Subject : Agricultural Science
Topic : DEFINITION AND CHARACTERISTICS OF WEEDS
Class : Basic 8 / JSS 1
Term : Third Term
Week : Week 3
Instructional Materials :
- Pictures of weeds
- samples of weeds
- Scheme of Work
- Online Information
- 9 Year Basic Education Curriculum
Previous Knowledge :
The pupils have previous knowledge of weeds in their previous lessons
CLASSIFICATION OF WEEDS
Behavioural Objectives : At the end of the lesson, the pupils should be able to
- say uses of weeds
- mention methods of weed control
- enumerate the characteristics of weeds
- list economic importance of weeds
- differentiate between weeds and crops
USES OF WEEDS; METHODS OF WEED CONTROL
- Uses of Weeds
- Methods of Weeds Control
Uses of Weeds
Weeds are used for the following purposes:
- Some weeds are used as cover crops.
- Weeds are used as forage grasses and legumes for feeding animals e.g. elephant grass.
- Weeds are used to prepare compost and green manure e.g. mucuna, pueraria.
- They are used to control soil erosion e.g. bahama grass.
- They serve medicinal purposes e.g. lemon grass.
- Weeds are used as fuel (firewood) for cooking.
- Weeds are used as flowers to beautify our environment e.g. morning glory.
- They are used to purify the environment by removing carbon dioxide from the atmosphere.
- Some weeds are edible e.g. water leaf.
Methods of Weed Control
Weed control refers to any deliberate effort made by man to eliminate weed seeds or stop weed growth.
Weeds can be controlled by using following methods:
- Physical/mechanical weed control
- Chemical weed control
- Cultural weed control
- Biological weed control
- Physical/mechanical weed control. It may be in the following forms:
(i) Hand pulling: The use of hand to remove weeds. It is most effective in the control of annual and biennial weeds of row crops
(ii) Hoeing: It entails using hoes to remove weeds.
(iii) Mowing: The use of mower to clear weeds.
(iv) Slashing: This is the use of cutlass to cut the weeds. In this method, only the shoot is destroyed, the roots are left intact in the soil. These may sprout again.
(v) Tillage: (ploughing and harrowing)
Other methods include flooding, smothering, heat treatment, etc.
- Chemical weed control: This involves the use of chemicals to control weeds.
The chemical used to control weeds are called herbicides. The machine used to apply herbicides is called a sprayer.
Herbicides may be categorised based on;
(a) The mode of action and
(b) Time of application.
(a) The mode of action: Under this, we have the following:
(i) Selective herbicides: These are herbicides that kill a particular type of weed. Examples are 2,4-D (2,4 Dichlorophenoxy acetic acid), MCPA, Atrazine, Simazine etc.
(ii) Non-selective herbicides: These are herbicides that kill both broad leaf and narrow leaf weeds. Example are paraquat or grammozone, Glyphosate and Roundup, Weed off.
(iii) Systemic herbicides: These are herbicides which when applied directly on the plants, the active ingredient goes into the system of the weed and ensures total eradication of the weeds. They are used in controlling perennial weeds.
Examples are galex, primextra, imazaquine, metribuzine alachlor, atrazine, fusillade super etc.
(iv) Contact herbicides: These are herbicides which have immediate effect on the weeds upon contact with them. They burn the weeds immediately they come in contact with it. They are effective in controlling annual weeds. Examples are paraquat, diquat etc.
(b) Time of application: Under this, we have the following:
(i) Pre-plant herbicides: These are herbicides which are applied before seeds of crops are planted.
(ii) Pre-emergence herbicides: These are applied before the planted seeds and weeds emerge on the land. The herbicides are applied immediately or at most three days after planting.
Examples are galex, for maize, cotoran for cotton, dirruon for cassava and yam, imazaquine for okra.
(iii) Post-emergence herbicides: These are applied after the emergence of both crops and weeds. Post-emergence herbicides are contact in action. That is , all contact herbicides are post emergence.
Benefits (Advantages) of Using Herbicides
- Herbicides are easy to apply.
- They are fast in action.
- They cover large areas of land.
- They are highly efficient and effective.
- They save labour.
- Cultural weed control: It involves the use of routine farm practices without the use of tools. These practices are:
(i) Mulching: Dry grasses are applied on ridges, vegetables beds or flat land. These will suppress or smothers weeds that have grown on such land and prevent others from sprouting.
(ii) Crop rotation: This is effective in controlling weeds associated with specific crops e.g milk weed is a serious problem on soyabeans and cowpea, but ineffective on cassava and maize. When such crops are moved away from the plot, the life cycle of the weed is interrupted.
(iii) Flooding: This method is effective in controlling weeds with under-ground perennial organs. The root of weeds are subjected to anaerobic condition (they will be deprived of oxygen) thereby killing them. E.g rice field.
(iv) Burning: This involves setting the vegetation on fire during land preparation. This method is effective in controlling weeds propagated by seeds and aerial portions.
(v) Cover cropping: This involves planting fast growing leguminous crops such as mucuna utilis and calopo on land to act as cover and smother weeds. This method is effective in controlling weeds in plantation farm.
Others include the following Use of clean seeds, Close spacing and Close season.
- Biological weed control: This is the use of natural enemies or living organisms e.g plants, animals, insects and micro-organisms to control weeds.
Preventive Weed Control
Under preventive method of weed control, the most effective is quarantine.
Quarantine are laws or regulations put in place by the government in order to prevent the entry of foreign pests, diseases, and weeds from one country to another.
All imported agricultural materials must undergo quarantine by subjecting them to certain tests and inspections to be sure they do not contain weed seeds, fruits or their propagules before they are allowed entry into the country.
Integrated Weed Management (IWM)
In this method, all or some of the measures discussed above are combined together and used to combat weeds as no single method is absolute. Hence, it is the most effective of weed control method.
The topic is presented step by step
The class teacher revises the previous topics
He introduces the new topic
The class teacher allows the pupils to give their own examples and he corrects them when the needs arise
- List four general methods of controlling weeds.
- Explain how mulching and crop rotation can be used to control weeds.
- Differentiate between pre-emergence and post-emergence herbicides.
- State five uses of weeds.
The class teacher wraps up or conclude the lesson by giving out short note to summarize the topic that he or she has just taught.
The class teacher also goes round to make sure that the notes are well copied or well written by the pupils.
He or she does the necessary corrections when and where the needs arise.
Prepare for the next lesson by reading about
CLASSIFICATION OF WEEDS