Agriculture Reference
In-Depth Information
Agricultural
weeds
can
hurt
crop
yields
or
Around the world, more human labor is
expended in hand weeding than in any other
agricultural task, and most cultivation and tillage
practices are designed to aid in weed control. The
chemical industry manufactures herbicides,
which, next to fertilizers, account for the largest
volume of chemicals applied to crops. Among
pesticides used for the management of pests, her-
bicides account for 65 % (IFPRI
1998
; USDA
1999
).
Most analyses concur that in a changing
climate, weeds may become even more active
than they are currently, thus posing the threat of
greater economic losses to farmers (IPCC
1996
;
Coakley et al.
1999
). While the majority of weeds
are invasive species from temperate zones, other
weeds in temperate regions originated in tropical
or subtropical regions, and in the current climate,
their distribution is limited by low temperature.
Such geographical constraints will be removed
under warm conditions. Warmer temperature
regimes have been shown to increase the maxi-
mum biomass of the grass weeds signifi cantly. In
crop monocultures, undesirable competition is
controlled through a variety of means, including
crop rotations, mechanical manipulations (hoe-
ing), and chemical treatment (herbicides).
increase costs of production by:
• Competing directly for light, nutrients,
moisture, and space
• Releasing natural substances that inhibit crop
growth (allelopathy)
• Physically hindering crop growth and
development, especially climbing vines like
morning glories,
Ipomoea
spp., and hedge
bindweed,
Calystegia sepium
• Hosting pests or pathogens that may attack
crops
• Promoting disease by restricting air circulation
around the crop
• Interfering with or contaminating crop harvest
• Reproducing prolifi cally, resulting in a greater
weed problem in the future
• Parasitizing crops directly (e.g., dodders,
Cuscuta
spp., and witchweed,
Striga
asiatica
)
10.1
Crop Losses
The most recent and comprehensive efforts to
provide a measure of global crop losses by weeds
are those made by Oerke (
2006
) (Table
10.1
).
Weeds produced the highest potential loss (34 %).
In the USA, annual losses in crop production due
to weeds have been valued at approximately US$
12 billion, amounting to some 10 % of potential
production (Patterson and Flint
1990
). Large
efforts are made to limit these damages through a
variety of weed control measures.
10.2
Climate Change and Weeds
Weeds grown in association with the crops are
exposed to increasing concentrations of CO
2
(
370 ppm) and other active greenhouse gasses
(CFCs, CH
4
, N
2
O, NO
2
, etc.) that have led to
global warming and the associated greenhouse
effect. They are equally and may be differently
infl uenced due to these changes as different spe-
cies of weeds grow together with a crop. The
responses of crop and weeds to increased CO
2
level and temperature are being studied world-
wide. The variations in the responses of C4 and
C3 plants (both crops and weeds) have been
noted under such situations. Composite weeds in
the crop fi elds having variable proportions of C4
and C3 plants are likely to undergo dynamics in
weeds insurgence and shift of weeds in favor of
certain species in course of time. Increased CO
2
≥
Table 10.1
Estimated potential of weeds and actual
losses due to weeds in six major crops worldwide, in
2001-2003 (Oerke
2006
)
Crop losses (%) due
to weeds
Potential
Attainable
production
(million tons)
Crop
Actual
Wheat
785.0
23.0 (18-29)
7.7 (3-13)
Rice
933.1
37.1 (34-47)
10.2 (6-16)
Maize
890.8
40.3 (37-44)
10.5 (5-19)
Potatoes
517.7
30.2 (29-33)
8.3 (4-14)
Soybeans
244.8
37.0 (35-40)
7.5 (5-16)
Cotton
78.5*
35.9 (35-39)
8.6 (3-13)
*Seed cotton
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