2023 Cambridge IGCSE Agriculture (0600) 模擬試題連答案詳解

Aphids are sap-sucking insects. Systemic insecticides are absorbed by the plant tissues and transported throughout the plant via the vascular system (sap stream). When aphids feed on the sap, they ingest the systemic pesticide, making it highly effective against them while minimizing direct contact damage to beneficial predatory insects.

Award 1 mark for identifying that a systemic insecticide (Option B) is the correct choice. Reject other options: contact fungicides (A) target fungi; selective post-emergence herbicides (C) target weeds; stomach insecticides applied to surfaces (D) are less effective against sucking pests that do not bite and chew leaf surfaces.

Agricultural supply is typically inelastic in the short term. Because crops have a fixed growing season and require significant lead time and investment to establish, farmers cannot immediately increase or decrease production in response to sudden market price changes.

Award 1 mark for the correct answer (Option A). Reject other options: perfectly elastic demand (B) refers to consumers; diminishing marginal returns (C) refers to input productivity; high opportunity cost (D) is not the primary economic concept describing this production lag.

The upward movement of water and dissolved mineral ions in the xylem is mainly driven by the transpiration pull. As water evaporates from the leaves through the stomata, a tension (negative pressure) is created, pulling the continuous water column up through the xylem vessels from the roots.

Award 1 mark for identifying the correct driver (Option B). Reject active transport in phloem (A) as phloem transports assimilates; root pressure (C) is a minor factor and not driven by translocation; guttation (D) is a symptom of high root pressure under low transpiration, not a driver of upward movement.

Rotational grazing allows pasture plants in rested paddocks a recovery period to regrow, photosynthesize, and restore their root reserves. This prevents overgrazing, improves pasture yield, and maintains forage quality over a longer period compared to continuous grazing.

Award 1 mark for the correct advantage (Option C). Reject other options: rotational grazing reduces soil compaction overall compared to overgrazed continuous systems (A); it does not eliminate the need for veterinary care (B); and it increases fencing costs due to paddock division (D).

Feeds with a crude fiber content of more than 18% are classified as roughages (or forages). These include grass, hay, silage, and straw, which are highly bulky and less energy-dense compared to concentrates.

Award 1 mark for identifying the feed type as roughage (Option A). Reject concentrates (B), which are low in fiber (<18%) and high in energy; reject protein supplements (C) and mineral additives (D), as these have specific composition characteristics not solely defined by high crude fiber.

Biological control involves using living organisms (predators, herbivores, parasitoids, or pathogens) to suppress pest populations. For example, using ladybirds to control aphids on a bean crop. A major risk is that the introduced biological agent may disrupt local ecosystems by preying on non-target species or outcompeting native beneficial species.

1.3 marks for describing biological control (e.g., introducing natural predator/parasite/pathogen to control a pest). 1.3 marks for identifying a valid risk (e.g., attacking non-target species, ecological imbalance).

According to the law of diminishing returns, successive increases in an input (such as fertilizer) to a fixed amount of land will eventually yield smaller and smaller increases in output (maize). Beyond the optimum point, the cost of the additional fertilizer exceeds the value of the extra crop produced, and excessive fertilizer can even cause toxicity and reduce yield.

1.3 marks for describing the initial phase where inputs increase yield. 1.3 marks for explaining the point where additional inputs result in smaller/negative returns.

A maintenance ration is the amount of feed needed to keep an animal's body functioning, maintaining its weight and body temperature in a non-productive state. A production ration is the feed supplied in addition to the maintenance ration to support productive functions like growth, lactation, reproduction, or work. Specific production purposes include milk secretion in dairy cows or egg laying in poultry.

1.3 marks for distinguishing maintenance from production ration. 1.3 marks for identifying a valid production purpose (e.g., milk production, pregnancy, growth, egg laying).

Translocation is the process by which organic nutrients, primarily sugars, are transported throughout the plant from sources (where they are made or stored, e.g., mature leaves) to sinks (where they are used or stored, e.g., growing roots, developing fruits). This transport occurs within the phloem tissue, and the primary sugar transported is sucrose.

1.3 marks for explaining translocation from source to sink in the phloem tissue. 1.3 marks for identifying sucrose as the primary form of carbohydrate transported.

In a zero-grazing (cut-and-carry) system, pasture grasses or legumes are harvested mechanically or manually and brought directly to animals housed in a stall or yard. This prevents animals from roaming. Advantages include higher pasture utilization (no trampling or fouling), better monitoring of food intake, and reduced risk of parasite infestation from pasture.

1.3 marks for defining zero-grazing (cutting forage and bringing it to housed animals). 1.3 marks for stating a valid advantage (e.g., reduced trampling, high yield per unit area, control of disease/parasites).

Ploughing along the contours (at right angles to the slope) creates horizontal ridges and furrows that act as small barriers to water flowing downhill. This slows down surface runoff, which reduces its capacity to transport soil particles. It also increases the contact time of water with the soil surface, promoting water infiltration rather than erosion.

1.3 marks for explaining that ploughing creates ridges/furrows across the slope (horizontal to the hill). 1.3 marks for explaining that this slows down runoff, increasing infiltration and reducing soil carrying capacity.

Nitrogen is a major constituent of amino acids, proteins, nucleic acids, and chlorophyll. It is vital for rapid vegetative growth (leaves and stems). When nitrogen is deficient, plants display stunted growth and chlorosis, which is a general yellowing of the leaves, starting with the older leaves as nitrogen is translocated to younger growth.

1.3 marks for stating nitrogen's role (protein/chlorophyll synthesis or vegetative growth). 1.3 marks for describing a deficiency symptom (e.g., yellowing/chlorosis of older leaves, stunted growth).

Good ventilation in a poultry house is crucial to maintain air quality by removing toxic gases (like ammonia from litter and carbon dioxide), excess moisture, and heat produced by the birds. This reduces the risk of respiratory diseases and heat stress. Physical features that facilitate ventilation include adjustable side shutters, wire mesh walls, ridge vents on the roof, or mechanical extractor fans.

1.3 marks for explaining the importance of ventilation (removing ammonia, moisture, heat, or preventing respiratory diseases). 1.3 marks for describing a physical design feature (e.g., ridge vents, extractor fans, mesh walls, shutters).

1. Define maintenance ration: It supports basic life processes (respiration, circulation, temperature regulation) without weight gain, weight loss, or production.
2. Explain production ration requirements: Additional energy, protein, and minerals (especially calcium and phosphorus) are needed for fetal tissue development and daily milk production.

1.3 marks: Accurate definition of maintenance ration (mentioning life maintenance/no loss or gain in weight/no production).
1.3 marks: Clear explanation of why lactation/pregnancy increases demand (fetal growth and milk synthesis require additional energy/nutrients/production ration).

Translocation refers to the movement of manufactured food materials through the phloem. The main transport sugar is sucrose, which is soluble and relatively unreactive. Movement occurs from source organs (e.g., mature leaves) to sink organs (e.g., developing tubers, roots, flowers) via active transport mechanisms.

0.6 marks: Identification of phloem as the tissue involved.
1.0 mark: Identification of sucrose as the transport carbohydrate.
1.0 mark: Explanation of source-to-sink movement.

Biological control operates by introducing a living organism (the control agent) that preys upon or parasitizes the pest, keeping its population below the economic injury level. If an exotic agent is introduced, it may lack natural competitors in the new environment and expand its host range to native, non-target species.

1.3 marks: Clear explanation of the biological mechanism (using predators, parasites, or pathogens to control pests).
1.3 marks: Identification of a valid ecological risk (non-target predation, becoming an invasive pest, or ecological imbalance).

The law highlights that input-output efficiency decreases after a certain point. Profit is maximized when the marginal cost (MC) of the input equals the marginal revenue (MR) of the output. Beyond this point, any further input application increases costs more than it increases crop revenue, reducing net profit.

1.3 marks: Definition of the law of diminishing returns (increasing variable inputs yields smaller increments of output).
1.3 marks: Explanation of finding the optimal point (matching marginal cost of input to marginal revenue of additional yield).

Rotational grazing maximizes pasture production and quality. Dividing pasture into paddocks prevents selective overgrazing and allows rested pasture to recover structurally. It also breaks the life cycle of internal parasites because larvae die in empty paddocks before animals return.

1.0 mark: Outline of management (dividing pasture into paddocks and systematically rotating animals).
1.6 marks: Two distinct advantages stated (0.8 marks each, e.g., pasture recovery, prevention of selective overgrazing, parasite control, higher carrying capacity).

Water erosion is accelerated by steep slopes where water flows quickly. Contour ploughing acts as miniature dams across the slope. For the vegetation-based method, cover crops (such as legumes) provide immediate canopy cover and root binding to stabilize soil particles.

1.3 marks: Explanation of contour ploughing (ploughing horizontally/across the slope to slow surface water runoff and promote infiltration).
1.3 marks: Correctly naming a vegetative conservation method (e.g., cover cropping, windbreaks, grass barrier strips, agroforestry).

Legumes (e.g., beans, peas, clover) possess root nodules containing symbiotic Rhizobium bacteria. These bacteria break the strong triple bond of atmospheric nitrogen, converting it into plant-available forms. Post-harvest, plow-down of the legume green manure adds nitrogen-rich organic matter directly back to the soil profile.

0.6 marks: Correctly naming 'Rhizobium' or 'nitrogen-fixing bacteria'.
1.0 mark: Describing the symbiotic relationship within root nodules.
1.0 mark: Explaining how the nitrogen becomes available to subsequent crops (fixation and post-decomposition release).

In intensive housing, waste accumulation generates noxious gases. Ventilation dilutes pathogens and prevents respiratory stress. Passive cooling utilizes thermal convection (hot air rising and escaping through ridge vents) and conduction prevention (insulating the roof or painting it white to reflect solar radiation).

1.3 marks: Explanation of the importance of ventilation (removal of noxious gases/moisture/pathogens or supply of fresh oxygen).
1.3 marks: Description of one passive temperature-control design feature (e.g., ridge vents, wide open-sided walls with adjustable curtains, reflective/insulated roof, strategic shade tree placement).

Systemic pesticides are absorbed by the plant and translocated throughout its tissues. Disadvantages include: 1. Chemical residue persistence: The active ingredients may remain in the edible parts of the crop at harvest, presenting health hazards to consumers. 2. Pest resistance: Frequent use of a single chemical selector increases the selection pressure, leading to the rapid evolution of resistant pest populations. In contrast, biological control is self-sustaining and chemical-free.

Award 1.3 marks for each valid disadvantage described (maximum of 2.6 marks). Acceptable points: Pest resistance / resurgence; Toxic chemical residues in crops; Bioaccumulation in the food chain; High purchasing costs. Reject: 'takes a long time to act'.

A tractor is a capital asset and its cost (such as depreciation or initial purchase cost) is a fixed cost because it is incurred regardless of whether the farmer produces one bag of maize or one thousand bags. Synthetic fertilizer is a variable cost because the total expenditure on it depends directly on the area planted and the target yield (the volume of production).

0.6 marks for correctly classifying both (tractor = fixed, fertilizer = variable). 1.0 mark for explaining that fixed costs do not vary with the level of output. 1.0 mark for explaining that variable costs change in proportion to the level of output.

A maintenance ration covers the animal's basal metabolic rate, temperature regulation, and basic tissue repair without any weight change or production. A production ration is the extra feeding given on top of the maintenance ration to support active outputs like milk secretion, fetal development during gestation, or muscle growth.

1.3 marks for defining maintenance ration (focus on survival, tissue repair, constant weight). 1.3 marks for defining production ration (focus on extra nutrients for milk, meat, or pregnancy).

Transpiration relies on the diffusion of water vapor from the humid air spaces inside the leaf (high water potential) to the drier atmosphere outside (low water potential). When atmospheric humidity increases, the external air holds more moisture. This reduces the concentration gradient of water vapor between the inside and outside of the leaf, which slows down the rate of diffusion through the stomata.

1.0 mark for identifying that the transpiration rate decreases/slows down. 1.6 marks for explaining the mechanism: a smaller/reduced water vapor concentration gradient (or water potential gradient) between the internal leaf tissue and the external atmosphere.

Rotational grazing involves moving livestock through a series of enclosed pastures (paddocks) in sequence. Two main benefits are: 1. Pasture recovery: Giving paddocks rest periods prevents overgrazing, allowing palatable grass species to regenerate root reserves and foliage. 2. Parasite control: Many internal parasites rely on direct ingestion of larvae from the pasture. Moving animals out of a paddock for several weeks allows the larvae to die before hosts return, breaking the parasite life cycle.

Award 1.3 marks for each clear benefit explained (maximum of 2.6 marks). Accept: prevention of selective overgrazing, improved pasture recovery/regrowth, reduction of soil compaction, control of internal parasites, even distribution of manure.

Ploughing along the contours (at right angles to the slope) rather than up and down creates a series of horizontal ridges and channels. These act as mini-dams that intercept rainwater flowing downslope. By slowing down the surface runoff, the water loses its kinetic energy to detach and carry soil particles, and more water is absorbed into the soil profile.

1.0 mark for stating that ploughing across the slope/at right angles to the slope creates physical barriers. 1.6 marks for explaining that this reduces the velocity of surface runoff and increases water infiltration/absorption, preventing soil wash.

Leguminous plants (e.g., beans, peas, clover) host symbiotic Rhizobium bacteria in specialized root nodules. These bacteria absorb atmospheric nitrogen gas from the soil pores and convert it into ammonia and nitrates, which the plant uses. When the legume crop is harvested and its crop residue or green manure is incorporated into the soil and decays, it releases this fixed nitrogen, significantly increasing the nitrate content and overall fertility of the soil for the next crop.

1.0 mark for mentioning symbiotic nitrogen-fixing bacteria (Rhizobium) in root nodules. 1.6 marks for explaining that they convert atmospheric nitrogen into usable nitrogen compounds (nitrates/ammonium) which enrich the soil during/after the growth of the leguminous crop.

In intensive poultry systems, birds are housed closely together, generating significant heat, moisture, and waste (droppings). Droppings release toxic ammonia gas. Good ventilation provides a continuous flow of fresh air, which: 1. Dilutes and removes harmful gases (ammonia and carbon dioxide) that damage respiratory tracts. 2. Regulates temperature and removes body heat to prevent fatal heat stress. 3. Reduces moisture/humidity, keeping litter dry and suppressing bacterial and fungal pathogens.

Award 1.3 marks for each distinct reason/mechanism explained (up to a maximum of 2.6 marks). Accept: removal of ammonia/carbon dioxide, temperature control/prevention of heat stress, reduction of relative humidity/keeping litter dry, and reducing the spread of airborne respiratory pathogens.

Legume plants have a symbiotic relationship with nitrogen-fixing bacteria, specifically Rhizobium, which live in the root nodules. These bacteria take nitrogen gas from the soil air and convert it into organic nitrogen compounds that the plant can use. When the legume crop is harvested, dies, or is ploughed back into the soil as green manure, soil decomposers break down the plant material, converting organic nitrogen into plant-available nitrates (nitrification), which enriches the soil for subsequent crops.

Award 1 mark for identifying the symbiotic relationship with nitrogen-fixing bacteria (Rhizobium) in root nodules. Award 1 mark for explaining that these bacteria fix atmospheric nitrogen into usable nitrogen compounds. Award 0.6 marks for explaining that the decay or ploughing-in of the legume tissue releases these nitrogen compounds into the soil as nitrates for the next crop.

Part (a): Extensive pasture management involves large areas of land with low stocking rates, minimal inputs (like fertilizer or irrigation), and low productivity per hectare (e.g., hill sheep farming). Intensive pasture management involves smaller areas of land with high stocking rates, high inputs of fertilizers, irrigation, and high productivity per hectare (e.g., dairy farming). Part (b): Rotational grazing is set up by dividing a large pasture into smaller enclosures called paddocks using fences. Livestock are moved from one paddock to another in a planned sequence. Animals graze one paddock intensively for a short period, then are moved to the next paddock, allowing the grazed paddock a period of rest and recovery to grow back before being grazed again. Part (c): Advantages of rotational grazing: 1. Higher carrying capacity as forage is used more efficiently. 2. Better control of weeds and pests (disrupts life cycles). 3. Reduces soil erosion as pasture has time to recover root systems. Disadvantages: 1. High initial capital cost for fencing and water supply in each paddock. 2. Higher labor and management requirement to monitor pasture and move stock. 3. Risk of overgrazing if livestock are kept in a paddock too long.

Part (a) [Max 4 marks]: 1 mark for extensive definition (large area, low stocking rate); 1 mark for extensive example/input level (low inputs/cost); 1 mark for intensive definition (small area, high stocking rate); 1 mark for intensive input/production level. Part (b) [Max 5 marks]: 1 mark for dividing pasture into paddocks; 1 mark for moving livestock in a planned sequence; 1 mark for high-intensity short-duration grazing; 1 mark for allowing a rest/recovery period for the pasture; 1 mark for mentioning water supply/fencing infrastructure. Part (c) [Max 6 marks]: 1 mark for each valid advantage explained (up to 3 marks) - accept higher yield, weed/pest control, reduced erosion, more uniform grazing; 1 mark for each valid disadvantage explained (up to 3 marks) - accept high cost of fencing/water, high labor requirement, risk of overgrazing if managed poorly.

Part (a): Cultural pest control involves modifying standard farming practices to make the environment less favorable for pests. Example: Crop rotation, deep plowing, or weeding. Biological pest control involves introducing or encouraging natural predators or parasites to control the pest population. Example: Introducing ladybugs to feed on aphids. Part (b): Integrated Pest Management (IPM) is implemented by: 1. Monitoring and identifying pests regularly to assess population levels. 2. Establishing economic injury levels (only treating when damage exceeds cost of treatment). 3. Using cultural and physical controls as the first line of defense. 4. Employing biological controls to keep pests below threshold levels. 5. Using chemical pesticides only as a last resort, using selective chemicals that target only the specific pest. Part (c): Three disadvantages of relying solely on chemicals: 1. Development of chemical resistance in pest populations, making pesticides ineffective over time. 2. Non-selective toxicity, which kills beneficial insects (e.g., pollinators, predators of pests) and disrupts the ecosystem. 3. Environmental pollution and health risks, including chemical residues in soil, water, and food crops.

Part (a) [Max 4 marks]: 1 mark for explaining cultural control; 1 mark for a valid cultural example; 1 mark for explaining biological control; 1 mark for a valid biological example. Part (b) [Max 5 marks]: 1 mark for pest identification/monitoring; 1 mark for setting economic action thresholds; 1 mark for prioritizing non-chemical methods (cultural/physical); 1 mark for incorporating biological agents; 1 mark for using targeted/selective chemicals as a last resort. Part (c) [Max 6 marks]: 2 marks for each well-explained disadvantage (1 mark for identifying the disadvantage, 1 mark for explaining its consequence/impact) up to 3 disadvantages: e.g., chemical resistance, destruction of beneficial organisms, bioaccumulation/residues, environmental pollution, high financial cost.