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The updated edition of the classic, fundamental book on weed science

Weed Science provides a detailed examination of the principles of integrated weed management with important details on how chemical herbicides work and should be used. This revised Fourth Edition addresses recent developments affecting weed science. These include the increased use of conservation-tillage systems, environmental concerns about the runoff of agrochemicals, soil conservation, crop biotechnology, resistance of weeds and crops to herbicides, weed control in nonagricultural settings and concerns regarding invasive plants, wetland restoration, and the need for a vastly improved understanding of weed ecology.

Current management practices are covered along with guidance for selecting herbicides and using them effectively. To serve as a more efficient reference, herbicides are cross-listed by chemical and brand name and grouped by mechanism of action and physiological effect rather than chemical structure. In addition, an introduction to organic chemistry has been added to familiarize readers with organic herbicides. Also included are guidelines on weed-control practices for specific crops or situations, such as small grains, row crops, horticultural crops, lawns and turf, range land, brush, and aquatic plant life.

Generously supplemented with 300 drawings, photographs, and tables, Weed Science is an essential book for students taking an introductory course in weed science, as well as a reference for agricultural advisors, county agents, extension specialists, and professionals throughout the agrochemical industry.

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Product Details

  • ISBN-13: 9780471370512
  • Publisher: Wiley
  • Publication date: 1/28/2002
  • Edition description: REV
  • Edition number: 4
  • Pages: 688
  • Product dimensions: 5.90 (w) x 9.40 (h) x 1.70 (d)

Meet the Author

THOMAS J. MONACO is Professor and Head of the Horticultural Science Department at North Carolina State University in Raleigh, North Carolina.
STEPHEN C. WELLER is a Professor of Weed Science in the Horticulture and Landscape Architecture Department at Purdue University in West Lafayette, Indiana.
FLOYD M. ASHTON is Professor Emeritus in the Department of Botany at the University of California at Davis.

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Read an Excerpt

Weed Science

Principles and Practices
By Thomas J. Monaco Steve C. Weller Floyd M. Ashton

John Wiley & Sons

ISBN: 0-471-37051-7

Chapter One

Introduction to Weed Science

Weed science is the scientific discipline that studies plants that interfere with human activity. Areas of study range from basic biological and ecological investigations to the design of practical methods of managing weeds in the environment. The overall goal of weed management is to design the most appropriate methods in a variety of situations that ensure a sustainable ecosystem and a minimum influence of nuisance weeds.

The first question is "What is a weed?" Before a plant can be considered a weed, humans must provide a definition. Many varying definitions have been developed for weeds, depending on each particular situation where they occur and the plants involved. For the purpose of this book, we define a weed as a plant growing where it is not desired, or a plant out of place-some plant that, according to human criteria, is undesirable. We decide for each particular situation which plants are or are not desired in terms of how they affect our health, our crops, our domesticated animals, or aesthetics. For example, some people consider a dandelion in a lawn a weed and want to control it, whereas others feel the dandelion is desirable and do not control it. The same thinking is involved for any weed situation, whether in a crop field, a pasture, abody of water, or in a noncropland or natural site.

Weeds are also classed as pests and included with insects, plant diseases, nematodes, and rodent pests. A chemical used to control a pest is called a pesticide, and a chemical used specifically for weed control is known as a herbicide.

Weed control is the segment of weed science that most people are familiar with and where the greater part of education and training is focused. The methods employed to manage weeds vary, depending on the situation, available research information, tools, economics, and experience. Improved agricultural technology over the centuries has contributed greatly to increased food production (Warren, 1998) and a related increase in our standard of living. Advances in weed control practices have been an important part of these gains.

Weed control in human endeavors is as old as the growing of food crops and has progressed from intense human inputs to methods involving less human energy and increasing inputs from other sources (Figure 1-1). For thousands of years humans have achieved amazing advances in weed control. Before 10,000 B.C., weeds were removed from crops by hand. The efforts of one person could hardly feed that person, and starvation was common. Later, farmers substituted a sharp stick or other wooden tools for fingers. By 1000 B.C., crude hoes dragged by an animal through a field helped reduce human labor in seedbed preparation, and, later, metal hoes dragged by a horse or ox through a field became common, although subsistence farming was still the norm. In 1731, in his book, Horse-Hoeing Husbandry, Jethro Tull proposed planting crops in rows to permit "horse-hoeing" and was among the first to use the word weed with its present spelling and meaning. With this advancement, one farmer could now provide food for 4 people. Less than 200 years later (by 1920), tractors started to replace horses in most agricultural situations and one farmer could now produce enough food for 8 people. Progressively, and with increasing momentum, humans learned to use their bare hands, hand tools, horsepower, and tractor power to manage weeds. All these methods still used brute force to control weeds. However, with the introduction of herbicides in 1947, one farmer could now feed 16 people. During the intervening years, many new herbicides have been developed and extensively used, resulting in chemical energy becoming the major tool of weed control in the United States and other countries (Figure 1-2). In 1990, one farmer could feed 75 people. This means that multitudes of people who previously worked on farms mainly hoeing weeds have been able to pursue other jobs and provide inputs into a wide variety of goods and services that have helped to increase our standard of living. As we continue to investigate new approaches to weed management, additional chemical, cultural, biological, and biotechnology-based practices will provide ever improved tools to permit a sustainable agriculture.


In the future, weed control methods presently being intensively researched will allow expanded weed control options beyond herbicides and mechanical methods in both agricultural and nonagricultural weed management. Biological control by insects and plant disease organisms, predictive modeling of weed/crop interactions, and the use of herbicide antidotes, more competitive crops, allelopathy, and genetic engineering/genomics will become more common as their reliability is improved.

The overall objective of additional approaches is to discover new, more environmentally acceptable weed management tools that not only control weeds effectively, but improve our understanding of weed ecology/biology and allow more sustainable management of the agroecosystem. Biological control of weeds by insects and plant disease organisms has had considerable success in several weed management situations, and ongoing research will lead to additional uses of biologicals. Considerable biological research involves the potential introduction of natural control species from an invasive weed species site of origin (Watson, 1993). The use of herbicide antidotes or safeners (Hatzios and Wu, 1996) to protect crop plants has been successful for some herbicides in certain crops-for example, chloroacteamide herbicides in corn and sorghum. One of the greatest recent changes in weed control has occurred through the genetic transformation of crops with herbicide-resistant genes and the incorporation of herbicide resistance through conventional breeding. In 1999 and 2000, more than 50% of the U.S. soybean acreage and more than 30% of the corn acreage was planted to cultivars resistant to one of several herbicides. Genetic engineering offers tremendous potential in all areas of weed science for improved understanding of plants and of weed control. Genetic engineering, along with recent advances in sequencing the genome of Arabidopsis (and in the future, other plants), will allow a clear determination of specific gene function. Such knowledge will permit gene manipulation and modification in our agricultural endeavors, such as the discovery of genes that contribute to weediness, competitiveness, allelopathy, dormancy, or a plant's being a perennial, among functions (Weller et al., 2001; Gressel, 2000). Genes of interest in weed science, once discovered, may then be engineered into crops or used to manipulate weeds to achieve a desirable effect in crop productivity and reduced weed influences. One area in particular where genetic engineering may play a role is allelopathy. Allelopathy (Rizvi and Rizvi, 1992) results from any direct or indirect inhibitory or stimulatory effect by one plant (including microorganisms) on another through the production and release into the environment of a chemical compound. Although no commercial breakthroughs have yet occurred in engineering plants to produce higher levels of allelochemicals, several such genes have been identified in Arabidopsis. Genetic engineering of crop plants or cover crops with genes for allelochemicals could allow major strides in developing plants useful in weed management. The future for weed science is exciting, as there are many opportunities for challenging basic and applied approaches for weed management in our environment, as reviewed by Hall et al. (2000).


Weeds should be everybody's business, as they affect everyone in one way or another. They not only reduce crop production and increase the cost of agricultural products, but they also cause problems for the general public in many other ways-for example, in regard to health and maintaining home landscaping recreational areas and other noncrop areas. Specific problems include lower crop and animal yields, less efficient land use, higher costs of insect and plant disease control, poorer-quality products, more water management problems, and lower human efficiency.

Lower Plant and Animal Yields

Weed control is an expensive but necessary part of agricultural production, directly affecting the price of food and other agricultural products. However, such products would be less abundant and more expensive without modern agricultural and weed science technology. Weeds reduce yields of plant and animal crops. Plant yields are primarily reduced by competition between the weed and the crop for soil water, soil nutrients, light, and carbon dioxide. Certain weeds may also reduce plant yields by releasing allelopathic compounds into the environment (Figure 1-3).

Livestock yields may be reduced by weeds, whose growth allows less pasture or range forage, or by poisonous or toxic plants that cause slower growth or death in animals (Figure 1-4).

Less Efficient Land Use

The presence of weeds on a given piece of land can reduce the maximum efficiency of the use of that land in a number of ways. These include increased costs of production and harvest, reforestation, and noncropland maintenance, as well as reduced plant growth, root damage resulting from cultivation, limitation of the crops that can be grown, and reduced land values.

Higher Costs of Insect and Plant Disease Control

Weeds harbor insect and disease organisms that attack crop plants. For example, the carrot weevil and carrot rust-fly may be harbored by the wild carrot, only later to attack the cultivated carrot. Aphids and cabbage root maggots live in mustard and later attack cabbage, cauliflower, radish, and turnips. Onion thrips live in ragweed and mustards and may later prey on an onion crop. The disease of curly top on sugar beets is carried by insect vectors that live on weeds in wastelands. Many insects overwinter in weedy fields and field borders.

Disease organisms such as black stem rust may use the European barberry, quackgrass, or wild oat as a host prior to attacking wheat, oats, or barley. Some virus diseases are propagated on members of the weedy nightshades. For example, the virus causing "leaf roll" of potatoes lives on black nightshade. It is thought that aphids carry the virus to potatoes. A three-way harboring and transmission of a mycoplasma disease from weeds to citrus has been discovered in California. Leaf hoppers transmit the disease organism, citrus stubborn disease (Spiroplasma citri), to and from diseased periwinkle and to and from London rocket (Sisymbrium irio). These weedy plants act as a source of the disease organism to infect citrus trees.

Poorer-Quality Products

All types of crop products may be reduced in quality. Weed seeds and onion bulblets in grain and seed, weedy trash in hay and cotton, spindly "leaf crops," and scrawny vegetables are a few examples.

Livestock products may be lower priced or unmarketable because of weeds; for example, onion, garlic, or bitterweed flavor in milk, and cocklebur in wool, reduce the quality of the products. Poisonous plants may kill animals, slow their rates of growth, or cause many kinds of abnormalities (Figure 1-4).

More Water Management Problems

Aquatic weeds can be a major problem in irrigation and drainage systems, lakes, ponds, reservoirs, and harbors. They restrict the flow of water (Figure 1-5), interfere with commercial and recreational activities, and may give off undesirable flavors and odors in domestic water supplies. Their control is often difficult and expensive. Terrestrial weeds growing at the edges of aquatic sites can also be a problem. Chapter 27 is devoted to the weed problems and control methods on these sites.

Lower Human Efficiency

Weeds have been a plague to humans ever since they gave up the hunter's life. Traveling in developing nations, one may feel that half the world's population work in the fields, stooped, moving slowly, and silently weeding. These people are a part of the great mass of humanity that spends a lifetime simply weeding. Many young people doing such work in Africa, Asia, and Latin America can never attend school; women do not have time to prepare nutritious meals or otherwise care for their families. Modern weed-control methods integrated into the economies and cultures of developing nations provide relief from this arduous chore and give nations the opportunity to improve their standards of living through more productive work.

Weed control constitutes a large share of a farmer's work required to produce a crop. This effort directly affects the cost of crop production and thus the cost of food. It affects all of us, whether we farm or not.

Weeds reduce human efficiency through allergies and poisoning. Hay fever, caused principally by pollen from weeds, alone accounts for tremendous losses in human efficiency every summer and fall. Poison ivy, poison oak, and poison sumac cause losses in terms of time and human suffering; children occasionally die from eating poisonous plants or fruits.


The cost of weeds to humans is much higher than generally recognized. Because weeds are so common and widespread, people do not fully appreciate their significance in terms of losses and control costs. Although relatively accurate estimates have been made of losses and control costs on farms in the United States, other areas of economic impact are much more difficult to estimate. The latter include noncropland, recreational areas, homesite maintenance, aquatics, livestock, and human efficiency, as well as many others.

Weeds are common on all 485 million acres of U.S. cropland and almost one billion acres of range and pasture. In U.S. agriculture, weeds are estimated to reduce yields by 12% annually, or approximately $36 billion in lost revenue (USBC, 1998). In addition, another $4 billion is spent each year on herbicides to control these weeds (Pimentel et al., 1999), and more than $3 billion for cultural and other methods of control.

Bridges (1992) surveyed weed losses in 46 crops in the United States. The annual monetary loss caused by weeds in crops using current Best Management Practices (BMP) with herbicides was estimated at $4.1 billion, and this cost increased to $19.6 billion, or a 4.9-fold increase if herbicides were not available. For both categories of weed control, approximately 82% of the monetary loss occurred in field crops, 5% in noncitrus fruit crops, 3% in citrus crops, 1% in tree nuts, and 9% in vegetables (Bridges, 1992).

Other areas where weeds are costly include pastures and rangelands, lawns, gardens and golf courses, and aquatic sites. Estimated costs of controlling weeds were $5 billion in pastures and rangelands, $1.5


Excerpted from Weed Science by Thomas J. Monaco Steve C. Weller Floyd M. Ashton Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Preface ix

I Principles 1

1 Introduction to Weed Science 3

2 Weed Biology and Ecology 13

3 Integrated Weed Management 44

4 Herbicide Registration and Environmental Impact 84

5 Herbicides and the Plant 98

6 Herbicides and the Soil 127

7 Formulations and Application Equipment 146

II Herbicides 181

8 Chemistry and Classification of Herbicides by Mechanism of Action 183

9 Photosystem II Inhibitors 198

10 Pigment Inhibitors 223

11 Membrane Disrupters 235

12 Cell Growth Disrupters and Inhibitors 256

13 Cellulose Biosynthesis Inhibitors 284

14 Growth Regulator Herbicides 291

15 Lipid Biosynthesis Inhibitors 311

16 Inhibitors of Amino Acid Biosynthesis 326

17 Miscellaneous Herbicides 362

18 Herbicide Resistance in Crops and Weeds 379

III Practices 397

19 Small Grains and Flax 399

20 Field Crops Grown in Rows 413

21 Small-Seeded Legumes 443

22 Vegetable Crops 449

23 Fruit and Nut Crops 484

24 Lawn, Turf, and Ornamentals 500

25 Pastures and Rangelands 522

26 Brush and Undesirable Tree Control 531

27 Aquatic Weed Control 546

28 Industrial Vegetation Management 564

29 Diagnosis of Herbicide Injury 573

30 Weed Science in the Future 592

Appendix 599

Index 649

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