Weeds Concept, Classification & Agricultural Importance [ 2007-12-21 ]
Dharma Raj Dangol, Ph. D.
Agricultural fields are dynamic and "living" biocoenosis where we can
find the interrelationship between four components: man, cultivated
plants, weeds and environment. The biotic components and the
environment interact with each other and create a dynamic system. This
ecological system can be called agroecosystem. In this section we will
deal about weeds as plants that always associate with the crops (or
desired plants).

Concept of Weeds
Much time has been spent by scientists to solve the problem of how to
define the term "weed". The difficulty may be as a result of the
different kinds of weeds associated with crops; the involvement of
weed scientists with different backgrounds, and different classes of
farmers. The definition provided by agronomists is not satisfactory
for ecologists, the concept of "weed" held by big farmers does not fit
to that of the small farmers. A simple definition of weeds provided by
Blatchley in 1912 as '"unwanted" plants "of out of place" becomes
questionable to the people who use the weeds from their fields as
food, medicine, forages, fibers and even in industries. Ten year
earlier, Aldrick and Kremer defined a weed as "a plant that originated
under a natural environment and, in response to imposed and natural
environments, evolved and continues to do so as an interfering
associate with our desire plants and activities". Sometimes, extreme
definitions may come like the one of Harlan in 1995. According to him,
weeds are organisms adapted to human disturbance and the definition
needs not be confined to plants.

Going through Harlan's definition, it is clear that "previous weed
boundaries" have been broken down (as Berlin wall did!) to accommodate
"organisms" like house sparrows, starlings, pigeons, sewer rats house
flies, etc. As yet I am a beginner, my taboos still do not allow me to
incorporate all "organisms adapted to human disturbances" inside my
"weed boundaries", which only allow me to accommodate plants growing
in "khetland" and "bariland" habitats without being cultivated. I also
do hope, if I go "further and gather more knowledge" in this field,
one can not say that my taboos will not be broken down in future.

Classification
Classification is the process of recognizing and delimiting groups to
produce a system. The taxonomic groups, the taxa, include, from the
highest rank to the lowest, are kingdoms, divisions, subdivisions,
class, order, family, genus and species (subspecies or varieties). The
rank levels used most often in weed science are families, genus, and
species. There are about 450 families of flowering plants and well
over 35,000 different species. Apart from the botanical (taxonomic)
classification, weeds are classified in different ways and into
different groups.

Agrestals (segetals or cropland) Weeds
Agrestals are weeds of tilled, arable (crop) land and can be
subdivided into "cereal weeds", "pulse weeds", "rice weeds", "wheat
weeds", "pineapple weeds", "tea weeds", "cotton weeds". Besides that,
for practical purposes, agrestals are subdivided into: annuals and
perennials (according to basis of life-cycle); major and minor (after
their degree of weediness), and broad-leaved and narrow-leaved
(according to susceptibility to herbicides). The type of weed
occurrence depends on the frequency and time of soil cultivation.
Annual weeds are prevalent more in annual field crops whereas more
perennials are found in plantations where the soil is not cultivated
each year.

Ruderal Weeds
Ruderals are plants occurring on earth heaps, dunghills, trash
deposits, roadsides, railway lines, roofs, margins of waste water
ditches, temples, old houses and so on. The ruderals are a most
heterogeneous weed group ranging from tiny, short leaving annuals
(Lemna minor) to tree-species (like Ficus benghalensis, Ficus
religiosa). Many agrestals, such as Cynodon dactylon, Eleusine indica,
Galinsoga ciliata, Ageratum conyzoides, Amaranthus spinosus and so on,
also very harmonically grow in ruderal habitats. Other examples of
such plants which are common in rice fields and ruderal sites include
Eichhornia crassipes, Monochoria hastata, Marsilea quadrifolia,
Vernonia anagallis-aquatica, etc.

Grassland (Meadow, Permanent Pasture and Mown pasture) Weeds
Grassland weeds are the plants that have a negative influence on
livestock or on their product or are not palatable and possess a high
competitive power to other desired species (Holzner, 1982). These
group includes mainly perennial species.

Aquatic (Water) Weeds
Aquatic plants may be wanted or unwanted. Aquatic weed can be
categorized according to their growth habit into three groups. They
are free-floating (Eichhornia crassipes, Salvia natans), submerged
(Hydrilla verticillata, Potamogeton nodosus) and emerged (Typha
angustifolia, Alternanthera philoxeroides).

Forestry Weeds
Plant species are considered forestry weeds when they interfere with
tree nurseries, afforestated species or desired species of the natural
forest vegetation. Forestry weeds also range from annual agrestals to
big trees (which are undesired because of their slow growth, many
branches, wood difficult to sell).

Environmental Weeds
Environmental weeds are introduced, aggressive species that colonize
natural vegetation and suppress the native species to a certain
extent. In Europe, Robinia pseudacacia, Prunus serotina, Amelanchier
species are considered as environmental weed trees. Elodea canadensis
and Elodea nuttallii are two aquatic environmental weeds. In Nepal,
Parthenium hysterophorum can be taken as an example of this type as it
dislodges mainly Cassia tora in the Terai and Inner Terai and Cannabis
sativa and Artemisia species in the Kathmandu valley.

Poisonous weeds
The plants that cause itching and swelling such as poison ivy
(Toxicodendron radicans), poison oak (Toxicodendron toxicarum), and
that cause hay fever such as goldenrod (Solidago sp.), big sagebush
(Artemisia tridentata), ragweed (Ambosia spp.) come under this group,
which we could call "medicinal weeds".

Parasitic Weeds
Phanerogamic plants that parasitize on crops or desired plants are
referred to as parasitic weeds. These weeds depend on the crop for
part or all of their water and nutrients and can consequently do vastl
damage than other weeds. They attach to the crop, either to the shoots
as in dodders (Cuscuta species) and mistletoes (Loranthaceae and
Viscaceae), or to the roots as in broomrape (Orobanche spp.) or
witchweeds (Striga species).

Agricultural Importance of Weeds Negative Effects Competition Weeds
compete with cultivated plants for nutrients, water, and light which
may result in a reduction of the crop yields. Yield is reduced to 91%
in upland rice, 10-70 % in transplanted rice, 24-64% in maize, 12-37%
in wheat, 29% in rape mustard and 21% in potato. The degree of
competition depends on the weeds (species, density, pattern,
duration), crops (species, variety, planting date, planting rate,
planting pattern, and environmental factors like weather, soil type,
soil fertility, pests.

Allelopathy
The term means: the injurious effect of one upon another, it was
coined for the production of chemical compounds (allelochemicals) by
one plant, mostly secondary metabolites, that can induce of suffering
in, or give benefit to, another plant. The role of allelopathy may be
stimulatory or inhibitory. In an agroecosystem, allelopathic
interactions may be crop-crop, weed-weed and crop-weed. If the
allelochemicals of one weed inhibit germination, growth, and
development of another weed, they can be called as natural herbicides.
A crop may be stimulatory to growth and development of other crop.
Such crops with stimulatory potential can be used as intercrops or as
rotational crops. A great number of data is accumulating on
Allelopathy. It is new to Nepal, only a few studies have been carried out.

Hosts for diseases, insects, nematodes
Weeds serve as hosts for many diseases, insect pests and nematodes and
increase the costs of protection and reduce yield. For example,
Alopecurus myosuroides weed hosts fungus called ergot (Claviceps
purpurea), Stellaria media weed hosts cucurbit mosaic virus, and
Chenopodium album weed harbors black bean aphid (Aphis fabae)

Adulteration in seeds
Weed seeds reduce the quality as well as the value of the
crop seeds. Seeds of madilo (Ischaemum rugosum,), Sama (Echinochloa
colona, E. glabrescens, E. crus-galli,), and mothe (Cyperus difformis,
Cyperus iria) in rice grains; of Gahunko mama (Phalaris minor) and
Kutilkosa (Vicia seeds) in wheat seeds; Kutilkosa (Vicia spp.), Methi
jhar (Medicago lupulina,) and Sukul jhar (Polygonum plebeium) in lentil.

Allergies due to weeds
Weeds are reported as a causal agent for allergies. Pollen grains from
weeds may cause hay fever, irritation of skin, souring of eyes,
ruining of nose and other similar problems. Pollens from Prosopis
spp., Cassia spp., Azadirachta indica, Albizia lebbak cause asthma.
Some weeds with skin allergy properties include: Parthenium
hysterophorus, Urtica dioica, and Mucuna puriens.

Reduction of quality of livestock products
A number of weeds possess hard and sharp structures such as spines or
thorns, or awns that can produce mechanical injury to livestocks. Some
of the more common mechanical injury-causing weeds are Avena fatua,
Bromus tectorum, Cenchrus longispinus, Salsola kali, and Tribulus
terrestris. Some weeds such as Allium vineale (wild garlic), A.
canadense (wild onion), A. tricoccum (wild leek) and Thlaspi arvense
(fanweed) cause off-flavors in milk and other diary products. Weeds
often damage wool, hides and carcasses, thereby lowering the quality
of animal products.

Problem in harvesting and threshing
Spiny weeds like Cirsium arvense, Carthamus oxycantha, Cirsium spp.,
Argemone mexicana, create great problems during the harvest of the
crops. Similarly very tall weeds like Echinochloa crus-galli, E.
glabrescens, and climbers like Ipomoea purpurea hamper the threshing
of rice.

Farm losses from weeds
The total annual monetary loss due to weeds was $ 6,408, 183, 000 in
the 13 field crops (corn, cotton, peanut, grain sorghum, soybean,
sugar beet, sugarcane, barley, flax, oat, rice, rye, and wheat), $
619,072 in vegetables and $ 441, 449 in fruit and nuts for the period
1975 to 1979.

According to Chandler, about $ 3.1 billion are spent annually in
equipment and labor for inter-row tillage of crops and tillage to
control weed between cropping season. It has been estimated that total
expenditures for herbicides used on croplands in the US: are
approximately $ 2.1 billion with application of these herbicides
costing $ 938 million annually.

Land value depreciation
Purchasers or bankers may reduce the value of land if it is full of
perennial (Convolvulus arvensis, Sorghum halepens, Eltrygia repens)
and parasitic (Cuscuta reflexa) weeds, because they are recognized as
a loss of productive potential.

Increase in cost of production
Weeds increase labors to control, to harvest and post harvest works
like separating contaminated seeds from crop seeds. As a result the
cost of production rises.

Positive Aspects of Agrestal Weeds Beneficial impact on the soil Weeds
such as Digitaria have very good ability to protect the soil from
erosion with an intensive root system and procumbent culms, rooting at
the nodes, while their competing effect is small because of their low
growth and late emergence. According to him, Convolvulus arvense
increase the fertility of the top soil by bringing up nutrients from
deep soil layers.

Impact on the population of pests
Weeds not only harbor harmful insects, as mentioned above, but also
serve as suitable habitat for predators. The result is the reason
poorer survival of noxious caterpillars on weedy than on hoed plots.

Beneficial impact on crops
All the weeds growing in cultivated fields are not harmful.
Experimental results tell us that the weed Agrostemma githago has a
positive influence on the protein production of wheat. Nepalese
farmers classify the weeds of their fields into malilo (fertile) and
rukho (infertile) weeds depending on their importance. Malilo weeds
are said to increase soil fertility and the other type is in contrast
to it.

Beneficial effects for agriculture
We have to look weeds from an ecological point of view. For instance,
in an agroecosystem, weeds not only compete with crops but also
compete with their own population. If we control weeds by herbicides,
the results are resistant weeds which only compete with crops. Because
of this reason Austrian Professor

Holzner forwarded his idea about ecological management saying "it
would seem reasonable and subject to aim at a weed control achieving a
weed population in the fields that is rich in species but poor in
individuals and that can be easily controlled, mainly by mechanical
means and by crop rotation, and does not tend to the negative effects
of compensation..." in order words "not total eradication of weeds but
management of the weed population, using tolerable species to control
other more noxious ones..."

The importance of weeds for man
Weeds indicate for soil and climate as all plants and can be used as
means for biogeographical mapping for planning purposes. In former
days, in Nepal, farmers engaging in weeding activity decide their
tiffin time in fields observing the flowers of Monochoria vaginalis.
If flowers stop opening, they take their tiffin (snacks).

(The writer is Associate Professor in the Department of Environmental
Science, Institute of Agriculture and Animal Science, Rampur, Chitwan,
Nepal)

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