2025 Cambridge IGCSE Agriculture (0600) 模拟试题及答案详解

1 (a) Biting and chewing pests physically ingest plant tissue, leading to visible damage such as chewed leaf margins, shot-holes in leaves, severed plant stems, or hollowed-out seeds and tubers.

(b) (i) A systemic insecticide is an active chemical substance that is absorbed into the plant's vascular tissues and transported throughout the plant body in its sap flow.
(ii) When sap-feeding pests (such as aphids) feed by piercing the plant, they naturally ingest the toxic sap. The chemical functions as an internal stomach poison, destroying the pest without requiring direct skin contact during spraying.

(c) IPM employs a variety of biological, physical, and cultural controls to minimize chemical residues on harvested crops, lower environmental toxic risks to beneficial insects/pollinators, and reduce the speed at which pests acquire genetic resistance to chemicals.

1 (a) Award 1 mark for each correct sign of damage (max 3):
- Holes in leaves / ragged edges
- Entire leaves consumed / defoliation
- Hollowed out or bitten tubers/roots
- Severed stems / girdled trunks

(b) (i) Award 1 mark for correct definition:
- Chemical pesticide absorbed into plant tissues and translocated via vascular system/sap [1]
(ii) Award 1 mark for each point (max 2):
- Translocated through xylem or phloem [1]
- Ingested by sucking/piercing pests as they feed [1]
- Acts as a stomach poison within the pest [1]

(c) Award 1 mark for each described advantage (max 2):
- Delays development of chemical resistance [1]
- Less harmful to beneficial insects / biological control agents [1]
- Minimizes chemical pollution of groundwater / soils [1]
- Lower residue levels on final food product [1]

2 (a) Gestation period refers to the time period between fertilization of the egg cell and parturition (birth), during which the offspring develops in the uterus.

(b) (i) Colostrum.
(ii) Newborn mammals are born with no natural disease protection. Colostrum provides them with ready-made maternal antibodies, creating passive immunity. It also provides high levels of energy/vitamins to sustain body temperature and acts as a laxative to clear the digestive tract.

(c) To ensure a safe birth, the farmer should relocate the pregnant animal to a clean, isolated, well-bedded calving/lambing pen. Her feed ration should be adjusted (steaming up) to reduce bulky feed and increase concentrated nutrients for peak energy. She should be checked regularly for clinical signs of impending labor, such as udder enlargement, vaginal discharge, and nesting behaviour.

2 (a) Award 1 mark for correct description:
- Length of time a fetus/embryo is carried in the uterus / duration of pregnancy [1]

(b) (i) Award 1 mark for correct name:
- Colostrum [1]
(ii) Award 1 mark for each point (max 3):
- High concentration of antibodies providing passive immunity [1]
- Rich in highly digestible energy/nutrients [1]
- Acts as a laxative to expel the meconium (first feces) [1]
- Must be ingested early before the intestinal wall becomes impermeable to antibodies [1]

(c) Award 1 mark for each described management practice (max 3):
- Separate to a sanitized parturition pen / isolation stall [1]
- Implement 'steaming up' / balance ration with high-energy concentrates [1]
- Monitor closely for visible signs of parturition [1]
- Provide clean bedding and accessible clean water [1]

3 (a) Roughages are bulky feeds containing a high proportion of crude fiber (>18%) and low concentration of digestible energy (e.g., pasture grass, dry hay, or crop residues). Concentrates are dense feeds with low crude fiber and high levels of digestible carbohydrates or proteins (e.g., commercial dairy meal or crushed grains).

(b) Lactation drains immense amounts of calcium and phosphorus directly into milk production. If dietary mineral intake is insufficient, dairy cows will deplete their skeletal reserves, leading to parturient paresis (commonly known as milk fever) and a sharp decline in milk production.

(c) Depriving livestock of fresh water leads directly to dehydration (loss of skin elasticity), rapid decrease in appetite and dry matter intake, dark concentrated urine, sunken eyes, and lethargy.

3 (a) Award 1 mark for each correct distinction and example (max 4):
- Roughage: high fiber / bulky / low digestible nutrient concentration [1]
- Example of roughage: grass / silage / hay / straw / legume forage [1]
- Concentrate: low fiber / nutrient-dense / high digestible nutrients [1]
- Example of concentrate: maize meal / grain / protein cake / dairy mash [1]

(b) Award 1 mark for each point (max 2):
- Large amounts of calcium and phosphorus are excreted in the milk [1]
- Prevents hypocalcaemia / milk fever [1]
- Protects skeletal integrity / prevents weak bones [1]

(c) Award 1 mark for each correct symptom (max 2):
- Skin tenting / loss of skin elasticity [1]
- Reduced feed consumption [1]
- Sunken eye appearance [1]
- Rapid weight loss / extreme drop in yield [1]
- Hard, dry feces [1]

4 (a) Wind-pollinated flowers possess feathery, exposed stigmas to catch passing pollen grains and lack colorful petals, scent, or nectar. Insect-pollinated flowers feature large, brightly colored petals, distinct scents, and nectaries to lure insects, and their stigmas are typically sticky and protected within the flower.

(b) Vegetative propagation ensures that offspring are clones of the parent plant, preserving elite characteristics (such as fruit quality or disease resistance) without genetic segregation. Additionally, asexual cuttings or tubers establish quickly and bypass the vulnerable early seedling phase, allowing crops to mature and produce yields much earlier.

(c) Many self-pollinating species rely on cleistogamy, meaning fertilisation occurs before the flower buds even open. Homogamy also ensures selfing by synchronising the physical position and exact maturation times of both the pollen grains and the receptive stigma within the same closed flower.

4 (a) Award 1 mark for each contrasted structural difference (max 2):
- Wind-pollinated has dull/absent petals AND insect-pollinated has brightly colored/large petals [1]
- Wind-pollinated has protruding/feathery stigmas AND insect-pollinated has sticky/internal stigmas [1]
- Wind-pollinated has long dangling filaments/stamens AND insect-pollinated has short, rigid stamens [1]
- Wind-pollinated lacks nectaries/scent AND insect-pollinated contains nectaries/scents [1]

(b) Award 1 mark for naming an advantage and 1 mark for its explanation (max 4):
- Clonal uniformity [1]; retains specific genetic benefits/yield capacity of parent [1]
- Faster growth/maturity [1]; crop bypasses seed germination stage to harvest quicker [1]
- Stronger survival [1]; larger food storage in cuttings/tubers compared to seeds [1]

(c) Award 1 mark for each described mechanism (max 2):
- Cleistogamy / flowers fertilise before opening [1]
- Synchronous maturation of anthers and stigma (homogamy) inside the same blossom [1]
- Proximity of male and female reproductive structures prevents external pollen entry [1]

5 (a) Nitrification is a two-step aerobic process performed by soil bacteria. First, Nitrosomonas species oxidise ammonium (\(\text{NH}_4^+\)) ions into nitrites (\(\text{NO}_2^-\)). Next, Nitrobacter species further oxidise these nitrites into highly soluble nitrates (\(\text{NO}_3^-\)), which plant roots readily absorb.

(b) Ammonium fertilizers generate hydrogen ions during nitrification, leading to soil acidification. Increased acidity decreases the availability of essential plant macronutrients like phosphorus, and can destroy the populations of beneficial soil microbes and earthworms.

(c) Sandy soils are loose and have poor water retention. Organic manure decomposes into humus, which coats sand particles and acts as a biological glue to form stable aggregates (crumb structure). This structure holds moisture and introduces negative surface charges, allowing the soil to capture and store dissolved nutrient cations.

5 (a) Award 1 mark for each point (max 3):
- Aerobic environment required [1]
- Nitrosomonas bacteria convert ammonium compounds to nitrites [1]
- Nitrobacter bacteria convert nitrites to nitrates [1]
- Plants absorb nitrogen primarily as nitrates [1]

(b) Award 1 mark for each correct reason (max 2):
- Releases hydrogen ions causing soil acidification [1]
- Low pH locks up key nutrients (e.g., phosphorus, calcium) [1]
- Suppresses populations of earthworms and beneficial bacteria [1]
- Accelerates nutrient imbalances [1]

(c) Award 1 mark for each explanation point (max 3):
- Humus binds loose sand particles to build a stable crumb structure [1]
- Increases water retention capacity of porous sandy soils [1]
- Elevates Cation Exchange Capacity (CEC) / reduces nutrient leaching [1]

6 (a) An allele is an alternative sequence or form of a specific gene located at a given locus on a chromosome.

(b)
- Heterozygous sheep: genotype is Ww, phenotype is white wool.
- Homozygous recessive sheep: genotype is ww, phenotype is black wool.

(c) Crossing Ww (ram) with ww (ewe):
Gametes from ram: W and w.
Gametes from ewe: w.

Cross analysis (Punnett Square):
| | W | w |
|---|---|---|
| w | Ww | ww |

This yields genotypes Ww and ww in equal proportion.
Genotypic ratio: 1 Ww : 1 ww (or 50% heterozygous and 50% homozygous recessive).
Phenotypic ratio: 1 White wool : 1 Black wool (or 50% white and 50% black).

6 (a) Award 1 mark for definition:
- Alternative/different version of a gene [1]

(b) Award 0.5 marks for each part (max 2):
- Heterozygous genotype: Ww [0.5]
- Heterozygous phenotype: white wool [0.5]
- Homozygous recessive genotype: ww [0.5]
- Homozygous recessive phenotype: black wool [0.5]

(c) Award up to 5 marks for diagram and ratios:
- Parents identified correctly as Ww and ww [1]
- Correct gametes shown: W, w from ram, and w from ewe [1]
- Punnett square drawn and filled out correctly [1]
- Genotypic ratio given as 1 Ww : 1 ww (or 1:1) [1]
- Phenotypic ratio given as 1 White : 1 Black (or 1:1) [1]

7 (a) Endoparasites reside internally within tissues or organs (e.g., Liver fluke, Hookworms). Ectoparasites inhabit the skin surface or fleece (e.g., Sheep ticks, Blowfly blow, biting lice).

(b) Internal parasites harm host animals by feeding on tissues (such as boring tunnels in the liver), drawing blood from the intestinal walls (which induces chronic anaemia), or consuming digested nutrients in the alimentary canal, leading to wasting and starvation.

(c) Rotational grazing moves livestock through segregated grazing paddocks, resting used fields for several weeks. Larvae hatched from dung are left without sheep hosts, causing them to die of starvation and dry conditions before the herd returns to graze, breaking the transmission loop.

7 (a) Award 1 mark for each point (max 4):
- Endoparasite: lives inside the body of the host [1]
- Example of endoparasite: liver fluke / roundworm / tapeworm [1]
- Ectoparasite: lives on the outer surface of the host [1]
- Example of ectoparasite: tick / mite / louse / blowfly strike [1]

(b) Award 1 mark for each described effect (max 2):
- Damage internal organ structures / tissues (e.g., liver damage) [1]
- Compete for and absorb digested nutrients causing weight loss [1]
- Consume blood causing anaemia / weaknesses [1]
- Physical blockage of internal tubes (bile duct/intestines) [1]

(c) Award 1 mark for each point (max 2):
- Resting pasture prevents larvae from finding a host [1]
- Larvae die off naturally, breaking the life cycle of the parasite [1]

8 (a) Agro-chemicals must be kept inside a dedicated, locked chemical cabinet with appropriate ventilation. They should always be kept in their original containers with intact labels, and must be situated far from feed stores, kitchens, and water storage.

(b) Keeping poisonous fluids in soft drink or water bottles represents a major cause of accidental ingestion, particularly among children or workers who cannot read the markings. Furthermore, non-specialized plastics can degrade or leak when exposed to concentrated chemicals.

(c) To safely handle empty containers, the farmer should perform a triple-rinse and dump the rinse water directly back into the spray tank. Next, puncture or crush the container to prevent any secondary use, and dispose of it at an authorized toxic waste collection site or bury it in a designated pit well away from surface water and domestic wells.

8 (a) Award 1 mark for each precaution (max 3):
- Store in a secure / locked cabinet or room [1]
- Keep out of reach of children / unauthorized personnel [1]
- Keep in original container with visible label [1]
- Store away from human food, animal feeds, and water [1]
- Ensure storage is dry, cool, and ventilated [1]

(b) Award 1 mark for each point (max 2):
- High risk of accidental poisoning / ingestion by mistaking chemical for drink [1]
- Beverage bottles lack children safety caps and standard hazard warnings [1]
- Chemicals can react with and dissolve common drink plastics [1]

(c) Award 1 mark for each step (max 3):
- Triple-rinse container and add rinse water to sprayer [1]
- Puncture/crush the plastic/metal container to render it unusable [1]
- Hand over to licensed pesticide disposal collection / bury safely away from water tables [1]

(a) A dominant allele is always expressed in the phenotype when present (even in heterozygous individuals), whereas a recessive allele is only expressed when the individual is homozygous recessive (no dominant allele is present). (b) - homozygous dominant bull (PP): polled (hornless) - heterozygous bull (Pp): polled (hornless) - homozygous recessive bull (pp): horned (c) Parent Genotypes: Pp x pp. Gametes from bull: P and p; Gametes from cow: p. Punnett square crossing produces: Pp (polled) and pp (horned) in equal numbers. Phenotype Ratio: 1 polled : 1 horned (or 50% polled, 50% horned).

(a) [2 marks] - Dominant allele is always expressed / expressed in heterozygote [1] - Recessive allele is only expressed when dominant allele is absent / homozygous [1] (b) [3 marks] - homozygous dominant: polled / hornless [1] - heterozygous: polled / hornless [1] - homozygous recessive: horned [1] (c) [3 marks] - Correct parental genotypes (Pp x pp) and gametes identified [1] - Correct offspring genotypes (Pp and pp) shown in diagram [1] - Correct phenotype ratio (1 polled : 1 horned / 50% polled : 50% horned) [1]

(a) The rumen is a large fermentation vat where mutualistic microorganisms (bacteria, protozoa, and fungi) break down complex plant fibers like cellulose. These microbes ferment the carbohydrates to produce volatile fatty acids (VFAs), which are absorbed through the rumen wall and serve as the animal's primary energy source. Additionally, the microbes synthesize high-quality microbial protein and essential B-group vitamins, which are later digested in the lower digestive tract. The rumen also acts as temporary storage, allowing the animal to ingest food rapidly and regurgitate it later for further chewing (rumination).

(b) A sheep is a ruminant with a complex, four-chambered stomach, whereas a pig is a monogastric non-ruminant with a simple, single-chambered stomach. In sheep, microbial fermentation occurs in the forestomach (rumen) before food enters the true stomach (abomasum) and small intestine. This allows for highly efficient digestion of cellulose. In contrast, a pig lacks a rumen and cannot digest cellulose in its stomach. Any cellulose digestion in the pig occurs via limited microbial fermentation in the caecum and large intestine, which is far less efficient, meaning pigs cannot utilize high-fiber diets as effectively as sheep.

(c) Three factors determining water requirements are:
1. Environmental temperature: In hot conditions, animals lose water via sweating and panting for thermoregulation, increasing their water needs.
2. Diet composition: Animals fed dry feeds like hay or concentrates require significantly more drinking water than those grazing on lush, moisture-rich pastures.
3. Physiological state: Lactating animals require massive amounts of water to produce milk, and pregnant animals need extra water to support fetal growth.

Part (a) [Max 4 marks]:
- Acts as a fermentation vat / contains microorganisms (bacteria, protozoa, fungi) [1]
- Microorganisms break down cellulose / complex plant cell walls [1]
- Produces volatile fatty acids (VFAs) absorbed as energy [1]
- Synthesizes microbial protein [1]
- Synthesizes B-group vitamins [1]
- Storage for regurgitation / rumination / chewing the cud [1]

Part (b) [Max 5 marks]:
- Sheep is a ruminant, pig is a non-ruminant / monogastric [1]
- Sheep has a four-chambered stomach, pig has a simple stomach [1]
- Sheep performs fermentation in the forestomach / rumen before the true stomach [1]
- Pig lacks a rumen and cannot digest cellulose in its stomach [1]
- Pig has limited fermentation occurring in the caecum / large intestine [1]
- Sheep are much more efficient at extracting energy from fibrous diets than pigs [1]

Part (c) [Max 6 marks]:
- 1 mark for each identified factor (max 3) and 1 mark for each corresponding explanation (max 3):
- Factor: Ambient temperature / climate [1]; Explanation: Higher temperatures lead to increased water loss through panting/sweating, needing replenishment [1].
- Factor: Type of feed / diet dryness [1]; Explanation: Dry feeds require more supplementary water, whereas fresh pasture supplies a high percentage of dietary moisture [1].
- Factor: Production / physiological state (e.g., lactation, pregnancy) [1]; Explanation: Lactation requires large volumes of water for milk production; pregnancy requires fluid for the amniotic sac and fetal growth [1].
- Factor: Animal age / body mass [1]; Explanation: Larger animals or rapidly growing young stock have higher metabolic rates and fluid needs [1].

(a) Sheet erosion is the uniform removal of a thin, even layer of fertile topsoil from a large area of land, often triggered by surface runoff and raindrop splash. It is a slow, gradual process that is difficult to detect initially. Gully erosion, on the other hand, is the removal of soil along concentrated drainage lines, forming deep channels or ravines (gullies) that cut into the subsoil. Gully erosion is highly visible, caused by rapid, high-volume water runoff, and severely restricts the movement of farm machinery.

(b) Three farming practices increasing erosion risk are:
1. Overgrazing: Keeping too many livestock on a pasture removes the protective vegetative cover, exposing bare soil to water and wind. Hoof action also compacts the soil, reducing infiltration and increasing surface runoff.
2. Up-and-down slope cultivation: Ploughing or planting crops parallel to the slope creates continuous channels that guide runoff water down the hill, accelerating its speed and erosive power.
3. Clean tilling / lack of cover crops: Leaving the soil bare between crop seasons directly exposes the topsoil to heavy rainfall impact and wind.

(c) Three conservation methods are:
1. Contour farming: Ploughing and planting across the slope following the natural contour lines. This creates mini-ridges and furrows that act as barriers, slowing down water runoff and encouraging infiltration.
2. Terracing: Constructing step-like flat platforms along the slope. This breaks a long, steep slope into several short, flat sections, reducing the volume and velocity of surface runoff.
3. Strip cropping: Alternating strips of row crops with dense cover crops across the slope. The cover crop strips trap soil particles carried by runoff water and slow down the water flow.

Part (a) [Max 4 marks]:
- Sheet erosion removes soil uniformly in thin layers over a wide area [1]
- Sheet erosion is caused by rain splash / shallow surface runoff [1]
- Gully erosion creates deep channels / ravines in the landscape [1]
- Gully erosion is caused by rapid, concentrated flow of water [1]
- Comparison of visibility / damage (sheet is subtle/gradual, gully is highly visible/prevents machinery use) [1]

Part (b) [Max 5 marks]:
- Max 3 marks for describing practices, max 2 marks for explaining the mechanism leading to erosion:
- Overgrazing / overstocking livestock [1] -> removes vegetative cover / exposes bare soil / compacts soil, increasing runoff [1].
- Cultivating / ploughing up and down a slope [1] -> creates furrows that act as runoff channels, speeding up water flow [1].
- Excessive tillage / over-cultivation [1] -> breaks down soil structure into fine particles that are easily carried by wind/water [1].
- Continuous monoculture without cover crops / leaving soil fallow and bare [1] -> lacks root systems to bind soil and canopy to absorb rain impact [1].
- Burning crop residues / slash-and-burn [1] -> destroys organic matter that aggregates soil particles [1].

Part (c) [Max 6 marks]:
- 1 mark for each method (max 3) and 1 mark for the explanation of how it reduces erosion (max 3):
- Contour ploughing / planting [1]; Explanation: Creates horizontal barriers that slow down water runoff and increase water infiltration [1].
- Terracing [1]; Explanation: Replaces a steep slope with flat steps, reducing the speed and volume of descending water runoff [1].
- Strip cropping [1]; Explanation: Dense crops trap soil eroded from row crops and act as a vegetative filter [1].
- Mulching / cover crops [1]; Explanation: Cover protects soil from physical raindrop impact and improves soil organic matter to bind particles [1].
- Agroforestry / planting windbreaks [1]; Explanation: Tree roots bind the soil firmly, and leaves intercept heavy rain and reduce wind velocity [1].