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This is the complete lecture note for CPS405 Weed Science and Control. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS • Course code: CPS 405 • Course title: Weed Science and Control • Course credit load: 2 Series 1: Weed Characteristics and Classification Series outline • Part 1: Defining Weeds o Sub-Part 1.1: General Characteristics of Weeds o Sub-Part 1.2: Distinguishing Weeds from Crops • Part 2: Classification of Weeds o Sub-Part 2.1: Morphological Classification o Sub-Part 2.2: Ecological and Life Cycle Classification • Part 3: Weed Biology o Sub-Part 3.1: Growth and Reproduction o Sub-Part 3.2: Adaptation and Survival Strategies Introduction Weeds are often seen as unwanted plants, yet their presence in agricultural fields has far-reaching consequences for crop productivity and farm management. They compete with crops for nutrients, water, light, and space, and their persistence can significantly reduce yields. Understanding the characteristics and classification of weeds is therefore a vital foundation for weed science, as it equips learners with the knowledge to identify, categorise, and anticipate weed behaviour in farming systems. This Series sets the stage for effective weed control by first examining what weeds are and how they differ from crops. The aim of this Series is to provide you with the ability to define weeds clearly, classify them according to recognised systems, and explain their biological traits that make them resilient and competitive in agricultural environments. We shall commence this Series by exploring the definition and general characteristics of weeds, including how they differ from cultivated crops. Next, we will examine the classification of weeds, considering both morphological and ecological perspectives. Finally, we will wrap up by studying weed biology, focusing on their growth, reproduction, and survival strategies. This sequential
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS approach will help you build a strong conceptual foundation for later Series on weed control methods and herbicide applications. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1 define weeds and explain their general characteristics, • SLO 1.2 distinguish weeds from crops using specific criteria, • SLO 1.3 classify weeds based on morphological features, • SLO 1.4 classify weeds according to ecological and life cycle patterns, • SLO 1.5 analyse the biological traits of weeds in terms of growth and reproduction, and • SLO 1.6 evaluate the adaptation and survival strategies that enable weeds to persist in farming systems. 1.1 Defining Weeds 1.1.1 General characteristics of weeds A weed is any plant that grows where it is not wanted and interferes with the utilisation of land and water resources. Weeds are characterised by their ability to compete aggressively with crops for nutrients, water, light, and space. They often possess rapid growth rates, high seed production, and efficient dispersal mechanisms. Many weeds also exhibit strong adaptability to diverse environments, making them difficult to eradicate. Weeds are typically hardy plants with survival traits such as resistance to drought, pests, and diseases. Their persistence in agricultural fields is a major factor contributing to reduced crop yields. For example, weeds like Imperata cylindrica (spear grass) can spread quickly through underground rhizomes, making them difficult to control. Fill in the blank: A plant is considered a ______ when it grows in an area where it is not wanted and competes with crops for essential resources.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Diagram showing common characteristics of weeds such as rapid growth, seed dispersal, and adaptability. Figure 1.1: General characteristics of weeds 1.1.2 Distinguishing weeds from crops While crops are deliberately cultivated for food, fibre, or other economic uses, weeds are unwanted plants that reduce productivity. The distinction lies in their purpose and impact. Crops are selected for desirable traits such as yield, nutritional value, and market demand, whereas weeds are identified by their negative effects on farming systems. Weeds often have traits that make them more competitive than crops, such as faster germination, tolerance to poor soils, and the ability to produce seeds over extended periods. Crops, on the other hand, usually require more care and management to thrive. For instance, maize is cultivated for its grain, but if Striga hermonthica (witchweed) grows alongside it, the weed parasitises the crop roots and drastically reduces yield. Fill in the blank: Crops are cultivated for their ______ value, while weeds are identified by their negative impact on farming systems.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Photograph showing a maize field with visible weed infestation. Plate 1.1: Distinguishing weeds from crops in a maize field Pilot questions 1.1 1. Which of the following best defines a weed? a) A plant cultivated for food b) A plant growing where it is not wanted c) A plant resistant to drought d) A plant with high nutritional value 2. Which characteristic is typical of weeds? a) Low seed production b) Slow growth rate c) High adaptability to environments d) Dependence on human care 3. What is the main distinction between crops and weeds? a) Crops are unwanted plants, weeds are cultivated b) Crops are cultivated for economic value, weeds reduce productivity c) Crops grow faster than weeds d) Crops are more adaptable than weeds 4. Which weed parasitises maize roots and reduces yield? a) Spear grass (Imperata cylindrica) b) Witchweed (Striga hermonthica) c) Water hyacinth (Eichhornia crassipes) d) Nut grass (Cyperus rotundus) 5. Which resource do weeds compete with crops for? a) Market demand b) Nutritional value c) Light, water, and nutrients d) Storage facilities 1.2 Classification of Weeds 1.2.1 Morphological classification Weeds can be classified according to their morphology, which refers to their external form and structure. This system groups weeds based on visible features such as leaf shape, stem type, and
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS root system. For example, broadleaf weeds like Amaranthus species have wide leaves with distinct veins, while grasses such as Imperata cylindrica possess narrow leaves and fibrous root systems. Morphological classification is practical because it allows farmers and agronomists to quickly identify weeds in the field without specialised equipment. However, it may sometimes be limited, as different species can share similar external features. Fill in the blank: Weeds with wide leaves and distinct veins are commonly referred to as ______ weeds. Diagram comparing broadleaf weeds and grass weeds. Figure 1.2: Morphological classification of weeds 1.2.2 Ecological and life cycle classification Another important system is ecological classification, which groups weeds based on the environment in which they thrive. For instance, aquatic weeds such as Eichhornia crassipes (water hyacinth) grow in water bodies, while terrestrial weeds like Chromolaena odorata dominate dry land areas. Weeds can also be classified by their life cycle, which refers to the duration of their growth and reproduction. Annual weeds complete their life cycle within one year, biennial weeds take two years, and perennial weeds persist for several years through structures such as rhizomes or tubers. This classification is crucial for designing effective control strategies, since perennial weeds often require more intensive management than annual weeds. Fill in the blank: Weeds that complete their life cycle within one year are known as ______ weeds. [Insert Table here] Table 1.1: Classification of weeds by life cycle
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Pilot questions 1.2 1. Which feature is used in morphological classification of weeds? a) Growth duration b) External form and structure c) Soil type d) Water requirement 2. Broadleaf weeds are characterised by: a) Narrow leaves and fibrous roots b) Wide leaves with distinct veins c) Tubers and rhizomes d) Aquatic growth habit 3. Which weed is an example of an aquatic species? a) Imperata cylindrica b) Chromolaena odorata c) Eichhornia crassipes d) Amaranthus spp. 4. Annual weeds complete their life cycle in: a) One year b) Two years c) Several years d) A few months only 5. Which type of weed requires more intensive management due to persistence? a) Annual weeds b) Biennial weeds c) Perennial weeds d) Morphological weeds 1.3 Weed Biology 1.3.1 Growth and reproduction Weeds exhibit remarkable abilities in growth and reproduction, which are key reasons for their persistence in farming systems. Growth refers to the increase in size and biomass of the plant, while reproduction is the process by which weeds produce offspring to ensure continuity of their species. Many weeds grow faster than crops, enabling them to dominate fields quickly. Reproduction in weeds can occur through seeds, vegetative structures such as rhizomes, stolons, tubers, or bulbs. For instance, Cyperus rotundus (nut grass) reproduces through underground
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS tubers, making it difficult to eradicate. Seed production is often prolific, with some species producing thousands of seeds per plant. These seeds may remain viable in the soil for years, forming what is known as a seed bank, which ensures future infestations even after control measures. Fill in the blank: The underground storage structures that enable nut
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