Prepared by Megan McPhee
(1998)

Dandelions, wild oats, tufted vetch, clover - no matter what part of the world a person comes from, they have seen weeds. Weeds have had a huge impact on agriculture through the ages, and they continue to effect agricultural practices today. Agriculture pushes succession back to its early stages, providing prime colonizing opportunities for weeds, and hence weeds will continue to be present in agricultural pursuits of the future.

Weeds have been defined many times, and in many ways. Anthrophyllic plants, weeds are plants said to grow where they are not wanted, and to possess virtues as yet undiscovered. A weed belongs to the first plant community which appears after a disturbance and paves the way for further stages of succession. A weed may be either a ruderal or an agrestal. For the purposes of this paper, primarily agrestals will be considered, although much of the information pertains to both types of weeds.

There are over 200 species of important weeds world wide. Of these, about 80 species actually interfere with man's ability to grow crops (Streibing, 1988). About half of the 206 important weed species belong to only 3 families, Poaceae, Cyperaceae, and Asteraceae. The world's worst weed, Cyperus rotundus originated in India, and is now the world's most widespread weed. Other highly successful noxious weeds include Avena fatua (wild oat) and Chenopodium album (common lambsquarters). As with many weeds, these species originated in one area, then quickly spread to other areas around the globe. In fact, often half of the weed flora on arable land is of alien origin. There are several conditions which facilitate weed invasion; these are described at *CONDITIONS FOR WEEDS

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Photo shows wild oats (left) and domesticated oats (right) grown in pots in the greenhouse. The wild oats resemble the domestic types in all respects except that they have shattering rather than non-shattering influorescences. Weeds of this type are called "crop mimics", and are the most difficult types of weeds to control chemically.

Many of the important crop species today have evolved from weeds. The genetic basis of characteristics has allowed scientists to manipulate and select for desirable characteristics in order to increase yield and maximize profits. It is useful to study the genetic differences between weedy and non weedy species to order to gain insight as to why weeds remain successful competitors in agricultural systems.

Eupatorium microstemon exists as both weedy and non weedy species. The weedy species has successfully colonized many new areas due to its plasticity in each environment. The weedy species is an annual, quick flowering, and is photoperiodically neutral (it can survive in areas with 8 to 16 daily hours of sunlight). It is self compatible and displays an economical use of pollen. All of these characteristics are typical of successful weed species. (Baker, 1965).

Weeds are characterized by the shortness of the vegetative phase between germination and flowering (Baker, 1965). Flowering is often independent of environmental controls. Self pollination is common in weeds (which is ideal for invading species, where the invasion of one individual may lead to the establishment of an entire community). Self pollination also leads to economical pollen production, so that the energy saved from pollen production may be used for other purposes, such as growth and offspring production.

Many weeds are annuals, with a wide environmental growth tolerance. Weeds often exhibit size plasticity with respect to environmental variation, with traits such as reduced leaf size in colder climates. This plasticity is often attributed to a general purpose genotype. This general purpose genotype is also associated with a high degree of selfing, and a low chromosome number (Baker, 1965). As self pollination is slow to produce recombinants which are finely adapted to survive in a new environment, the general purpose genotype allows the plant to exist in a wide variety of environments, as long as there is no fierce competition. This is ideal in allow weeds to colonize disturbed areas and new sites. The general purpose genotype is said to be a jack-of-all-trades, yet a master of none.

There is no simple rule which may be used to relate chromosomal change to the development of weediness. It has however been hypothesized that a number of the more successful weeds were formed by hybridization. Their ploidy values are often odd numbers. Examples are found in Taraxacum officinale (Dandelion) and Oxalis corymbosa (Bermuda buttercup). Self pollination allows these plants to reproduce, and allelic diversity is maintained by having several duplications of the same gene on a single chromosome (Baker, 1965). Where outcrossing does occur, the intermingling of crop and weed genes may increase desirable weed characteristics in crops (Jasieniuk et. al., 1994)

Within many genera, one species seems to have evolved a suitable reproductive system and a general purpose genotype which allows them to be very successful weeds. The following list are characteristics which seem to favour weediness. If a plant were to evolve which had all of the characteristics, it would likely be a very successful weed indeed (Baker, 1965).

• no special environmental requirements for germination
• self-controlled, discontinuous germination and great longevity of seed
• only a short time spent in vegetative period before beginning to flower
• continuous seed production maintained for as long as growing conditions permit
• self-compatible, but not obligatorily self-pollinated or apomictic
• cross pollination may be achieved by a nonspecialized flower visitor or by wind
• very high seed output in favourable environmental circumstances
• production of some seed in a wide range or environmental circumstances; high tolerance or and often plasticity in face of climatic and edaphic variation
• special adaptations for short- and long-distance dispersal
• specialized features for competition, ex. rosette formation, etc.

If the weed is a perennial, in addition to the above, the following characteristics contribute to weediness:

• vigorous vegetative reproduction
• brittleness at the lower nodes, rhizomes, or rootstocks
• ability to regenerate from several portions of the rootstock

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Traditionally, weeds have been controlled in order to improve crop yield and quality. Weeds compete with the crop for the available water, nutrients, light and space. In North America, the majority of arable land is treated with herbicide at least once per season (Hurle, 1988). Increasing the amount of herbicide is thought to reduce the amount of physical labour required for weed control. Not all weeds have a negative effect on the crop, and it is important to consider the individual system when assessing the impact and losses due to weeds in order to determine the ideal treatment.

At low densities, weeds often don't affect the yield (Altieri, 1988). Over the last few decades, effective weed control has reduced the risk of weed infestation. There is now a low risk to the crops in tolerating a slight weed infestation. There are several items to consider when deciding on weed treatment. Firstly, the effect of the weed must be assessed. In many areas, weeds have both positive and negative effects. It is often possible to leave weeds with the crop for a certain period of time, the period threshold before using control methods.

Weed control is costly to the farmer. It is important to weigh the various methods available, and to consider the costs and benefits of each. In doing so, the economic threshold must also be considered, as it may not be worthwhile to control the weeds if the payoff is not sufficient.

Weeds have been known to carry and transmit pathogens, insects and viruses. Weeds are also thought to produce negative effects on crops through allelopathy. In some systems, reducing the degree of weed infestation is indeed desirable. There are several non-chemical treatments and practices which can lead to reduced weed infestation. The seed dispersal of the weed has a greater influence on crop yield than the actual competitive ability of the weed (Maxwell, 1992). Harvest machines should be used as weed seed predators instead of agents of dispersal. Other factors which affect weed infestation include

• tillage
• regulation of water supply
• supply of nutrients
• pH, which is influenced by liming and draining
• date of sowing/planting of the crop
• vegetative period of the crop
• shading intensity/period
• collection of seeds at harvest
• purity crop seed
• types of field implements (some spread weeds)

source: Hurle, 1988.

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Not all weeds are undesirable. In many systems, particularly in the tropics, weeds are useful elements in agricultural systems. Certain weeds have been shown to increase the profitability of a system (Showler et. al., 1991). Weeds are adapted to colonize disturbed sites. As such, they are excellent for soil cover and in erosion control. Weeds are often used in tropical systems as COVER CROPS AND LIVING MULCHES
and to minimize nutrient and soil loss through erosion and leaching. In agricultural systems around the world, weeds play a number of beneficial roles. Some of these roles include

• raising the level of soil organic matter
• reducing pest outbreaks; weed polyculture can have a stabilizing effect which combats frequent and severe pest outbreaks (Showler et. al., 1990).
• improving the level of nitrogen availability; leguminous weeds can increase crop yield by fixing nitrogen (Altieri, 1988).
• conserving moisture in the soil
• serving as alternate food sources; weeds may be used as animal fodder, and are also alternative sources of food for humans before harvest or if the crop is destroyed or yield is smaller than anticipated.
• influencing herbivore density;. weeds have been shown to alter the movement and searching behavior or certain herbivores, thus reducing herbivore damage to crops (Altieri, 1988).
• protecting the soil surface from solar radiation
• enhancing crop resistance and adaptive range: many weeds around crops are wild relatives, and are able to breed with the crops; interbreeding leads to an increase in gene flow, and thus to increased variability and genetic diversity - that can improve crop performance, especially in marginal areas. Distinct and fertile hybrids can preserve genes, and protect against the possible detrimental effects of a monoculture (Altieri, 1988).
• improving the soil microclimate, including the temperature and moisture level
• providing shelter and food for beneficial organisms; the presence of weeds can increase the presence and diversity of arthropod prey and predators, the natural enemies of certain insects, which may minimize insect damage to the crops (Showler et. al, 1990, 1991).
• other miscellaneous uses, such as ceremonial materials, medicinal uses, raw material for paper, ornamental purposes, crop fertilizer, source of biogas, important agents in tertiary sewage treatment; For further uses, see the table below, Buen Monte/Mal Monte.

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Many weeds have both useful and detrimental characteristics. Interviews with rural farmers in Mexico revealed that a number of weeds were classified as "buen monte" (good weeds) or "mal monte" (bad weeds). Treatment of the weed, whether the weed was completely eradicated or where it was cut, depended on the classification of buen or mal monte (Gliessman, 1988). The following table follows the same principles, giving both good and bad characteristics of a number of common weeds.

WEED SPECIES	BUEN MONTE	MAL MONTE	SOURCE
Taraxacum officinale	traditional medicinal uses widely used as food for humans and livestock	annoying lawn weed	Alteri, 1988.
Eichornia crassipes	serves as raw material for paper useful as a crop fertilizer (nitrogen provider) source of biogas agent which can remove heavy metal agents in polluted water systems stop salt deposition by evaporation on the surface increase soil organic matter decrease noxious weed growth	.	Altieri and Gliessman, 1988
Euphorbia heterophylla	plant is easy to cut cools the soil	poisonous	Gliessman, 1988
Lagascea mollis	helps control noxious weeds serves as animal fodder safe to grow with corn and beans	.	Gliessman, 1988
Hyptis verticillata	easy plant to cut has medicinal properties	can harm corn due to strong dominance in specific areas	Gliessman, 1988
Guazuma ulmifolia	stimulatory growth effects on corn serves as animal fodder improves soil helps control other weeds	.	Gliessman, 1988
Scleria setuloso-ciliata	produces a large amount of organic matter which improves the soil	leaf edges easily cut fingers	Gliessman, 1988
Beruda erecta and Jucus, sp.	leaves used as mulch which prevents other weed growth through allelopathy and shading	.	Gliessman, 1988.
Chenopodium ambrosioides	medicinal plant useful in nematode control	allelopathic potential with strong dominance	Gliessman, 1988
Stizolobium, sp.	when planted in rotation with corn, decrease fallow time from 4-5 years to 6 months toasted seeds may be used as a coffee substitute	.	Gliesman, 1988
Melanthera nivea	loosens the soil does not damage crop	can possibly harm crop due to strong dominance in some areas	Gliessman, 1988
Cynodon dactylon	used as feed for chickens and turkeys	roots compact the soil in corn crops	Gliessman, 1988

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VARIABLE	VALUE & UNITS	SOURCE
Typical weed seed density in cultivated soil	6000-75,000/m2	Hill et. al., 1998; Streibing, 1988
Typical no. weed species in seedbank	20-100/m2	Hill et. al., 1988
Yearly costs in U.S. of chemical weed control	$3.6 billion U.S.	Altieri, 1988
Loss of agricultural crop production attributed to competitive effect of weeds	10%	Altieri, 1988

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agrestal

a plant which enters agricultural land  

allelopathy

the positive or negative response of a plant which is mediated through chemicals produced by another plant  

economic threshold

the weed density at which the cost of control is equal to the expense of leaving the weeds in the field  

general purpose genotype

a genotype which allows a plant to grow in a wide range of environments and which confers tolerance to various climates and edaphic situations  

period threshold

the time after crop emergence before weed control is necessary to prevent yield losses, and the time after which no further control measures are required (Hurle, 1988).  

ruderal

a plant which occurs in waste places and along roadsides  

weed

a plant which, in any specified geographical area, belongs to a population which grows entirely or predominantly in situations markedly disturbed by man. This does not include cultivated plants. (Baker, 1965)

Contents

• Altieri, M.A., and Liebman, M. (ed.) 1988. Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, Florida, United States.
• Boyetchko, S.M. 1996. Impact of soil microorganisms on weed biology and ecology. Phytoprotection 77: 41-56.
• Cousens, R., and M. Mortimer. 1995. Dynamics of Weed Populations. John Wiley & Sons, Toronto.
• Engelken, L.K., W.B. Showers, and S.E. Taylor. 1990. Weed management to minimize black catworm (Lepidoptera: Nocuidae) damage in no-till corn. Journal of Economic Entomology 90: 1058-1063.
• Hobbs, R.J., and S.E. Humpries. 1995. An integrated approach to the ecology and management of plant invasions. Conservation Biology 9: 761-771.
• Holm, L., J. Doll, E. Holm, J. Pancho, and J Herberger. 1997. World Weeds - Natural Histories and Distribution. John Wiley & Sons, Inc., Toronto.
• Holzner, W., and M. Numata. (ed.) 1982. Biology and Ecology of Weeds. Dr. W. Junk Publishers, London.
• King, L.J. 1966. Weeds of the World - Biology and Control. Interscience Publishers, Inc., New York.
• Labrada, R., J.C. Caseley, and C. Parker. (ed.) 1994. Weed Managemnet for Developing Countries. Food and Agricultural Organization of the United Nations, Rome.
• Radosevich, S.R., and J.S. Holt. 1984. Weed Ecology - Implications for Vegetative Management. John Wiley & Sons, Inc., Toronto.
• Wyass, E. 1995. The effects of weed strips on aphids and aphidophagous predators in an apple orchard. Entomologia Experimentalis Applicata 75: 43-49.

Contents

• WEED IMAGES AND DESCRIPTIONS
  (http://www.rce.rutgers.edu/weeds/index.html ) Rutger's University, Spons. (2001 January 9; Viewed Feb 2 2001)
  This site is supported by Rutgers, the University of New Jersey. The site features pictures and information on a large variety of North American weeds.

• WHERE DO WEEDS COME FROM?
  (http://www.ianr.unl.edu/pubs/Weeds/g807.htm) University of Nebraska, Spons. (1996 September; Viewed February 2 2001)  This site discusses weed entry and loss from the system. Topics include density and diversity of the soil weed seed population.

• WEED MANAGEMENT
  (http://www2.ncsu.edu/ncsu/cals/sustainable/peet/IPM/weeds/c07weeds.html) Dr. Mary Peet, Auth. (1997, July 9; Viewed Feb 2 2001)
  Options for weed management are discussed, including integrated pest management and consideration of the weed-free period.

• COVER CROPS AND LIVING MULCHES
  (http://www2.ncsu.edu/ncsu/cals/sustainable/peet/cover/c02cover.html) Dr. Mary Peet, Auth. (1997, July 9; Viewed Feb 2 2001)
  The importance of cover crops is discussed. How to use cover crops in the production cycle is presented, including selection of the` cover crop, crop management, establishment, and incorporation in to the system. A description of a number of potential cover crops is presented, including the pros and cons of each species.

• WEEDS - WEED INFORMATION AND IDENTIFICATION
  (http://www.greensmiths.com/weeds.htm) Greensmiths, Spons. (1996, Viewed Feb 2 2001)
  This site presents definitions of weeds, and aids in weed identification and control options. An index of common weeds is included, with a picture and description of each species.

Contents

• Altieri, M.A. 1988. The impact, uses, and ecological role of weeds in agroecosystems. In: Altieri, M.A., and M. Liebman. (ed.) Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, Florida, United States. pp. 1-6.
• Baker, H.G. 1965. Characteristics and modes of origin of weeds. In: Baker, H.G., and G.L. Stebbins. (ed.) The Genetics of Colonizing Species. Academic Press. pp. 147-168.
• Gliessman, S.R. 1988. Ecology and Management of weeds in traditional agroecosystems. In: Altieri, M.A., and Liebman, M. (eds.) Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, Florida, United States. pp. 237-244.
• Hurle, K. 1988. How to handle weeds? -Biological and economic aspects. Ecological Bulletin . 39:63-68.
• Jasieniuk, M., and Maxwell, B.D. 1994. Population genetics and the evolution os herbicide resistance in weeds. Phytoprotection . 75:25-35.
• Maxwell, B.D., and Ghersa, C. 1992. The influence of weed seed dispersion versus the effect of competition on crop yield. Weed Technology . 6:196-204.
• Showler, A.T., and Reagan, T.E. 1991. Effects of sugarcane borer, weed, and nematode control strategies in Louisiana sugarcane. Environmental Entemology . 20:358-370.
• Showler, A.T., Reagan, T.E., and Knaus, R.M. 1990. Sugarcane weed comunity interactions with arthropds and pathogens. Insect Science and Its Applications . 11:1-11.
• Streibing, J.C. 1988.Weeds - the pioneer flora of arable land. Ecological Bulletins . 39:59-62.

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