Fundamentals of Weed Science

By Robert L Zimdahl

This book addresses herbicides and their use as an important aspect of modern weed management and strives to place them in an ecological framework. Many weed scientists believe agriculture is a continuing struggle with weeds--without good weed control, good and profitable agriculture is impossible. Each agricultural discipline sees itself as central to agriculture's success and continued progress, and weed science is no exception. While not denying the importance of weed management to successful agriculture, this book places it in a larger ecological context. The roles of culture, economics, and politics in weed management are also discussed, enabling scientists and students to understand the larger effects on society.

• Information on New herbicides included, along with the old herbicides that are important for understanding the history
• New section on weed resistance to herbicides and genetic engineering
• New information on invasive plants
• Expanded chapters on Biological Control, Pesticide Legislation and Regulation, Weed Management Systems, and more
• Instructor resources can be found at http://textbooks.elsevier.com/web/Login.aspx, and it is password protected. Please contact your sales representative at [e-mail] for access to the instructor resources. The insturctor site consists of chapter questions, essay questions, an exam and images from the book

• Language: English
• Publisher: Academic Press
• Release date: Aug 9, 2013
• ISBN: 9780123978189

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

Preface

Approximately 50 years ago, the Monsanto Company distributed a picture that hangs in my study. It shows four books¹ 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, textbooks written for undergraduate weed science courses lack an ecological-management perspective on 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, but 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. Continuing the discussion is not a primary purpose of this book.

Science, of all kinds, is not in favor these days. Scientists, including weed scientists, eagerly accepted the credit when after World War II, advances in societal development were widely regarded as, and in fact were, contributions of science. 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 rather 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 a mingling of awe and fear. The practice of science is constrained because while 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). Weed science is not atypical, and because of its close identification with herbicides, it may be regarded with more fear than some other areas of agricultural science. A few of the 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, the agricultural community, disturbingly often, 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, 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, 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 also to explore and discover other views. It is an engagement in public scholarship, whereby original, peer-reviewed intellectual work is fully integrated with the social learning of the public (Jordan et al., 2002).

This book includes herbicides³ and their use as an important aspect of modern weed management and strives to place them in an ecological framework. Any book that purports to discuss present 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 is 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 as 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 fourth edition, is not a complete revision of the third and earlier editions, but it has been changed in several significant ways while maintaining an overall ecological framework.

Some references in the first edition have been omitted, and 260 new references have been added, 124 of which are work published after 2007 from the ecological literature. The literature review for this edition was completed in early 2012.

The chapters are arranged in a logical progression. Chapter 1 discusses and answers the question, Why study weeds and why are they important? The second chapter pursues discussion of the definition of weed begun in Chapter 1, and it 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 (i.e., undesirable) in all places. Many plants have uses known to many and are studied by ethnobotanists. Weed reproduction and dispersal and the very 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, Weed Population Genetics, was written by Dr. Michael Christophers, Associate Professor, Department of Plant Sciences, North Dakota State University, Fargo. It is a new addition to the book and describes the role, increasing importance, and understanding 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 co-author is Dr. Cynthia S. Brown, Associate Professor, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins. 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 weed management. For many, this is the essence of weed science, the fundamental topic: How are weeds controlled? Weed problems are created, and 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. A new chapter (11) on the challenges of weed management in organic cropping systems has been added. Chapter 11 continues discussion of control by biological techniques. Chapter 12 introduces important concepts related to biological control of weeds. Chapter 13 introduces herbicides and chemical control of weeds. It is important to note that 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 of the interactions of 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 mode of action and their chemical family. Herbicide formulation is covered in Chapter 17.

Chapter 18, new in this edition, deals with the problems and opportunities of herbicide-resistant crops and the problems and research challenges caused by the development of herbicide resistance among weeds. The influence of molecular biology on weed management is included. Chapter 19 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 20 is a brief presentation of the U.S. legislative decisions required to address some of the questions raised in Chapter 19. Chapter 21 brings things together by discussing weed management systems, many of which are largely conceptual and not yet prescriptive. A section on weed management decision aids completes the chapter. The last chapter (22) 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 objective of this book is 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 selection of methods to control weeds in a broad range of crops, on non-croplands, 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 hope the book conveys some of the challenges of the world of weeds, their management, and the importance of weed problems to agriculture, 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, USDA/ARS, University of Missouri; Dr. David L. Mortensen, The Pennsylvania State University; Dr. Robert F. Norris, University of California at Davis; Dr. Gregory L. Orr, Fort Collins, CO; Dr. Keith Parker, Syngenta Corp., Greensboro, NC; 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, NC. The book has been improved because of their efforts and the comments of eight anonymous reviewers on my proposal for the fourth edition.

I express my gratitude to Ms. Maggie Hirko and Ms. Janet Dill, who have made my task more pleasant and easier by their courtesy, regular assistance, and tolerance of my, perhaps too frequent, requests for assistance.

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.

Endnote: Agricultural Examples

• The mid-1960s controversy over the real and suspected hazards of 2,4,5-T, 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 and permanently injuring 30,000 people.

• Pesticide poisoning: No one knows for sure, but it is estimated that between 1 million and 5 million cases of pesticide poisoning occur every year throughout the world, resulting in 20,000 deaths. Developing countries use 25% of pesticides but experience 99% of the deaths. (See Chapter 19.)

• The 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. The concern for weed science is the widespread adoption of herbicide-resistant technology.

• Air and water pollution and animal suffering from Confined Animal Feeding Operations (CAFOs).

• 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

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

2. Zimdahl RL. My view. Weed Sci. 1999;47:1.

---

¹The books are: Ahlgren, G.H., Klingman, G.C., Wolf, D.E., 1951. Principles of Weed Control, Wiley, New York; Fernald, M.L., 1970. Gray’s Manual of Botany, eighth ed. American Book, New York; King, L.J., 1966. Weeds of the World: Biology and Control, Interscience, New York; and Muenscher, W.C., 1935. Weeds, Macmillian, New York.

²Davidson’s comments were made several years ago and cited by Kirschenmann, F., 2010. Some things are priceless. Leopold Lett. 22 (1), 5.

³Common names of herbicides are used throughout the text except in some tables in which they may be paired with one or more trade names.

Robert L. Zimdahl

Fort Collins, Colorado, 2012

Chapter 1

Introduction

The primary question is why should we study weeds? What is it about weeds that makes them worthy of study? Emphasis is given to the formidable obstacles, including weeds, that have been placed between humans and a continuing food supply. Nature does not recognize weeds as a separate category of plants. Weed science will not solve the world’s agricultural problems, but they will not be solved without careful study on weeds.

Keywords

Food supply; nature; study; weed; weed science

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.

—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 much attention, but they have never been respected or understood well. 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 difficult 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. This is not usually 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 or the generalship 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 and show how they have changed with time and lead to the future of weed management.

A brief review will lead to the conclusion that the story of agriculture is a story of struggle. It is the story of the struggles that have ensued in consequence of the sudden overturning of established conditions, and the substitution therefore 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 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, including the following:

• Physical constraints: lack of good highways, adequate transportation, and functional markets. These are food distribution, not food production, problems.

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

• Environmental constraints: too much or too little water, too short a growing season, poor soil, and highly erodible soil.

• Biological constraints: soil fertility, plant varieties not well adapted to a place, soil pH, and salinity.

• Political constraints: 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 crops are lost to pests, diseases, and inadequate storage before they reach the market. Surveys by the Food and Agriculture Organization of the United Nations (FAO, 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 (approximately $65 billion) and the world’s potato¹ crop (approximately $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. These losses were only up to harvest and do not include damage during storage—another large sum. Current, less complete estimates show that losses due to pests of all kinds have increased since the first FAO estimates were made. The dominant pests in the FAO and similar reports are insects and plant diseases. They are important. Weeds are also important but receive far less publicity.

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

• The desert locust (Schistocerca gregaria), a pest since biblical times. They fly in unexpectedly and can strip a field bare in 1 hour. They prefer grasses, but they 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 approximately one-third of the world’s land surface (Hill, 1994).

• Late blight [Phytophthora infestans (Mont.) D. By.], which caused the Irish potato famine of 1845–1849.

• 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 270 million 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. In fact, the battle has become more intense. Agriculture has evolved from many small 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 growing world human population (7 billion in 2012, projected to grow to 9 billion) has increased the need and the demand for ever greater quantities of high- quality food.

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 but 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 parable (Matthew 13:24–30) 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 a different species.

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)

Four major advances in agriculture 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.

A 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 beginning, 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 the 1900s, while searching the literature to confirm their own discoveries, produced enormous positive benefits in agriculture.

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.

A fourth major advance in agriculture has been the use of pesticides and plant growth regulators. These moved beyond mechanization to the chemicalization of agriculture and led to the development and growth of weed science. Weed science did not develop exclusively because of herbicide development, nor is its continued development dependent on herbicides, although they are an important part of knowledge concerning weeds and their management.

The dandelion is considered a weed by many.

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 johnsongrass 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 Figure 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 9 billion humans want or need. Therefore, we humans successfully modify the environment—the natural world—in many ways. All one has to do is look around to see what we have wrought. One of the most significant changes began more than 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—was slowly abandoned as we learned how to dominate and subdue,² which many began to and still interpret as a God-given right—even a duty. We made what Diamond, (1987) called the worst mistake in the history of the human race. The immediate success of settled agriculture resulted in a catastrophe. It allowed the human population to grow, perhaps beyond the limits of what the earth can support.

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

Population growth continues and we do not know for sure if we have exceeded the earth’s capacity. The limit may be approximately 1 billion people at the United Nations High Income Countries standard of living.³

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. And, for many humans, a better—indeed a good—life. We are the only species able to modify the natural world. It is no longer humans against nature. We decide what nature is and act to make it what we want it to be. All other species must adapt to the environment as it is.

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, 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?

Nature knows no such category as weed. The definition accepted by the Weed Science Society of America (Buchholtz, 1967) is that a weed is a plant growing where it is not desired. It is important to note the anthropocentric dimension of this definition. Desire is a human trait and 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 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 the crop 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.

This book discusses 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, currently 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 of which dealt with killing, controlling, or eradication. As you proceed, you will find that the 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. Figure 1.2 shows the world’s tropical and subtropical areas, their major crops, and the percentage of the world’s total crop grown in each area. The region’s ability to control weeds is shown in Figure 1.3 with data for the world and four major areas. Each segment in Figure 1.3 is divided into good, moderate or acceptable, low, and very poor weed management. The world’s tropical and subtropical regions (Figure 1.2) are home to approximately 66% of the world’s people. The regions that extend roughly from the Tropic of Capricorn on the north to the Tropic of Cancer on the south, produce most of some of the world’s most important crops. In the developed, primarily temperate, world, production agriculture and the ability to manage a large array of pests has made remarkable progress since 1971 when Dr. L. Holm of the University of Wisconsin prepared Figure 1.2. However, the areas identified still suffer from underdevelopment of weed science and other agricultural technology (Figure 1.3). Seventy countries in Asia, Africa, and Latin America were included in a more recent United Nations/FAO survey (Labrada, 1996). The countries had approximately 6.1×10⁸ (44%) of the world’s 1.4×10⁹ of arable land and, of most importance to our subject, inadequate weed control technology and knowledge (Figure 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. U.S. 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. For most Americans (although, unfortunately, not all), this is true. Not only is it true but also 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 variables, 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. Many had imagined but never seen such a picture before. Space exploration opened exciting new vistas and opportunities for exploration, but, for now, we are confined to this planet. About 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 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 it is often slipping behind. About 10% of the world’s 33 billion acres of land are arable, and although 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 is certainly 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 use intelligently all present agricultural technology and continue to develop better, safer, equally effective technology. Shared technology and knowledge will permit our neighbors in this world to farm in ways that create opportunities to realize 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. 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.

Literature Cited

1. Aldrich RH, Kremer RJ. Principles in Weed Management second ed. Ames, IA: Iowa State Univ. Press; 1997; pp. 455.

2. Appleby AP. A history of weed control in the United States and Canada—A sequel. Weed Sci. 2005;53:762–768.

3. Bailey LH. The Survival of the Unlike: A Collection of Evolution Essays Suggested by the Study of Domestic Plants London: Macmillan; 1906.

4. Buchholtz KP. Report of the terminology committee of the weed science society of America. Weeds. 1967;15:388–389.

5. Cato, the Elder. De Agri Cultura. 2nd century B.C.

6. Cox, S., 2006. Civilization’s weedy roots, In The Land Report 84 (Spring), pp. 8–10.

7. De Morgan, A.C., 1850. A Budget of Paradoxes, item 662.1, International Thesaurus of Quotations, 1970. Compiled by R.T. Tripp, pp. 453.

8. Diamond J. The worst mistake in the history of the human race. Discover. 1987;May:64–66.

9. Food and Agriculture Organization of the United Nations, 1963. Production Yearbook.

10. Food and Agriculture Organization of the United Nations, 1975. Production Yearbook.

11. Goodwin DC, Johnstone PH. A brief chronology of American agricultural history. U.S Dep Agric Yearb Agric. 1940;1184–1196.

12. Grant L. Too Many People: The Case for Reversing Growth Santa Ana, CA: Seven Locks Press; 2000; pp. 102.

13. Hill DS. Agricultural Entomology Portland, OR: Timber Press; 1994; pp. 635.

14. Holm L. The role of weeds in human affairs. Weed Sci. 1971;19:485–490.

15. Labrada R. Weed management status in developing countries. Second Int Weed Cont Congr. 1996;2:579–589.

16. Liebig and After Liebig, 1942. American Association for Advancement of Science. Washington, DC, pp. 111.

17. Malthus TR. An Essay on the Principle of Population as It Affects the Future Improvement of Society, with Remarks on the Speculations of Mr Godwin, M Condorcet, and Other Writers Macmillan 1798; 1966 ed., pp. 396.

18. Markham E. The man with the hoe. In The Pocket Book of Verse. 1940 New York: Pocket Books; 1899; pp. 303–304.

19. Moldenke HN, Moldenke AL. Plants and the Bible New York: Dover; 1952; pp. 70–72, 133–134, and 153.

20. Rasmussen WD. Agriculture in the United States: A Documentary History New York: Random House; 1975; 4 vols.

21. Shaw, W.C., Hilton, J.L., Moreland, D.E., Jansen L.L., 1960. Herbicides in plants. pp. 119–133, in The Nature and Fate of Chemicals Applied to Soils, Plants and Animals. USDA, ARS, pp. 20–29.

22. Timmons FL. A history of weed control in the United States and Canada. Weed Sci. 1970;18:294–307 Republished—Weed Sci. 53, 748-761.

23. Williams, H., 1937. The Poems of Jonathan Swift. Vol. II. On Poetry: A Rhapsody. pp. 639–659. Specific reference on p. 651.

24. World Development Report. Development and Climate Change Washington, DC: The World Bank; 2010; pp. 417.

25. Zimdahl RL. A History of Weed Science in the United States London: Elsevier; 2010; pp. 207.

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¹Common and scientific names of all crops and weeds mentioned in the text are paired in Appendix 1 and Appendix 2, respectively.

²And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping animal that creepeth upon the earth. So God created man in his [own] image, in the image of God created he him; male and female created he them. And God blessed them, and God said to them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living animal that moveth upon the earth (Genesis 1:26-28)

³Grant (2000) provided a debatable, intellectually challenging answer to the nagging question: How many people can the earth support? First, one must ask, At what level? He suggests dividing the world gross income ($US) by the average gross income of those of us who live in the United Nations High Income Countries. Using 2008 data (World Development Report, 2010), the answer is slightly more than 1 billion people. Using 2002 data, the answer was 1.25 billion people.

Chapter 2

Weeds—The Beginning

Chapter 2 discusses and answers several questions. What is a weed? How are they defined? What are their characteristics? How are they harmful? What do they cost?

Keywords

Characteristics; cost; definition; harm

Fundamental Concepts

• The most basic concept of weed science is embodied in the term weed.

• Weeds are defined in many ways, but most definitions emphasize behavior that affects humans.

• Weeds share some characteristics.

• There are at least nine ways weeds express their undesirability.

• There are no completely accurate estimates of what weeds cost in the United States or the world. Losses due to weeds in the United States surely exceed $8 billion per year.

Objectives

• To understand why weeds are defined as they are.

• To know the characteristics that weeds share.

• To understand how weeds cause harm.

• To appreciate how estimates of the cost of weeds are made and the magnitude of costs.

The Beginning

… and nothing teems

But hateful docks, rough thistles, kecksies, burs,

Losing both beauty and utility.

And as our vineyards, fallows, meads, and hedges

Defective in their natures, grow to wildness;

Even so our houses, and ourselves, and children,

Have lost, or do not learn, for want of time,

The sciences that should become our country.

W. Shakespeare, King Henry V, Act V, Scene 2. The Duke of Burgundy addressing the Kings of France and England

I will go root away the noisome weeds,

which without profit suck the soil’s fertility from wholesome flowers.

W. Shakespeare, Richard II, Act III, Scene III A gardener speaking to a servant in the Duke of York’s garden

There are laws in the village against weeds

The law says a weed is wrong and shall be killed

The weeds say life is a white and lovely thing

And the weeds come on and on in irrepressible regiments.

Sandburg, C. 1922. Weeds. p. 241 in Smoke and Steel. Harcourt Brace & Howe, New York

It is indisputable that farmers have always been aware of weeds in their crops, although evidence for their awareness and concern is nearly all anecdotal. It just makes sense that farmers had to be aware of weeds, even though they could not do much to control them. Clark and Fletcher (1923) suggested that the annual losses due to the occurrence of pernicious weeds upon farm lands, although acknowledged in a general way, are far greater than is realized. They thought this was because farmers gave little critical attention to the weeds growing among their crops. They did not deny that farmers were aware of the weeds. Their book has carefully drawn color pictures of 71 weeds and 100 seeds. It is interesting to note that many of the same weeds are shown in most current weed identification books. The original hardback book cost $2.00. There are very few older books about weeds. Most are identification books as is Clark and Fletcher’s book. Weed science is relatively new among the agricultural sciences that study pests. It cannot claim the historical lineage of entomology or plant pathology. No one disputes that weeds have been present as long as other pests, but they have not been studied as long.

Timmons (1970) reported that available literature indicates that relatively few agricultural leaders and farmers became interested in weeds as a problem before 1200 AD or even before 1500 AD. One cannot be certain how he selected the dates, but his claim has not been disputed. We can be certain that the critical attention Clark and Fletcher thought was absent increased, but not rapidly, primarily because the general attitude seemed to be that weeds were a curse which must be endured, and about which little could be done except by methods which were incidental to crop production, and by laborious supplemental hand methods (Timmons, 1970). In 1731, Tull (1829) appealed for greater attention to weeds:

It is needless to go about to compute the value of the damage weeds do, since all experienced husbandmen know it to be very great, and would unanimously agree to extirpate their whole race as entirely as in England they have done the wolves, though much more innocent and less rapacious than weeds.

Farmers, however, were bound by their inability to do much about weeds except by the laborious hand methods Timmons mentioned.

Insects caused obvious human and crop problems. Weeds, with a few exceptions, do not cause direct harm to humans. Those that do, (e.g., poison ivy and poison oak) can be avoided. Neither was widespread as a weed of crops nor of great concern to the majority of people. Some weeds aggravated human allergies, but many other plants are also allergenic. Insects and insecticides were respectively causes of and solutions to human disease problems. Weeds and herbicides were not, and less attention was paid to them. Weeds were agricultural problems, not organisms of general societal concern. Before World War II (1940–1945), only a few scientists were interested in the study of weeds and in developing techniques to reduce the crop losses caused by weeds.¹

It is true that weeds were not and in most situations are still not of general societal concern (lawns and golf courses are notable exceptions to this generalization). However, it is worthy of note and not generally recognized in weed science that weed control and the way agriculture was practiced have influenced societal structure and attitudes toward women: Cultural norms about the economic role of the sexes can be traced back to traditional farming practices (Economist, 2011). I claim, without supporting evidence, that more human labor is spent weeding crops than is spent on any other task people do, and most of the weeding is done by women. Boserup (1970) was among the first to argue that cultural norms about women’s roles and their place in society can be traced back to traditional farming practices. Alesina et al. (2011) affirm Boserup’s hypothesis. Traditional agricultural practices have had a strong influence on the gender division of labor and the evolution of gender norms. Today, societies that practiced what Alesina et al., and many others, call plough agriculture have significantly lower rates of female participation in the workplace, in politics, and in entrepreneurial activities, as well as a greater prevalence of attitudes favoring gender inequality. Plough agriculture is dominated by men, who work with plows, cultivators, and planting and harvesting machines to grow crops for sale. Plows and other machines require the greater upper body strength that men have to control the machines and the animals that power them. Women’s role in plough agriculture is limited. There is less need for weeding than in what Boserup calls shifting cultivation, which does not involve machines. It is labor-intensive and requires hoes and digging tools. Hoes are used primarily for weeding, which is done by women. In societies that were and in many cases still are dominated by shifting cultivation, women are much more likely to be employed in the modern non-agricultural labor force. Thus, the modern societal role of women can be traced back to how agriculture was practiced and who did the weeding.

Definition of the Word Weed

To be fully conversant with any subject, it is mandatory that one understand its basic concepts. The most basic concept of weed science is embodied in the word weed. Each weed scientist has a clear understanding of the term, but there is no universal definition shared by all scientists. In 1967, the Weed Science Society of America defined a weed as a plant growing where it is not desired (Buchholtz, 1967). In 1989, the Society’s definition was changed to define a weed as any plant that is objectionable or interferes with the activities or welfare of man (Humburg, 1989, p. 267; Vencill, 2002, p. 462). The European Weed Research Society (1986) defined a weed as any plant or vegetation, excluding fungi, interfering with the objectives or requirements of people. Although the definitions are clear, they are not accepted by all scientists. The definitions leave the burden and responsibility for specific identification and final definition with people. People determine when a particular plant is growing in a place where it is not desired or when it interferes with their activities or welfare.

The Oxford English Dictionary (Little et al., 1973) defines a weed as an herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegetation. The human role is again clear because it is we who determine use or beauty and which plants are to be regarded as superior. It is important that weed scientists and vegetation managers remember the importance of definitions as determinants of their views of plants and attitudes toward them.

How one defines something largely determines one’s attitude toward the thing defined, and, for the weed scientist and vegetation manager, the definition determines which plants are weeds are thus undesirable and to be controlled. Weeds, like other plants, lack consciousness and cannot enter the court of public opinion to claim rights. Humans can assign rights to plants and serve as their counsel to determine if they have rights and then advocate their rights or lack thereof. Our attitude toward weedy plants need not always be shaped by another’s definition. We do not always agree about what should be done when others define things:

Once in a golden hour,

I cast to earth a seed.

Upon there came a flower,

The people said a weed.

Read my little fable:

He that runs may read

Most can raise the flowers now,

For all have got the seed.

And some are pretty enough,

And some are poor indeed:

And now again the people

Call it but a weed.

A. Tennyson, The Flower

It is clear that there is disagreement about what a weed is and what plants are weeds. Harlan and de Wet (1965) assembled several definitions (partially reproduced herein) to show the diversity of definitions of the same or similar plants. The array of definitions emphasizes the care weed scientists and vegetation managers must take in equating how something is defined with a right or privilege to control.

Godinho (1984) compared the definition of the French words d’aventice and le mauvaise herbe with the English weed and the German unkraut. No single definition was found for weed and unkraut because both have two meanings:

1. In the ecological sense, weed, unkraut, and d’aventice mean a plant that grows spontaneously in an environment modified by man.

2. In the weed science sense, weed, unkraut, and malherbe (Italian) or le mauvaise herbe mean an unwanted plant.

In some languages, weeds are just bad (mal) plants. In Spanish, it is mala hierba or malezas, and in Italian, it is malherbe. One must agree with Godinho, Anderson (1977), Crafts and Robbins (1967), and Fryer and Makepeace (1977) that neither the word weed nor the weedy characteristics of the plants to which the word is assigned are easy to define.

Aldo Leopold (1943) made the point well in an article that was critical of the 1926 bulletin Weeds of Iowa. Many of Iowa’s native plants are in the bulletin, and Leopold noted that they, in addition to their inherent beauty, have value as wildlife food, for nitrogen fixation, or as creators of stable plant communities. He admits that many plants others call weeds are frequent in pastures but argued that soil depletion, overgrazing, and needless disturbance of advanced successional stages encourage expression of their weediness and create the need for control. Leopold argues that the definition of weed is part of the problem because not all plants that some call weeds should be blacklisted for general persecution. Leopold’s view is supported by McMichael (2000), who noted, with supporting evidence, that in many rural cultures, non-crop plants (often termed weeds) represent food, fodder, and medicine. See Chapter 4 for additional supporting citations.

About 3000 of the 350,000+recognized plant species have been or are cultivated somewhere in the world. It is incorrect to assume those that are not cultivated are weeds. That this is wrong is not debatable. However, when a new, unknown plant appears in a field or garden, one’s objectivity is difficult to maintain.

The ulterior etymology of weed is unknown, but an exposition of what is known was provided by King (1966). Weed has Germanic, Romance language, and Oriental roots. He concluded that weed is an example of language as an accident of usage. He was unable to find a common word or words, in ancient languages, for the collective term weed. The ultimate etymology of weed is unknown.

It is logical to assume that even if one cannot define weed, it should be possible to identify the origin of individual species and determine certain characteristics of weeds that come from native and naturalized flora. Some plants succeed as weeds because they evolve forms adapted to disturbed environments more readily than other species. Baker’s (1965) definition, repeated here, emphasizes success in disturbed environments, a point he reiterated in a later paper (Baker, 1991):

A plant is a weed if, in any specified geographical area, its populations grow entirely or predominantly in situations markedly disturbed by man (without, of course, being deliberately cultivated plants). Thus, for me, weeds include plants which are called agrestals by some writers of floras (they enter agricultural land) as well as those which are ruderals (and occur in waste places as well as along roadsides). It does not seem to me necessary to draw a line between these categories and accept only the agrestals as weeds (although this is advocated by some agriculturally oriented biologists) because in many cases the same species occupy both kinds of habitat. Ruderals and agrestals are faced with many similar ecological factors, and the taxa which show these distributions are, in my usage, weedy.

If one considers weeds in the Darwinian sense of a struggle for existence, they represent one of the most