Lesson Notes By Weeks and Term - Senior Secondary School 2

SUBJECT: AGRICULTURAL SCIENCE

CLASS: SS2

DATE:

TERM: 3RD TERM

REFERENCE BOOKS

• Essential Agricultural Science for Senior Secondary Schools, by O.A. Iwena
• WAEC PAQ
• Essential economics

WEEK ONE

TOPIC: COMMON WEEDS FOUND IN FARMS

CONTENT

• Meaning of Weeds
• Effects of weeds on crops and the Economy
• Various types of weeds and their botanical names
• Methods of weed control

MEANING OF WEEDS

A weed is any plant that grows in a place where it is not (expected to grow) wanted

ECONOMIC IMPORTANCE OF WEEDS

1. They compete with crops for space
2. Weeds compete with crops for sunlight
3. Weeds compete with crop for nutrient
4. Weeds habour crop pests and pathogens
5. Weeds compete with crop for soil moisture
6. It leads to loss in crop yields
7. Weeds compete with crop for soil oxygen
8. They reduce the quality of crops
9. They reduce the income of farmers
10. They cause reduction in the taste of some grasses
11. Some weeds are toxic to farm animals

BENEFITS OF WEEDS TO FARMERS

1. Weeds helps in checking soil erosion
2. Some weeds are useful as food for humans
3. Some weeds serve as food for animals
4. Certain weeds have medicinal values
5. They can be used as green manure
6. Some are used for making roots, mats and baskets
7. Weeds are used for mulching
8. Weeds are used for compost making

EVALUATION

1. What are weeds?
2. List five beneficial effects of weeds

CHARACTERISTICS AND GROWTH HABIT OF WEEDS

1. Production of large quantity of seeds
2. Some weeds reduced by seeds
3. Some weeds reduced by vegetative propagation
4. Growth of large leaves within a short time
5. Weeds growth is favoured during rainy season
6. Most weeds are wind pollinated
7. Production of large quantity of pollen grains

DISPERSAL OF WEED SEEDS AND FRUITS

Most seeds and fruits of weeds are generally dispersed by wind and animals. The mode of dispersal are;

1. Dispersal of seeds by wind: Such weeds have feathery, winged or powdery seeds e.g. Tridax (pwd weed), goats weed, guinea grass, elephant grass and many other species of grasses.
2. Dispersal of seeds by Animal: Animal dispersed weeds usually have sticky, hooked, edible or colourful fruits or seeds. The sticky and hooked types of fruits and seeds adhere to the coats of grazing animals and are carried from one place to another. Examples of such animals are; rabbits, rats squirrels, antelope and man. Pig weed and commelina spp are example of such weed.
3. Dispersal of seeds by water: Water dispered weeds have water proof cuticles, air spaces within the fruit and a light weight
4. Dispersal of seeds by explosive mechanism: Few plants and all leguminouns weeds are dispersed by explosive mechanism eg water leaf, centro, pueraria, calapo and mucuna.

COMMON WEE

EVALUATION

1. List and explain different weed dispersal method
2. List five weeds and their botanical names

GENERAL CONTROL OF WEEDS

1. Mechanical control: involves the use of machines such as bulldozer, tree puller, plough etc to get rid of unwanted plants.
2. Physical or manual control: This can be done in different ways which includes hand pulling, hoeing, slashing with cutlass etc.
3. Biological Control: This involves the introduction of some insects and herbivorous animals to farmland to eat up the leaves of the weed and even the entire plant in some cases.
4. Cultural control: This involves the use of farming system to get rid of wed. It can be achieved by practice of crop rotation and the growing of cover crops to suppress the growth of the weeds on the farm. Other types of cultural weed control methods are: flooding, bush burning, mulching
5. Chemical Control: This involves the use of chemical solution called herbicides to control weeds. Some herbicides are selective while some are non-selective or contact herbicide

REASONS WHY WEEDS ARE DIFFICULT TO CONTROL

1. They have vigorous growth, they are aggressive and persistent
2. Weeds have high reproductive capacity
3. Their seeds have long period of viability
4. Weeds have efficient dispersal method
5. They have efficient means of propagation
6. They have long period of dormancy
7. They produce many seeds
8. They have short life cycle

GENERAL EVALUATION

1. What are weeds?
2. Discuss the methods of weed dispersal
3. List five economic importance of weeds
4. List five common weeds and state their botanical names
5. Discuss the four methods of weed control
6. Give three reasons why weeds are difficult to eradicate

READING ASSIGNMENT

• Essential Agricultural Science for senior Secondary School by O.A. Iwena Chapter 28 pages 249-260

• Answer the following questions from WAEC PAQ 2005 theory question 5

WEEKEND ASSIGNMENT

1. A cowpea growing in a maize plot is a ________ A. plant B. weed C. crop D. animal
2. Seed of centro, calapo are usually dispersed by _____ A. wind B. water C. explosive mechanism D. animals
3. Which of the following is not a cultural weed control methods A. flooding B. burning C. mulching D. hoeing
4. Which of the following is not a physical method of weed control? A. hand pulling B. mulching C. hoeing D. slashing
5. Weeds have a _____ lifecycle A. long B. medium C. short D. average

THEORY

1. What are weeds
2. Describe four methods of weed control
3. List four benefits of weeds to farmers
4. State four reasons why weeds are difficult to control