WEEK FIVE
SUBJECT: AGRIC SCIENCE
CLASS: GRADE 7
TOPIC: WEEDS
IDENTIFICATION OF WEED:
(i) Forest zone and
wetter parts of the savannah: Siam weed (Chromolina
odoratum), Guinea grass (Panicum maximum).
(ii) Cultivated land,
pastures, lawns and waste land: Gbure (Talinum
triangulare), Wild spinach or Tete (Amaranthus spinosus and also Amaranthus
hybridus), Crowfoot grass (Eleusine indica), Croton lobatus, sensitive plant or
‘Patanmo (Mimosa asperata).
(iii) Oil palm
plantations, road sides and plantations generally: Spear
grass (Paspalum conjugation), Goat weed (Aegeratum conyzoides), PWD weed
(Tridax procumbens), Sorghum and maize, Elephant grass (Pennisetum purpureum).
(iv) Farmland
encouraged by burning and shifting cultivation: Giant
star grass ( Cynodon plectostachus), Sword grass (Imperata cylindrica), Bahama
grass (Cynodon dactylon).
(v) Abandoned
cultivated and forest clearings: Mucuna (mucuna
prurens).
METHOD OF WEED CONTROL: It is necessary to remove weeds from a farm
so that the plant crop can use all the food and water in the soil for its
growth. Weeds must be removed between two or three weeks after the crops have
been planted. Weed removal is generally carried out three or four times before
the crop is ready for harvest. The exact number of times that weeds are removed
from a farm will depend on the crop, its spacing, the rainfall in the area, the
fertility of the soil and how long it takes from the time the crop is planted
to the time it is harvested. For instance, maize crop is weeded three times
before it is harvested. The first one is done two to three weeks after planting
while two more times of weeding are done every fifteen days after the first
one. For a crop like cassava which takes nine to eighteen months from the time
of planting to harvest, four to six times of weeding must be done. It pays to
remove weeds before they produce seeds it you wait for weeds to grow and
produce seeds before you remove them, many of the weed seeds would have dropped
on the ground. They will later germinate, grow and produce many more weeds.
There are five main methods of controlling
weeds. These are:
Crop rotation: Weeds
can also be reduced on a farm if the farmer practices crop rotation. Crop
rotation is a scientific alternative to the traditional method of shifting
cultivation which leaves land fallow for long periods. It enables farmers to
crop their land every year. Crops are rotated i.e. different crops follow each
other on the same piece of land in a definite order or cycle in successive
seasons in order to maintain the fertility of the soil.
Use of herbicides: Weeds
can be killed with chemicals called herbicides or chemical weed killers. They
kill the weeds either when they touch them or after the weeds have absorbed the
chemicals. Herbicides can be applied before the planted crops germinate. These
are known as pre-emergence weed killers. Some other herbicides are applied only
after the planted crops have been established and are growing well in the
field. These are called post-emergence herbicides. Herbicides usually kill
weeds and not the planted crops, hence they are said to be selective.
Herbicides that are recommended for weed
control in cacao, kola, coffee and cashew farms are shown below:
(i)
Glyphosate (1.92 kg ai/ha)
(ii)
Paraquat (0.60 kg ai/ha)
(iii)
Asulam + ioxynil/2.4.D (3.40 +0.55 kg ai/ha)
(iv) 2,4-D
(0.55 kg ai/ha)
(v)
Folar 525 (glyphosate + terbuthylazine) (1.05 to 3.15kg ai/ha).
For tea
Gramoxone or rand up (0.56kg ai/ha)
Cultural weed control: In this
system, weeds are removed by hand, cutlass or hoe. Many farmers in West Africa
still pull out weeds from their farms by hand. This is possible where the farms
are small. The process is slow, back breaking, but very effective. Most
farmers, however, use a hoe or cutlass to remove weeds. A hoe does a better job
as it removes the shoot and part of the root of the weeds. The cutlass removes
only the shoot. Cultural methods of weed control also include cover cropping,
crop rotation, mulching and tillage.
Mechanical weed control:
Implements attached to tractors called 'cultivators' are used to remove weeds
from medium and large sized farms. Cultivators are used on row crops such as
maize, cowpeas, groundnuts, etc. They are used when the crops are young,
otherwise the cultivators will break and damage the stems of tall-growing
plants, such as maize.
Biological weed control: Weeds
can be prevented from growing on a farm by establishing other plants where the
weeds would have been. The usual practice is to sow legumes in between
widely-spaced perennial row crops such as rubber, oil palm, etc. The most
commonly planted legumes are mucuna, puereria, centrosema and calopogonium. You
may have seen goats grazing in an oil palm or rubber plantation. This is
another form of biological weed control since the animals eat up the weeds in
the plantation.
Burning: Burning
can be used to control harmful weeds during the peak of the dry season, though
it is becoming unpopular and is therefore not recommended. It however has
proved advantageous in the eradication of Eupatorium odoratum very common,
tough weed (Siam weed).
EFFECTS OF WEED CONTROL
METHODS ON VEGETATION AND SOIL:
(a) Removal of Soil Nutrients:
Hand
weeding or weeding by the use of mechanical implements leaves the soil bare and
liable to erosion thus causing the removal of organic matter and top soil,
especially if the farm is on sloping land. This also changes the vegetation or
the land as all other plants other than the planted crops are removed. Grazing
of livestock in oil palm or rubber plantations eliminates the original
vegetation of the particular area.
(b) Pollution of Soil and
Ground Water. It is known that when herbicides are sprayed
with a view to kill weeds, some of the herbicides make contact with the soil
and remain there. Some of the residue is also washed into the soil when it
rains. It has been shown that some herbicides have residual effects immediately
after they are applied. These do not allow the good growth of some crops such
as vegetables. In addition, weed killers pollute the soil and especially, the
ground water.
(c) Contamination of Livestock
Pasture: When herbicides are sprayed on pastures, to get rid of weeds
therein, livestock should not be allowed to graze those pastures immediately
after the application. It is safer to allow at least one heavy rain to fall to
wash away the herbicide residues, which in most cases could be poisonous to
livestock.
ASSIGNMENT
OBJECTIVE:
1.
The chemical used for the control of weeds is known as A. fungicide B.
herbicide C. insecticide D. nematicide E. rodenticide
2.
The following are cultural methods of weed control EXCEPT A. cover cropping B.
crop rotation C. mulching D. planting of clean seeds E. tillage
3.
Which of these is NOT a characteristics feature of weeds? They A. are resistant
to trampling B. can survive harsh conditions C. have luxuriant growth D. have
short period of viability E. produce many seeds
4.
Cultural control of weeds include the following EXCEPT A. cover cropping B.
burning C. flooding D. mulching E. slashing
5.
Weeds compete with agricultural crop for the following EXCEPT A. light B.
living organism C. plant nutrients D. space E. water
6.
A pepper plant that grows in a yam plot is regarded as A. crop B. flower C.
pest D. vegetable E. weed
THEORY:
1.
Mention two kinds of weed found in forest zone and wetter part of the savannah.
2.
List and explain six methods of weed control.
3. List three effects of weed control
methods on vegetation and soil.
4. List
five effects of weeds on crop growth and yield.
0 Comments