Lesson Notes By Weeks and Term v4 - SHS 3
HEALTH ISSUES IN CROP PRODUCTION
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HEALTH ISSUES IN CROP PRODUCTION
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Agriculture
Class: SHS 3
Term: 2nd Term
Week: 10
Grade code: 3.4.1.LI.2
Strand code: 4
Sub-strand code: 1
Content standard code: 3.4.1.CS.1
Indicator code: 3.4.1.LI.2
Theme: AGRICULTURE AND HEALTH
Subtheme: HEALTH ISSUES IN CROP PRODUCTION
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Weeds are a constant challenge for farmers across Ghana, from the maize fields in the Ashanti Region to the vegetable farms in the Greater Accra Region. They compete with our precious crops for sunlight, water, and nutrients, drastically reducing yields. But have you ever wondered how a stubborn weed like "Acheampong" (Siam weed) or speargrass appears in a newly cleared farm, seemingly from nowhere? The answer lies in weed dispersal – the process by which weeds travel and spread. Understanding how weeds move is the first critical step in developing effective strategies to manage and control them, thereby improving our farm's health and productivity.
A. What is a Weed? A weed is simply a plant growing where it is not wanted. It is a plant out of place. For example, a maize plant in a yam farm is considered a weed. Common Ghanaian Examples: Speargrass (*Imperata cylindrica*), Siam weed / Acheampong weed (*Chromolaena odorata*), Goat weed (*Ageratum conyzoides*), Water hyacinth (*Eichhornia crassipes*), Wire weed (*Sida acuta*). B. What is Weed Dispersal? Weed dispersal is the movement or transportation of weed seeds and other reproductive parts (like stems, roots, or bulbs, collectively called propagules) from the location of the parent plant to a new area where they can grow and establish a new population.
Think of it like people travelling from their hometown to a new city to start a new life. Weeds do the same to survive and multiply. C. Agents and Methods of Weed Dispersal Weeds do not have legs, so they rely on natural and man-made agents to move them around. The main agents and their methods are: Wind (Anemochory) This is dispersal by wind. Weeds that use this method have seeds with special features to catch the wind. Method: The wind blows light seeds or seeds with special structures over short or long distances. Characteristics of Weeds: Light and small seeds: They are easily picked up by the slightest breeze. Parachute structure (Pappus): Fluffy, hair-like structures that act like a parachute, allowing the seed to float in the air for a long time. Example: Siam weed, Goat weed, Tridax (Coat buttons). Winged seeds (Samara): Seeds have flat, wing-like extensions that help them spin and glide in the wind. Ghanaian Example: After weeding "Acheampong" (Siam weed), if you leave the flowers on the ground, a windy day can spread thousands of its tiny, fluffy seeds all over your farm and your neighbour's farm. Water (Hydrochory) This is dispersal by water. It is very common in riverine, coastal, and irrigated areas. Method: Seeds or plant fragments are carried by the flow of water in rivers, streams, irrigation canals, or surface runoff during heavy rains. Characteristics of Weeds: Buoyancy: Seeds have air-filled cavities or light, corky tissues that allow them to float. Waterproof seed coat: Protects the seed from getting waterlogged and rotting. Ghanaian Example: The rapid spread of Water Hyacinth on the Volta River and Tano River is a classic example. Fragments of the plant break off, float downstream, and start new colonies, choking the waterways. Animals (Zoochory) This is dispersal by animals, including birds, mammals, reptiles, and insects. Method 1: External Transport (Epizoochory) Seeds attach to the outside of an animal's body. Characteristics: Seeds have hooks, barbs, spines, or sticky surfaces that cling to fur, feathers, or wool. Ghanaian Example: When a goat walks through a patch of Beggar-ticks (*Desmodium* sp.), the flat, hooked seed pods stick to its fur. The goat carries them to a new location and they fall off or are rubbed off later. Method 2: Internal Transport (Endozoochory) Animals eat the fleshy fruits of a weed, and the hard seeds pass through their digestive system unharmed. The seeds are then deposited in a new location within the animal's droppings, which also act as a natural fertilizer. Characteristics: Weeds produce attractive, colourful, and fleshy fruits. The seeds have a very hard coat to resist digestion. Ghanaian Example: Birds and bats eat the fruits of Lantana camara. They then fly away and excrete the seeds, spreading this invasive weed into new areas. Humans (Anthropochory) Humans are the most effective and long-distance dispersers of weeds. This is often unintentional. Methods: Contaminated Crop Seeds: Buying grains or seeds from the market (e.g., maize, rice) that have weed seeds mixed in them. Farm Machinery & Tools: Mud containing weed seeds gets stuck on tractor tyres, ploughs, harrows, and even cutlasses and wellington boots. When this equipment is moved to another farm, the seeds are transported too. Livestock Manure: If livestock are fed with fodder contaminated with weed seeds, the seeds can pass through their guts undigested and end up in manure, which is then spread on the farm as fertilizer. Deliberate Introduction: Sometimes, plants are introduced into an area for ornamental purposes (e.g., as a beautiful flower) but later escape cultivation and become invasive weeds. Ghanaian Example: A tractor is hired to plough a field infested with speargrass. The rhizomes (underground stems) of the speargrass get chopped up and stuck to the plough. The tractor then moves to a clean field and starts ploughing, effectively "planting" speargrass all over the new farm. Mechanical/Explosive Dispersal (Autochory) This is self-dispersal, where the plant itself disperses its seeds without an external agent. Method: The fruit pod of the weed dries out, creating tension. It then splits open suddenly and violently, flinging the seeds away from the parent plant. Characteristics: The fruit pods are designed to dehisce (split open) explosively. Ghanaian Example: The Castor oil plant (*Ricinus communis*), which can be found growing wild, has seed capsules that explode when ripe, scattering the seeds over a few metres. D. Advantages and Disadvantages of Weed Dispersal
| Aspect | From a Farmer's/Agricultural Perspective (Mainly Disadvantages) | From an Ecological Perspective (Can have Advantages) | | :--- | :--- | :--- | | Disadvantages | 1. Spread of Infestation: Introduces weeds to clean, cultivated fields, increasing the area a farmer has to manage. 2. Increased Cost of Production: More money and time must be spent on weed control (labour for weeding, cost of herbicides). 3. Reduced Crop Yield & Quality: New weeds compete with crops, and their seeds can contaminate harvested produce, lowering its market price. | 1. Loss of Biodiversity: Invasive weeds can outcompete and displace native plant species, reducing local biodiversity. | | Advantages | *Generally, there are no direct advantages for a crop farmer.* | 1. Soil Conservation: Weeds are often "pioneer plants" that quickly cover bare ground (e.g., after a bushfire or construction). Their roots hold the soil together, preventing wind and water erosion. 2. Maintaining Genetic Diversity: Dispersal allows the weed species to find new habitats and survive environmental changes, preserving its genetic line. 3. Source of Food/Habitat for Wildlife: Some animals and insects rely on weeds for food and shelter. 4. Source of Ethnobotanicals: Many weeds are used in traditional Ghanaian medicine. Dispersal ensures they are available. |
Guided Practice (With Solutions)