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Lesson Note

Subject: Agricultural science

Topic: Weeds

Learning Objectives: By the end of the lesson, the learners should be able to:

  1. Define the term weeds,
  2. Identify some common weeds in West Africa,
  3. State the beneficial and armful effects of weeds,
  4. Identify the methods if weed dispersal
  5. List and explain the methods of controlling weeds.

Lesson Discussion

Concept And Definition Of Weeds

When the world came into existence, no single plant was classified as weed. The concept of weeds came to be when man started his selection of plants or crops that met his food and other needs. At this point, man realised the need to grow those plants that are of value to him to be harvested at a future date. The plants in this category he termed crops while those that grew on their own in the environment where the crops grew, even though they were not wanted, he termed weeds. Weeds therefore, have been defined as plants that grow where they are not wanted by man.

Commons Weeds In Agriculture

Below are some common weeds in West Africa

Common Weeds In Agriculture: itsmyschoollibrary.com

Effects Of Weeds In Agriculture

Weeds are generally perceived in negative sense. They are not wanted in the farms and where ever they grow, they create one form of problem or the other to the farmer.

Harmful Effects Of Weeds

  1. Contamination of Crops: Some weed seeds are inadvertently harvested and stored along with the crop seeds. These weed seeds contaminate the crop seeds and render them impure. They may also create moist conditions which predispose the crop seeds to infection.
  2. Unhealthy Competition: Weeds compete with crops for nutrients, light, water, space, and air. This competition exposes the crops to deprivation and finally leads to reduced yield.
  3. Some weeds are naturally poisonous to ruminant livestock such as Solanum nigrum. The consumption of such toxic material results in severe diarrhoea and in extreme cases, loss of farm animals.
  4. Weeds harbours dangerous insect pests and pathogens such as viruses, fungi, nematodes and bacteria. These injurious insects or pathogens infest and attack crops especially in the fields. The weeds harbour these organisms by serving as alternate hosts to them or by providing a hiding place for them.
  5. Some weeds are naturally parasitic to some crops and feed on them with specialized organs called haustoria. Cuscuta, Striga, Alectra etc are known to be common crop parasite.

Beneficial Effects Of Weeds

  1. Some weeds such as Cynodon, Oanicum, Aspilia, Centrosema, Pueraria etc, serves as livestock feed. In the same vein, Talinum, Pennisetum, etc, are used as leaf vegetables by man.
  2. Weeds such as Imperata cylindrical, Cassia Occidentalis, etc have medicinal value and are widely used in traditional medicare.
  3. Weeds recycle soil nutrients by drawing and using soil nutrients from lower soil horizons. The nutrients are used in building up the plants vegetative parts. Where the weeds die, they decompose and release the locked up nutrients in the vegetative parts into the rhizosphere. Humus from the decomposed weeds improves the soil structure thereby making it less prone to erosion.
  4. Some weeds such as Tagetes spp and Asparagus spp. Secret toxic exudates into the rhizosphere. These exudates immobilize the nematode and limit their infestation. Other weeds such as Crotolaria spp and Stylosanthes spp. allow nematode larvae to enter into them but will not allow the nematode to develop further. This helps to control nematode infestation on the crop plants.
  5. Weeds serves as food to many organisms in the ecosystem. They therefore, ensure biological diversity and maintain ecological balance.

Characteristics Of Weeds

Weeds have shown adaptations to varied ecological systems for agaes. They have certain features that enable them survive man’s efforts towards their eradication. These features are:

  1. Means of Dispersal: weeds are efficiently dispersed by wind, water, animals and self explosive mechanism.
  2. Efficient Means Of Propagation: Weeds are propagated by seedling and vegetatively from bulbs, tufts, roots, rhizome, stem etc.
  3. Production Of Large Number Of Seeds: plants generally regarded as weeds produce seeds profusely. For instance, Eupatorium, Pennisetum etc, produce a lot of seeds. A good number of thses seeds are bound to be lost but the number that survives is enough to perpetuate the species from year to year.
  4. Smothering Ability: A good number of weeds grow very fast especially at the early stage of their life. This enable them to develop vegetative parts earlier than the actual crops. With well developed vegetative parts, they smother our many crops for example Eupatorium spp.
  5. Seed Longevity: Buried weeds seeds remain viable and dormant in the soil for a long time without germination. Such weeds germinate only when they are exposed by weeding and tillage operations to conditions that favour germination. These conditions that are necessary for germination especially warmth, light, and oxygen are lacking in lower soil horizons.
  6. Early Maturity: Many weeds grow and mature faster that the actual crops. This makes their control difficult as the dry seeds could be scattered by weeding operations.
  7. Physical Resistance: Weed seeds have structures that enable them to develop physical resistance against prolonged wetness/dryness, burning and digestion by ruminants.
  8. Light Requirement: Light seeded weeds require light to germinate. When exposed to the right light conditions, they germinate but remain dormant if the are deeply buried by agronomic practices.

Methods Of Weed Dispersal

  1. Water: Flowing water carry seeds of weeds that are light enough to keep afloat in water e.g Stachytarpheta cayenensis.
  2. Wind: weed seeds dispersed by wind are usually small and light and have such structures as wings, floss, and pappus that enhance wind dispersal. The seeds of Tridax, Aspilia, Eupatorium, Imperata, Pennisetum, Combretum, etc are wind dispersed. Diagrams
  3. Animals: Weed seeds that have hairs or hoohooks that sticks to their skin hairs and furs or those that pass through their digestive system without being digested or losing viability. Man can disperse weed seeds that stick to their clothing and hairs. Examples of weeds that are dispersed in this way are Urena, Triumphetta, Desmodium, Ageratum etc.
  4. Weed Dispersal Mechanism: Leguminous weeds have forceful or explosive self dispersal mechanism. They have pods that are dry dehiscent. Weeds that belong to this group are Pueraria, Mucuna, Centrosema, Crotolaria, Calopogonum etc.

Methods Of Weed Control

1. Cultural Method
The cultural method include:

  • Burning: When farm lands are burnt in preparation for crop production, weed seeds are also destroyed.
  • Flooding: Flooding is an effective means of controlling weeds in rice fields. The weeds that survives flooding are those that enjoy prolonged wetness which favour rice production.
  • Tillage Operations: During tillage, hand tools such as how’s and cultivators are used to cut or uproot weeds in the farm. It is advisable to turn the slices of soil up-side down so that the weed roots are exposed to the scorching effect if the sun.
  • Crop Rotation: The growth of some weeds is enhanced by the growth of certain crops. The reason for this could be traced to the shape of such crops which enables the weeds to exploit the resources in the environment. A deliberate crop rotation changes the environment where the weeds grow and increases their competitive disability.
  • Weeding: Weeding is carried out with how’s and cutlasses. It involves cutting the weeds at the point of soil surface or uprooting the weeds from the soil. It is much more efficient if the exercise precede flowering and maturation of the weeds or if the weed residues are exposed to the scorching effect of the sun.
  • Mulching: Mulching physically obstruct weed emergence. It shields light living weed seeds from light that could have triggered off their germination.

2. Biological Control of Weed

  • Use of Cover Crops
  • Use of parasitic insects:

3. Chemical Control Method

Chemical control method involves the use of herbicides

4. Integrated weed Management

This involves the combination of cultural, biological and chemical method in controlling weed.


Done studying? See all previous lessons in Agriculture.

Take a quick test for this lesson

  1. Explain the concept of weeds.
  2. Give a brief origin of the term weeds.
  3. Mention some common weeds west Africa.
  4. What are the beneficial and harmful effects of weeds,
  5. State the methods of controlling weeds.

Questions answered correctly? Bravo!!

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