feed almost 240,000 more people than yesterday. Population growth has increased the need and the demand for ever greater quantities of high-quality food and meat.

One must respect the prescience of Swift (1677–1745, See Williams 1937) who said:

Hobbes clearly proves that every creature

Lives in a state of war with nature

…

So, Nat'ralists observe, a Flea

Hath smaller Fleas that on him prey;

And these have smaller Fleas to bite 'em:

And so proceed ad infinitum.

De Morgan (1850), who probably had read, but did not cite, Swift's poem, expressed the ubiquity of pests several years later:

"Great fleas have little fleas upon

their backs to bite 'em,

And little fleas have lesser fleas,

and so ad infinitum,

And the great fleas themselves, in

turn, have greater fleas to go on;

While these again have greater still,

and greater still, and so on.

The subject of this book is weeds—visible, unspectacular pests, whose presence is obvious nearly everywhere, but whose effects are not. Weeds have always been with us and are included in some of our oldest literature:

Cursed is the ground for thy sake;

in sorrow shalt thou eat of it all the days of thy life;

thorns and thistles shall it bring forth to thee;

and thou shalt eat the herb of the field.

Genesis 3:17–18

Ye shall know them by their fruits. Do men gather grapes

of thorns, or figs of thistles?

Matthew 7:16

And thorns shall come up in her palaces, nettles and

brambles in the fortresses thereof…

Isaiah 34:13

Weeds are also mentioned in the parables of Jesus (Matthew 13:18–23). The biblical thistles, thorns, and brambles are common weeds and have been identified as such by biblical scholars (Moldenke and Moldenke, 1952). They were and are serious threats in the continuing battle to produce enough food. The tares in the following parable (Matthew 13:25–30) are the common weed, poison ryegrass, a continuing problem in cereal culture:

The kingdom of heaven is likened unto a man which sowed good seed in his field: But while he slept, his enemy came and sowed tares among the wheat, and went his way. But when the blade was sprung up, and brought forth fruit, then appeared the tares also.

The Greek word tares is translated as darnel—a weed that grows in wheat. It is a grass resembling wheat or rye, but with smaller, poisonous seeds. The weed called tares in Europe today is one of several vetches native to Europe.

No agricultural enterprise or part of our environment is immune to the detrimental effects of weeds. They have interfered with human endeavors for a long time. In much of the world, including my garden, weeds are controlled by hand or with a hoe. A person with a hoe may be as close as we can come to a universal symbol for the farmer, even though most farmers in developed countries no longer weed with, or even use, hoes. For many, the hoe and the weeding done with it, symbolize the practice of agriculture. The battle to control weeds, done by people with hoes, is the farmer's primary task in much of the world.

Bowed by the weight of centuries he leans

Upon his hoe and gazes on the ground,

The emptiness of ages in his face,

And on his back the burden of the world.

Who made him dead to rapture and despair,

A thing that grieves not and that never hopes,

Stolid and stunned, a brother to the ox?

Who loosened and let down this brutal jaw?

Whose was the hand that slanted back this brow?

Whose breath blew out the light within this brain?

…

O masters, lords and rulers in all lands,

How will the future reckon with this man?

How answer his brute question in that hour

When whirlwinds of rebellion shake all shores?

Excerpt from The Man with the Hoe Edwin Markham (1899)

There have been four major advances in agriculture that have significantly increased food production.

First was the introduction of mineral fertilizer. Early work on plant nutrition and soil fertility proceeded directly from the pioneering studies of Justus von Liebig (see Liebig, 1942) who questioned prevailing theories of plant nutrition.

The introduction of mineral fertilizer increased food production.

The second major advance was rapid mechanization that began in the United States with development of Whitney's cotton gin in 1793, McCormick's reaper in 1834, and Deere's moldboard plow in 1837.

Mechanization has increased agricultural productivity.

Understanding and using genetic principles in plant and animal production was the third major advance for agriculture. The obscure Austrian monk, Gregor Mendel, pursued his studies quietly and in seclusion. He had no goal of pragmatic application or economic gain. The discoveries made from his research, most notably in development of plant hybrids, have had huge, generally positive, effects on our ability to produce food. The nearly simultaneous and independent rediscovery of Mendel's work by De Vries in Holland, Correns in Germany, and Tschermak in Austria in 1900 when searching the literature to confirm their own discoveries has resulted in enormous positive benefits to agriculture.

The fourth major advance in agriculture was the development and widespread use of fertilizers and pesticides. These heralded the chemicalization of agriculture and led to the development and growth of weed science. It is reasonable to posit that weed control has been always been part of agriculture and that weed science began coincident with herbicide development. Weed science did not develop because of herbicides although their dominance is undeniable. The preponderance of evidence is that they will continue to dominate weed science research. They are and will remain an essential part of the knowledge required to manage weeds.

Weed science is vegetation management—the employment of many techniques to manage plant populations in an area. This includes dandelions in turf, poisonous plants on rangeland, and Palmer amaranth in soybeans. Weed science might be considered a branch of applied ecology that attempts to modify the environment against natural evolutionary trends. Natural evolutionary or selection pressure tends toward the lower side of the curve in Fig. 1.1 (Shaw et al., 1960), toward what ecologists call climax vegetation, the specific composition of which will vary with latitude, altitude, and environment. A climax plant community does not and cannot provide the kind or abundance of food the growing world population wants and needs. Therefore, we humans successfully modify the environment—the natural world—in many ways. We are the only species able to modify the natural world. It is no longer humans against nature. We decide and act to make the natural world what we want it to be. All other species must adapt to the environment as it is. Bronowski (1973, p. 19) reminds us that humans are not figures in the landscape—we are shapers of the landscape. All one has to do is look around to see what we have wrought. One of the most significant changes began over 10,000   years ago when humans began altering the land and the plants and animals on it to grow what was wanted. Hunting and gathering—living from what the land offered—were slowly abandoned as we learned how to dominate and subdue, ⁴ which many began to and still interpret as a God-given right—even a duty. Diamond (1987) suggests that the transition to settled agriculture was the worst mistake in the history of the human race. The immediate success of settled agriculture resulted in what many regard as a continuing catastrophe. It allowed the human population to grow, perhaps beyond the limits of what the earth can support (see Malthus, 1798). In Diamond's view the mistake was choosing to increase the food supply (what agriculture has done so successfully) rather than limit the population. That choice has led, in his view, to exploitation of the earth, gross social and sexual inequality, (and) the disease and despotism that curse our existence. However, it has also yielded a better, indeed a good, life for many, unfortunately not all, humans.

Figure 1.1 The food productivity potential of vegetation (Shaw et al., 1960).

Table 1.1

Data from World Bank Development Report and Data_extract-from_world_development_indicators_HI_aggregate.

Population growth continues, albeit at a slower rate, and we don't know for sure if we have exceeded the earth's capacity. Grant (2000) suggested the limit may be about 1.06   billion people at the World Bank's High Income Countries standard of living. He provided a debatable, intellectually challenging, answer to the nagging question: How many people can the earth support? First one must ask: At what level? Using the World Bank Atlas from 1996, he divided the world's gross income ($US) by the average gross income of those of us who live in the 44 high income countries—a standard most would like to have. Using data from the World Bank's Development Report for 2000, 2010, and 2015 the answer was less than 2   billion (Table 1.1).

Another estimate is based on energy consumption. Holdren (1991) said the earth was consuming 13   terawatts (13   trillion watts) of human-generated energy, 75% of which was used by 1.5   billion people (about 23% of world's population) in industrialized countries, who produce about 85% of the world's economic product. Average consumption in developing countries was about 1   kW/person. Holdren projected that sometime in the 21st century demand would be an unachievable at eight times higher and asked what if demand could be held at 3   kW/person   =   3× what the poor used and one-fourth of 1991 US use. Then he asked what if we could use 9   terawatts without trashing the environment? If one allows for unforeseen consequences (a 50% error margin), 6   terawatts total and 3   kW/person is reasonable. Long division   =   2   billion people. These simple calculations are disturbing and challenging to all in agriculture. Bartlett's (1978) paper is relevant.

In the beginning there were no weeds. If one impartially examines the composition of natural plant communities, or the morphology of weed flowers, one can find beauty and great aesthetic appeal. The flowers of wild onion, poison hemlock, dandelion, Queen Anne's Lace, chicory, sunflower, and several of the morning glories are beautiful and worthy of artistic praise for symmetry and color. By what right do we humans call plants with beautiful flowers weeds? Who has the right to say some plants are unwanted? By what authority do we so easily assign the derogatory term weed to a plant and say it interferes with agriculture, increases costs of crop production, reduces yields, and may even detract from quality of life?

The flowers of many weeds are beautiful and have great aesthetic appeal. This is the flower of the wild carrot or Queen Anne's Lace.

Nature knows no such category as weed. In 1967 (Buchholtz), Weed Science of America defined a weed as a plant growing where it is not desired. The definition in the 1974 Herbicide Handbook (Hilton p. xxi) of the society added, Plants are considered weeds when they interfere with activities of man or his welfare. Desire was omitted in the 2014 Handbook (Shaner, p. 487), in which a weed was defined as Any plant that is objectionable or interferes with the activities or welfare of man. It is important to note the continuing anthropocentric dimension of the definition. Desire is a human trait and only we have the ability to object or claim interference. Therefore a particular plant is a weed only in terms of a human attitude. Ecologists speak of weedy plants but often their use of the term is affected by preconceptions of the role of vegetation on a particular site. People say that a plant, in a certain place, is not desirable and therefore arbitrarily assign it the derogatory term weed. Weeds are usually regarded as the lowest of the kingdom of flowering plants not because they are naturally harmful but because they are or are perceived to be harmful to us.

It is homeowners and neighbors who say that dandelions and crabgrass are unacceptable in lawns. Does grass really care what other plants live in the neighborhood? It is those who suffer from hay fever who say that ragweed or perhaps big sagebrush in the western United States are unacceptable. It is those who are allergic to poison ivy who say it is unacceptable in their environment and who want to get rid of it. Farmers say, with clear economic justification, that they want their crops to grow in a weed-free environment to maximize yield and profit. People decide what plants are weeds and when, where, and how they will be controlled.

The dandelion is considered a weed by many.

This book will discuss many aspects of weeds, their biology, and their control. It differs from other weed science texts in significant ways. The differences may not make it better, only different. For example, most, but not all, presently available weed science textbooks devote at least 50% of their content to herbicides and their use. In some it is as much as 75%. A notable exception is Aldrich and Kremer's book (1997), which does not include any major section on herbicides. Because of the undeniable success of chemical weed management, it is my view that it must be included in a complete weed science textbook. Omitting the topic will produce students who are only partially prepared for modern weed management. Therefore the book includes herbicides and their use, but only as part, albeit an important part, of the fundamentals of weed science. The book correctly claims that killing weeds with herbicides is the highly successful modern way. It is, but its very success has deterred understanding weed biology and ecology. Control has been the primary emphasis of weed science since its beginning. Several years ago the historical committee of the Weed Science Society of America identified 17 important early publications on weeds (from 1895 to 1965), 12 dealt with killing, controlling, or eradication. As you proceed, you will find that this book does not ignore these important topics, but understanding weeds is the primary emphasis.

One can establish a relationship between pesticide use and agricultural yield. Perhaps a better way to put it is that one can find a relationship between good pest management (regardless of how it is accomplished) and agricultural yield. One should not always equate good weed control with herbicide use. Good weed control depends on cultural knowledge—what a good farmer or plant grower knows. Cultural knowledge is different than the scientific knowledge that leads to herbicide development and successful use. Both kinds of knowledge—scientific to tell us what can be done and cultural to tell us what we ought to do—are essential for good weed management.

One can also postulate a relationship between the way weeds and other pests are controlled, the practice of pest management, and a nation's food supply. Fig. 1.2 shows the world's tropical and sub-tropical areas, their major crops, and the percent of the world's crop grown in each area. The region's ability to control weeds is shown in Fig. 1.3 with data for the world and four major areas. Each segment in Fig. 1.3 is divided into good, moderate or acceptable, low, and very poor weed management. The world's tropical and subtropical regions (Fig. 1.2) are home to about 66% of the world's people. The regions extend roughly from the Tropic of Capricorn in the south to the Tropic of some of the world's most important crops. In the industrialized, primarily temperate, world, production agriculture and the ability to manage a large array of pests has made remarkable progress since 1971 when Dr. Leroy Holm of the University of Wisconsin prepared Fig. 1.2. However, the areas identified still suffer from underdevelopment of weed science and other agricultural technology. Seventy countries in Asia, Africa, and Latin America were included in Labrada's 1996 UN/FAO survey, which, sadly, is still accurate. The countries had roughly 44% of the world's 1.4   billion acres of arable land and, of most importance to our subject, inadequate weed control technology and knowledge (Fig. 1.3).

Figure 1.2 Crop production in the world's tropics (Holm, 1971).

Figure 1.3 The level of weed control practices in the world and four regions (Labrada, 1996).

The founder of Latin prose, Cato the Elder, reminds us in his work on agriculture that it is thus with farming: if you do one thing late, you will be late in all your work. We are late in implementing appropriate weed management techniques in much of the world, and agriculture will not progress to its full potential without them.

The agricultural productivity of the developed world is not an accident. US agriculture and that of other advanced nations grew out of a propitious combination of scientific advancement, industrial growth, and abundant resources of soil, climate, and water. One should not regard it as just good fortune or God's benevolence that we, in the United States, can say that after the food bill is paid we have more money left over than most other folks in the world. ⁵ For most Americans (although, unfortunately, not all) this is true. It not only is true for US citizens, it is regarded as so common that it is treated as a right rather than as something that was created and must be maintained.

Weeds are controlled in much of the world by hand or with crude hoes. The size of a farmer's holding and yield per unit area are limited by several things, and paramount among them is the rapidity with which a family (most often its female members) can weed its crops. More human labor may be expended to weed crops than on any other single human enterprise, and most of that labor is expended by women. Weed control in the Western world and other developed areas of the world is done by sophisticated machines and by substituting chemical energy for mechanical and human energy. There is a relationship between the way farmers control weeds and the ability of a nation to feed its people. Weed science is part of that relationship. Good weed management is one of the essential ingredients to increase food production.

The early flights of the Apollo spacecrafts and subsequent space flights gave those of us bound to earth a view of the whole planet, floating in the great, black sea of space (Fig. 1.4). Many had imagined but had never seen such a picture before. My generation remembers Apollo 13. On April 11, 1970, James Lovell, Fred Haise, and John Swigert were launched toward the moon. On April 13, their oxygen system exploded. As the 1995 movie shows so well, they had only one option for survival—return to earth. We have the same option. We must inhabit this planet as one human society. There is no other place to go. There is only one land mass to live on, one ocean system, one sun, one atmosphere, one rain cycle. You might choose to regard life as having one major purpose—to sustain itself.

There are, of course, other purposes. People want more than existence (survival). We want a good life—a life that enables us to realize our full potential. We want purpose, hope, and a sense of the meaning of things. Yet in practical terms the first prerequisite is a place to live. We have inherited a place to live from those who preceded us; and if others are to live, we must pass it on, in habitable condition with a productive, secure, sustainable agricultural foundation.

Figure 1.4 National Aeronautics and Space Agency—Epic view of earth. http://www.nasa.gov/press-release/nasa-satellite-camera-provides-epic-view-of-earth.

Around 1965, world food production began to lose the race with an expanding population as Rev. T. R. Malthus (1798) predicted it would. Each year the Malthusian apocalypse he predicted is prevented, but it is a daily specter for many in the world. The world's population now exceeds 7.4   billion and it will continue to grow, albeit at a slower rate. More than 85% of the world's people live in poor, developing countries where about 95% of the population growth will occur. As world population expands, food production is barely keeping pace, and often slipping behind. About 10% of the world's 33   billion acres of land are arable and while the area devoted to productive agriculture can be expanded, the cost will be great. One must also recognize that the world may lack the social and political will to handle the complex problems that expansion onto previously untilled land will bring. Such expansion may be part of the solution to the world food dilemma, but an equally important solution is use of appropriate, available technology, and development of new technology. If all the world's people are going to enjoy higher standards of living and be able to watch their children mature without fear of debilitating disease, malnutrition, or starvation, we must intelligently use all present agricultural technology and continue to develop better, safer, and equally or more effective technology. Shared technology and knowledge will permit our neighbors in this world to farm in ways that create opportunities to realize the earth's full agricultural and human potential.

Weed science is not a panacea for the world's agricultural problems. The problems are too complex for any simple solution and students should be suspicious of those who propose simple solutions to complex problems. The caution of H.L. Mencken (1920) is worth noting: There is always a well-known solution to every human problem—neat, plausible, and wrong. In fact, the hope should be not to solve but to diminish, not to cure but to alleviate, and to at least anticipate the brute question and have some answers when whirlwinds of rebellion strike all shores. The work of the weed scientist is fundamental to solving problems of production agriculture in our world. Weeds have achieved respect among farmers who deal with them every year in each crop. Weeds and weed scientists have achieved respect and credibility in academia and the business community. The world's weed scientists are and will continue to be in the forefront of efforts to feed the world's people.

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