Weeds are unwanted plants, growing in competition with
the cultivated crop for space, light, nutrients, etc. They
can be classified on the basis of their leaf forms (narrow
leaf weeds or broad leaf weeds), life span (perennials)
and growth or habitat. Fig.W.7 shows Parthenium grass,
a common weed, covering a large area.
Fertilizers do not discriminate - they supply useful
nutrients to both the weed and the crop. The problem
becomes one of keeping the infestation of weeds below a
tolerable limit. Some fertilizers give the crop a
competitive advantage over some weeds, but also
increase the growth of tall and fast growing weeds,
especially perennials.
Frequent planting of winter cereals increases the
weed problem. plowing and soil cultivation, or using
chemical weed-killers (herbicides) are among the main
control measures used for destroying weeds and are parts
of weed management. Demand for herbicides is
constantly on the rise for combating perennial weeds.
There are also other aspects of weed management which
involve removing the weed seeds in the soil, and
resorting to, what may be broadly classified as, cultural,
biological, chemical and integrated weed control
methods.
Weeds can be a major problem in all rice growing
areas where a yield reduction of up to 30% is observed.
Weeds can be a major constraint in labor-intensive,
small-scale farm operations. For instance, on Nigerian
savannahs losses of 65 to 92% have been reported due to
weeds. A parasitic flowering plant, known as witch weed
(Striga sp), is a major pest in a large part of Sub-Saharan
Africa.
Weeds may act as hosts for plant pests, providing an
adjacent source of inoculums and allowing pests to
survive or proliferate even in the absence of crop plants.
Weeds may also serve as alternative hosts to allow for the
sexual recombination of specific diseases or as a means of
surviving a stress period.
Unwanted plants significantly increase the farmer's
work. Weeds can interfere with the yield and the quality
of crops, clog waterways and canals, poison livestock,
cause dermatitis, induce hay fever and other respiratory
problems in humans, create fire hazards, damage
pavements in parking lots and roadways, interfere with
utility lines and occasionally produce an attractive but
poisonous fruit which if eaten can lead to human health
problems or even death. Annual losses to society
worldwide arising from weeds are estimated to exceed 10
billion dollars.
Most weeds on land are grasses, sedges or broad
leaved. Parasitic weed plants like dodders, broomrape,
witch weed and certain mosses are among the important
weeds found along with agricultural, ornamental and
forest plants. Some examples of aquatic plant weeds are
cattails, arrowheads, water hyacinth, pondweeds and
algae.
