PART IPrinciples

1 Introduction to Weed Science

INTRODUCTION TO WEED SCIENCEWeed science is the scientific discipline that studies plants that interfere with humanactivity. Areas of study range from basic biological and ecological investigations to thedesign of practical methods of managing weeds in the environment. The overall goalof weed management is to design the most appropriate methods in a variety of situationsthat 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 forweeds, depending on each particular situation where they occur and the plantsinvolved. For the purpose of this book, we define a weed as a plant growing where itis 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 notdesired 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 andwant 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, apasture, a body 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 andwhere the greater part of education and training is focused. The methods employed tomanage weeds vary, depending on the situation, available research information, tools,economics, and experience. Improved agricultural technology over the centuries hascontributed greatly to increased food production (Warren, 1998) and a related increasein our standard of living. Advances in weed control practices have been an importantpart of these gains.

Weed control in human endeavors is as old as the growing of food crops and hasprogressed from intense human inputs to methods involving less human energy andincreasing inputs from other sources (Figure 1-1). For thousands of years humans haveachieved amazing advances in weed control. Before 10,000 B.C., weeds were removedfrom crops by hand. The efforts of one person could hardly feed that person, andstarvation was common. Later, farmers substituted a sharp stick or other wooden toolsfor fingers. By 1000 B.C., crude hoes dragged by an animal through a field helpedreduce human labor in seedbed preparation, and, later, metal hoes dragged by a horseor ox through a field became common, although subsistence farming was still thenorm. In 1731, in his book, Horse-Hoeing Husbandry, Jethro Tull proposed planting

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crops in rows to permit “horse-hoeing” and was among the first to use the word weedwith its present spelling and meaning. With this advancement, one farmer could nowprovide food for 4 people. Less than 200 years later (by 1920), tractors started toreplace horses in most agricultural situations and one farmer could now produceenough food for 8 people. Progressively, and with increasing momentum, humanslearned to use their bare hands, hand tools, horsepower, and tractor power to manageweeds. All these methods still used brute force to control weeds. However, with theintroduction of herbicides in 1947, one farmer could now feed 16 people. During theintervening years, many new herbicides have been developed and extensively used,resulting in chemical energy becoming the major tool of weed control in the UnitedStates and other countries (Figure 1-2). In 1990, one farmer could feed 75 people. Thismeans that multitudes of people who previously worked on farms mainly hoeingweeds have been able to pursue other jobs and provide inputs into a wide variety ofgoods and services that have helped to increase our standard of living. As we continueto investigate new approaches to weed management, additional chemical, cultural,

Figure 1-1. Energy sources providing weed control at different times. Data shown for 1920,1947, and 1990 are for the United States.

4 INTRODUCTION TO WEED SCIENCE

biological, and biotechnology-based practices will provide ever improved tools topermit a sustainable agriculture.

THE FUTURE

In the future, weed control methods presently being intensively researched will allowexpanded weed control options beyond herbicides and mechanical methods in bothagricultural and nonagricultural weed management. Biological control by insects andplant disease organisms, predictive modeling of weed/crop interactions, and the useof herbicide antidotes, more competitive crops, allelopathy, and geneticengineering/genomics will become more common as their reliability is improved.

The overall objective of additional approaches is to discover new, moreenvironmentally acceptable weed management tools that not only control weedseffectively, but improve our understanding of weed ecology/biology and allow moresustainable management of the agroecosystem. Biological control of weeds by insectsand plant disease organisms has had considerable success in several weed managementsituations, and ongoing research will lead to additional uses of biologicals. Considerablebiological research involves the potential introduction of natural control species from an

Figure 1-2. Crop energy output per farmer or the number of people fed by one farmer. Datafor 1920, 1947, 1980, and 1990 are for the United States.

THE FUTURE 5

invasive weed species site of origin (Watson, 1993). The use of herbicide antidotes orsafeners (Hatzios and Wu, 1996) to protect crop plants has been successful for someherbicides in certain crops—for example, chloroacteamide herbicides in corn andsorghum. One of the greatest recent changes in weed control has occurred through thegenetic transformation of crops with herbicide-resistant genes and the incorporationof herbicide resistance through conventional breeding. In 1999 and 2000, more than50% of the U.S. soybean acreage and more than 30% of the corn acreage was plantedto cultivars resistant to one of several herbicides. Genetic engineering offerstremendous potential in all areas of weed science for improved understanding ofplants and of weed control. Genetic engineering, along with recent advances insequencing the genome of Arabidopsis (and in the future, other plants), will allow a cleardetermination of specific gene function. Such knowledge will permit gene manipulationand modification in our agricultural endeavors, such as the discovery of genes that contributeto weediness, competitiveness, allelopathy, dormancy, or a plant’s being a perennial, amongfunctions (Weller et al., 2001; Gressel, 2000). Genes of interest in weed science, oncediscovered, may then be engineered into crops or used to manipulate weeds to achieve adesirable effect in crop productivity and reduced weed influences. One area in particularwhere 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 (includingmicroorganisms) on another through the production and release into the environment of achemical compound. Although no commercial breakthroughs have yet occurred inengineering plants to produce higher levels of allelochemicals, several such genes havebeen identified in Arabidopsis. Genetic engineering of crop plants or cover crops withgenes for allelochemicals could allow major strides in developing plants useful in weedmanagement. The future for weed science is exciting, as there are many opportunities forchallenging basic and applied approaches for weed management in our environment, asreviewed by Hall et al. (2000).

WEED IMPACTS

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 othernoncrop areas. Specific problems include lower crop and animal yields, less efficientland 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, directlyaffecting the price of food and other agricultural products. However, such productswould be less abundant and more expensive without modern agricultural and weedscience technology. Weeds reduce yields of plant and animal crops. Plant yields are

6 INTRODUCTION TO WEED SCIENCE

primarily reduced by competition between the weed and the crop for soil water, soilnutrients, light, and carbon dioxide. Certain weeds may also reduce plant yields byreleasing allelopathic compounds into the environment (Figure 1-3).

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

Less Efficient Land Use

The presence of weeds on a given piece of land can reduce the maximum efficiencyof the use of that land in a number of ways. These include increased costs ofproduction and harvest, reforestation, and noncropland maintenance, as well asreduced plant growth, root damage resulting from cultivation, limitation of the cropsthat 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, thecarrot weevil and carrot rust-fly may be harbored by the wild carrot, only later to attackthe cultivated carrot. Aphids and cabbage root maggots live in mustard and later attackcabbage, cauliflower, radish, and turnips. Onion thrips live in ragweed and mustards

Figure 1-3. Weeds often reduce the yield of crops through allelopathic effects by exudatesfrom roots or leaves. This figure shows the effect of leaf leachates from various grasses on thegrowth of peach trees. From left to right: Control (fertilizer water); leachate from fescue +fertilizer; leachate from common bermudagrass + fertilizer; leachate from coastal bermudagrass +fertilizer; leachate from hybrid turf bermudagrass + fertilizer. Trees were watered daily for 6weeks. Note trees watered with the various grass leachates all had reduced growth compared tothe control trees even though they received similar amounts of fertilizer.

WEED IMPACTS 7

and may later prey on an onion crop. The disease of curly top on sugar beets is carriedby insect vectors that live on weeds in wastelands. Many insects overwinter in weedyfields 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 virusdiseases are propagated on members of the weedy nightshades. For example, the viruscausing “leaf roll” of potatoes lives on black nightshade. It is thought that aphids carrythe virus to potatoes. A three-way harboring and transmission of a mycoplasmadisease from weeds to citrus has been discovered in California. Leaf hoppers transmitthe disease organism, citrus stubborn disease (Spiroplasma citri), to and from diseasedperiwinkle and to and from London rocket (Sisymbrium irio). These weedy plants actas 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 bulbletsin grain and seed, weedy trash in hay and cotton, spindly “leaf crops,” and scrawnyvegetables are a few examples.

Livestock products may be lower priced or unmarketable because of weeds; forexample, onion, garlic, or bitterweed flavor in milk, and cocklebur in wool, reduce the

Figure 1-4. Photo of a Friesan-Holstein cow with forage-induced photosensitization. Ingestionof St. Johnswort can cause this problem which can lead to reduced growth, reduced milkproduction and skin infection and overall loss of productivity. The nonpigmented skin becomesred and blistered with accompanying hair loss. Black skin is not affected. (S.B. Hooser, PurdueUniversity).

8 INTRODUCTION TO WEED SCIENCE

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), interferewith commercial and recreational activities, and may give off undesirable flavors andodors 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. Chapter27 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 workin the fields, stooped, moving slowly, and silently weeding. These people are a part ofthe great mass of humanity that spends a lifetime simply weeding. Many young peopledoing such work in Africa, Asia, and Latin America can never attend school; womendo not have time to prepare nutritious meals or otherwise care for their families.Modern weed-control methods integrated into the economies and cultures ofdeveloping nations provide relief from this arduous chore and give nations theopportunity to improve their standards of living through more productive work.

Figure 1-5. In St. Cloud, Florida, the State Mosquito Control Board sprayed ditches withdiuron to eliminate weeds and improve drainage. The weed-control program reduced theexpenses of mosquito control enough to permit a one-third savings in the total budget. Right:One year after hand weeding. Left: One year after chemical treatment with diuron. (E.I. du Pontde Nemours and Company.)

WEED IMPACTS 9

Weed control constitutes a large share of a farmer’s work required to produce acrop. 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 inhuman efficiency every summer and fall. Poison ivy, poison oak, and poison sumaccause losses in terms of time and human suffering; children occasionally die fromeating poisonous plants or fruits.

COST OF WEEDS

The cost of weeds to humans is much higher than generally recognized. Becauseweeds are so common and widespread, people do not fully appreciate theirsignificance in terms of losses and control costs. Although relatively accurateestimates 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 includenoncropland, recreational areas, homesite maintenance, aquatics, livestock, andhuman efficiency, as well as many others.

Weeds are common on all 485 million acres of U.S. cropland and almost one billionacres of range and pasture. In U.S. agriculture, weeds are estimated to reduce yieldsby 12% annually, or approximately $36 billion in lost revenue (USBC, 1998). Inaddition, 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 annualmonetary 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 $19.6billion, or a 4.9-fold increase if herbicides were not available. For both categories ofweed control, approximately 82% of the monetary loss occurred in field crops, 5% innoncitrus 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 billion in lawns, gardens, and golf courses,and $100 million for aquatic situations (Pimentel et al., 1999). A high percentage ofthese costs is related to control of nonindigenous (alien) plant species that have beenintroduced into the United States. There are more than 1000 alien plant introductionsthat have become major weed pests in cropping systems.

Another example of high costs related to the nonherbicide approaches to weedcontrol relates to a new policy enacted in 2000 by the Los Angeles County SchoolBoard (Wall Street Journal, 2000). A nonherbicide weed control policy for schooldistrict land was begun with an estimation that manual weed control would take onlyone-sixth of groundskeeping time and cost $650,000 for equipment and 15 full-timeweeders. The result was that in less than a year, more than 50% of the groundskeepingtime was spent torching, digging, or pulling weeds, at a cost of $1.5 million and

10 INTRODUCTION TO WEED SCIENCE

requiring the work of 37 full-time employees. These examples demonstrate that asignificant increase in efficiency of food production and land management can resultfrom effective, efficient, and integrated weed-control strategies on lands used forhuman activities.

PREVENTION, MANAGEMENT, AND ERADICATION

Prevention

Prevention, which means stopping a given weed species from contaminating an area,is often the most practical means of controlling weeds. Prevention in agriculture is bestaccomplished by (1) making sure that new weed seeds are not carried onto a farm incontaminated crop seeds, feed, or on machinery, (2) preventing weeds on the farmfrom going to seed, and (3) preventing the spread of perennial weeds that reproducevegetatively. Prevention is a method of weed management that, if properly employed,could greatly reduce weed problems worldwide.

Control

Control is the process of limiting weed infestations. In crops, the weeds are limited sothat they have minimal effect on crop growth and yield. The degree of control isusually a matter of economics, a balance between the costs involved and the increasein profits due to the control of the weeds and the types of production systems and toolsbeing used. Each farmer has the ability to decide what level of weed control is suitableto reach the objectives of the cropping system. On noncropland, it is often desirableto remove essentially all vegetation for a specific period of time. Weeds are thuslimited to a level that does not allow them to interfere with human activities.

Most biological control programs using a highly specific insect or plant diseaseorganism as a control agent are based on obtaining adequate economic managementof a weed, but not eradication.

Eradication

Eradication is the complete elimination of all living plants, including their vegetativepropagules and seeds. Eradication is much more difficult than prevention or control.In general, it is justified only for the elimination of a serious weed in a limitedarea—for example, a perennial weed in a small area of a field, around fuel storagetanks, or in railroad yards.

The most difficult part of eradication is the elimination of the vegetative propagulesand seeds in the soil. Seeds of many weeds may remain dormant for a number of years,and in this dormant state they are not usually killed by standard weed control practices.Vegetative propagules and many weed seeds can be killed by soil fumigation, such asby methyl bromide or other chemicals (Chapter 7). This practice is expensive andmethyl bromide will not be available after 2005 because of environmental concerns,which has necessitated research to find suitable alternatives. Persistent soil-applied

PREVENTION, MANAGEMENT, AND ERADICATION 11

herbicides are also used; however, because of their long soil persistence, they preventthe growth of desirable species for substantial periods of time.

LITERATURE CITED AND SUGGESTED READING

Bridges, D. C., ed. 1992. Crop Losses Due to Weeds in the United States, 1992. WSSA,Lawrence, KS.

Gressel, J. 2000. Molecular biology of weed control. Transgenic Res. 9:355–382.

Hall, J. C., L. L. van Eerd, S. D. Miller, M. D. K. Owen, T. S. Prather, D. L. Shaner, M. Singh,K. C. Vaughn, and S. C. Weller. 2000. Future research directions for weed science. WeedTechnol. 14:647–658.

Hatzios, K. K., and J. Wu. 1996. Herbicide safeners: Tools for improving the efficacy andselectivity of herbicides. J. Envir. Sci. Health. B31:545–553.

Herbicide Handbook, 7th ed. 1994, and 1998 Supplement. WSSA, Lawrence, KS.

Herbicide tolerant crops: 1994. Their value to world agriculture. Proc. Brighton CropProtection Conf.—Weeds 2:637–660.

Pimentel, D., L. Lach, R. Zuniga, and D. Morrison. 1999. Environmental and economic costsassociated with non-indigenous species in the United States. Bioscience 50:53–65.

Rizvi, S. J. H., and V. Rizvi. 1992. Allelopathy: Basic and Applied Aspects. Chapman and Hall,London.

USBC. 1998. U.S. Bureau of the Census, Statistical Abstract of the United States, 1996. 200thed. Washington, DC, U.S. Government Printing Office.

Warren, Susan. Tangled Up in Green: Weeds Run Amok on School Playgrounds. Wall StreetJournal, October, 05, 2000. Vol 236, Issue 67, p A1.

Warren, G. F. 1998. Spectacular increases in crop yields in the United States in the twentiethcentury. Weed Technol. 12:752–760.

Watson, A. K., ed. 1993. Biological Control of Weeds Handbook. WSSA Monograph Ser. #7.WSSA, Lawrence, KS.

Weller, S. C., R. A. Bressan, P. B. Goldsbrough, T. B. Fredenburg, and P. M. Hasegawa. 2001.The impact of genomics on weed management in the 21st century. Weed Sci. 49:282–289.

WEB SITES

Weed Science Society of America

http://www.wssa.net/

National Center for Food and Agriculture Policy (NCFAP)

Pesticide use data for U.S. agriculture

http://www.ncfap.org

Go to “Pesticide Use” icon.

For chemical use, see the manufacturer’s or supplier’s label and follow thesedirections. Also see the Preface.

12 INTRODUCTION TO WEED SCIENCE