2023 Cambridge IGCSE Agriculture (0600) 模拟试题及答案详解

The heterozygous white ram has the genotype Bb, while the black ewe has the genotype bb. Crossing Bb with bb yields offspring with genotypes Bb (white wool) and bb (black wool) in equal proportions, resulting in a phenotypic ratio of 1 white wool : 1 black wool.

Award 1 mark for the correct answer C.

Nitrosomonas bacteria are nitrifying bacteria that convert ammonium compounds into nitrites as the first stage of nitrification. Nitrobacter then converts these nitrites into nitrates.

Award 1 mark for the correct answer B.

Concentrates are highly nutritious, energy-dense feeds with low fiber. They are added to the feed during lactation as part of a production ration to provide the extra energy and protein required for high-volume milk synthesis.

Award 1 mark for the correct answer C.

Systemic pesticides are absorbed by the plant and translocated through its vascular tissues (phloem/xylem). Sap-sucking insects ingest the pesticide when they feed on any part of the plant, leading to their death.

Award 1 mark for the correct answer C.

Variable costs are expenses that change in direct proportion to the volume of agricultural production. The cost of hybrid maize seeds depends directly on the quantity of maize planted, making it a variable cost. Machinery purchases, depreciation, and loan interest are fixed costs because they do not change with the output volume in the short term.

Award 1 mark for the correct answer C.

At a low pH of 5.2, essential nutrients like phosphorus become locked or chemically bound to iron and aluminum in the soil, making them unavailable to plants. Applying agricultural lime (calcium carbonate) neutralises the acidic hydrogen ions, increasing pH. This treatment unlocks phosphorus and reduces toxic aluminum ions, which allows for healthy root elongation and nutrient uptake.

(i) 1 mark: Reduces the availability of phosphorus / locks up key nutrients. (ii) 2 marks: 1 mark for identifying agricultural lime / calcium carbonate; 1 mark for explaining that it neutralises acidity to promote root growth / reduce toxic aluminum.

Many soil-borne pests are host-specific, meaning they can only feed and reproduce on specific plant families. By planting a non-host crop in the next season, the pests are deprived of their primary food source. This breaks their reproductive cycle, causing their population in the soil to die out or significantly decrease.

1 mark for stating that it removes the host plant / deprives the pest of its food source. 1 mark for explaining that this breaks the pest's life/reproductive cycle.

(i) Roughage provides dietary fiber, which is essential to stimulate mechanical digestion, rumination (chewing the cud), and healthy microbial fermentation in the rumen. (ii) Milk production is highly energy-intensive. Dry cows only require maintenance energy, but lactating cows need vast amounts of energy and protein. Concentrates are highly digestible and nutrient-dense, allowing cows to meet these high demands without exceeding their physical rumen capacity.

(i) 1 mark for: stimulates rumen activity / rumination / peristalsis / provides fiber. (ii) 2 marks: 1 mark for identifying that lactation has high energy/nutrient demands; 1 mark for explaining that concentrates are nutrient-dense / highly digestible to meet these metabolic demands.

The dense foliage of a cover crop acts as a physical canopy that intercepts falling raindrops, absorbing their kinetic energy and preventing them from detaching soil particles on impact. Additionally, the extensive root system of the crop binds the soil structure together, making it harder to wash away, while the physical plant mass slows down the velocity of surface runoff, encouraging water infiltration rather than erosion.

1 mark for: canopy/leaves intercept raindrops / reduce kinetic impact energy of rain. 1 mark for: roots bind soil particles / slow down surface runoff speed.

(i) Phloem is the tissue responsible for transporting soluble organic compounds. (ii) A source is an organ that synthesises or releases carbohydrates (typically mature photosynthesising leaves). A sink is an organ that imports, utilises, or stores these carbohydrates for growth or reserves (such as growing fruits, seeds, roots, or storage organs like potato tubers).

(i) 1 mark: Phloem. (ii) 2 marks: 1 mark for defining and exemplifying source (e.g. leaf producing sugars); 1 mark for defining and exemplifying sink (e.g. tuber/root/fruit storing/consuming sugars).

(i) Carrying capacity is the maximum number of livestock units that can graze a given area of land for a specific period without causing deterioration of the pasture or soil. (ii) When exceeded (overstocking), livestock selectively eat the most palatable grasses before they can regenerate, causing these desirable species to die out and be replaced by weeds. Furthermore, heavy trampling from too many animals compacts the soil, reducing aeration and water infiltration, which stunts root growth and kills pasture plants.

(i) 1 mark: Maximum livestock a pasture can support sustainably without degradation. (ii) 2 marks: 1 mark for describing loss of desirable/palatable grass species / weed invasion; 1 mark for describing soil compaction reducing root growth / plant survival.

The cross is Bb (heterozygous black) x bb (homozygous recessive red). The gametes from the bull are B and b. The gametes from the cow are all b. The offspring genotypes are Bb (black phenotype) and bb (red phenotype) in a 1:1 ratio. Thus, the expected phenotype ratio of the offspring is 1 black : 1 red (or 50% black and 50% red).

1 mark for correct genetic cross working (showing gametes B, b and b producing Bb and bb genotypes). 1 mark for stating the correct phenotype ratio: 1 black : 1 red (or 1:1).

(i) Using natural predators or insects to control weeds is biological weed control. (ii) One major advantage is specificity; the insect will only attack the targeted weed, leaving other valuable pasture forage plants unharmed. Another advantage is sustainability; once established, the insect population self-regulates and continues to control the weed indefinitely without the recurring costs and labour of chemical sprays. It also avoids toxic residues in the soil, livestock, or local water systems.

(i) 1 mark: Biological control. (ii) 2 marks: Any two of: highly specific to target weed (no damage to other plants); self-sustaining/long-term solution; zero chemical residues/safe for livestock; cost-effective over time.

Cultural control methods disrupt pest life cycles without chemicals: 1. Crop rotation prevents specific pests from building up in the soil. 2. Sanitation (weeding) removes alternative wild hosts where insects multiply. 3. Adjusting planting dates allows the crop to avoid peak pest emergence periods.

Award 1 mark for each valid cultural method described up to a maximum of 3 marks: - Crop rotation / altering crop sequence [1] - Removal of weeds / crop residues / alternative host plants [1] - Changing planting or harvesting dates [1] - Using trap crops / companion planting [1] - Physical barriers / insect netting [1]

Factors causing a decrease in supply include: 1. Higher input costs (e.g. fertilisers, seeds) which reduce profit margins and discourage high-volume planting. 2. Unfavourable weather (e.g. drought or frost) which directly destroys harvest volume. 3. Pests and disease outbreaks which lower total crop yield.

Award 1 mark for each valid factor with explanation up to a maximum of 3 marks: - Rising production costs (e.g., fertilisers/seeds) reducing planting incentives [1] - Adverse weather conditions (e.g., droughts, frost, flooding) destroying crops [1] - Pest or disease epidemics reducing harvestable yield [1] - Shortage of agricultural labour [1]

A maintenance ration only provides enough nutrients to keep the animal alive and at a constant weight. A production ration contains additional concentrated energy (carbohydrates/fats), protein, and specific minerals to support output. Targeted products include milk, eggs, meat, or wool.

Award marks as follows: - 1 mark for explaining the nutritional difference (higher energy, protein, or mineral density in production ration compared to maintenance ration) [1] - 1 mark for each correct targeted product (max 2 marks) [2]. Accept: milk, meat, eggs, wool.

The soil water is dilute and has a higher water potential compared to the highly concentrated cell sap inside the vacuole of the root hair cell (which has lower water potential). Consequently, water moves down the water potential gradient by osmosis across the selectively permeable membrane of the root hair cell.

Award marks as follows: - State that soil water has a higher water potential OR cell sap has lower water potential [1] - Describe the movement by osmosis / across a selectively permeable membrane / down a gradient [1]

Changing to intensive rotational grazing benefits management by: 1. Preventing selective grazing, as animals are forced to eat less palatable plants uniformly. 2. Providing a recovery period for rested paddocks to regrow, improving pasture yield. 3. Helping control internal parasites by moving animals away before parasite eggs hatch.

Award 1 mark for each valid benefit described up to a maximum of 3 marks: - Avoids selective grazing / ensures uniform utilization of pasture [1] - Allows rested paddocks a period of regrowth and recovery [1] - Helps in parasite control / breaks parasite life cycles [1] - Higher carrying capacity / stocking density per unit area [1] - Easier monitoring of animal health and forage availability [1]

Mulch creates a physical barrier over the soil. It absorbs the kinetic energy of falling raindrops, which prevents splash erosion and soil particle detachment. It also physically obstructs water flow across the surface, slowing down surface run-off and allowing more water to infiltrate.

Award marks as follows: - Absorbs raindrop impact / prevents soil detachment / splash erosion [1] - Slows down surface water run-off / increases water infiltration [1]

Purple leaf colouration and poor root development indicate a phosphorus deficiency. This can be corrected by applying synthetic phosphate fertilisers (like superphosphates) and incorporating organic sources high in phosphorus, such as bone meal or farmyard manure.

Award marks as follows: - Identify phosphorus as the deficient nutrient [1] - State one valid corrective soil application (e.g. phosphate fertilisers, superphosphate, bone meal) [1] - State another valid corrective method (e.g. adding organic manure, adjusting soil pH to increase availability) [1]

Proper ventilation is vital because it removes toxic gases (ammonia, carbon dioxide) and excess humidity, reducing the risk of respiratory diseases and heat stress in cattle. It can be achieved naturally by incorporating open-sided walls, ridge vents, or mechanically by using extractor fans.

Award marks as follows: - Explanation: Removes toxic gases (ammonia / carbon dioxide) / moisture OR prevents respiratory infections / heat stress [1] - Method of achievement: Open-sided walls / ridge vents / extractor fans [1]

1. Well-rotted organic matter acts like a sponge, which increases the water-holding capacity of sandy soils that normally drain too quickly.
2. It helps bind loose, individual sand particles together to form stable soil crumbs, improving overall structure.
3. It improves soil aggregation and ground cover potential, which reduces the susceptibility of the dry sandy soil to wind erosion.

Award 1 mark for each valid physical improvement described (up to 3 marks):
- Improves water retention / water-holding capacity [1]
- Helps bind sand particles together / forms stable crumb structure [1]
- Reduces susceptibility to wind erosion / improves soil stability [1]
- Note: Do not accept chemical improvements such as 'adds nutrients/nitrogen' or 'increases soil pH' as the question specifies physical properties.

A maintenance ration is the minimum amount of feed required daily to keep the animal alive, healthy, and at a constant body weight without any productive work or yield. A production ration is the extra feed given above maintenance levels to supply the nutrients specifically needed for milk production (lactation).

Award marks as follows:
- 1 mark for correct definition/explanation of maintenance ration (keeps animal alive/healthy/constant weight) [1]
- 1 mark for correct explanation of production ration (additional feed for milk production/lactation/reproduction) [1]
[Max 2 marks]

1. Crop rotation: Growing a non-host crop in the subsequent season breaks the life cycle of pests specific to maize.
2. Timely planting: Sowing maize early allows the crop to establish and develop before insect pest populations peak.
3. Destruction of crop residues: Ploughing under or burning old maize stalks after harvest destroys overwintering sites or pupae of pests like the maize stalk borer.

Award 1 mark for each valid cultural practice described (up to 3 marks):
- Crop rotation (breaks pest life cycle by planting non-host crops) [1]
- Timely/early planting (allows crop to establish before pest numbers peak) [1]
- Removal/destruction of crop residues/field hygiene (destroys breeding sites/overwintering pests) [1]
- Trap cropping / companion planting (attracts pests away from the main maize crop) [1]

Initially, adding more nitrogen fertilizer leads to a significant increase in crop yield (increasing marginal returns). However, as more fertilizer is added to the fixed land area, a point is reached where each additional unit of fertilizer produces a smaller increase in yield than the previous unit (diminishing marginal returns). Eventually, further additions may cause no increase, or even damage the crop, reducing total yield.

Award marks as follows:
- 1 mark for stating that initial applications cause a large increase in yield [1].
- 1 mark for explaining that after a certain point, each additional unit of fertilizer results in a smaller increase in yield than the previous one [1].
- 1 mark for noting that excessive fertilizer can eventually lead to no further yield gain or a decrease in total yield (negative returns) [1].

As water evaporates from the spongy mesophyll cells and exits through the stomata, it creates a tension or negative pressure (transpiration pull) in the leaves. Because water molecules are cohesive, this pull is transmitted down the xylem vessels, drawing water and dissolved mineral salts up from the roots in a continuous transpiration stream.

Award marks as follows:
- 1 mark for evaporation of water from leaves creating a tension/transpiration pull / negative pressure [1].
- 1 mark for explaining that this tension draws water and dissolved mineral ions up through the xylem vessels from the roots [1].

1. Intercepting rainfall: The leaves and stems of the cover crop absorb the kinetic energy of falling raindrops, preventing them from shattering soil aggregates (splash erosion).
2. Binding soil: The extensive root system of the cover crop binds the soil particles together, making them harder to be washed away.
3. Slowing runoff: The physical barrier of the crop cover slows down the speed of surface runoff water, reducing its capacity to carry soil particles downhill.

Award 1 mark for each correct description of how cover crops prevent erosion (up to 3 marks):
- Canopy intercepts rainfall / reduces force of raindrops on bare soil [1]
- Root system binds soil particles together / increases stability [1]
- Foliage/stems slow down the velocity of surface water runoff [1]
- Increases water infiltration into the soil, reducing runoff volume [1]

The cross is Ww x ww.
Gametes from Ww: W and w
Gametes from ww: w
Offspring genotypes: Ww (heterozygous white) and ww (homozygous black) in equal proportions.
Therefore:
Genotypic ratio = \(1\text{ Ww} : 1\text{ ww}\)
Phenotypic ratio = \(1\text{ White} : 1\text{ Black}\)

Award marks as follows:
- 1 mark for correct phenotypic ratio: \(1\text{ white} : 1\text{ black}\) (or 50% white and 50% black) [1].
- 1 mark for correct genotypic ratio: \(1\text{ Ww} : 1\text{ ww}\) (or 50% Ww and 50% ww) [1].

1. Prevents overgrazing: Animals are confined to one paddock at a time, preventing them from selectively overgrazing and depleting the most palatable grass species.
2. Allows pasture recovery: While animals graze one paddock, other paddocks are rested, allowing the grass time to regrow and restore its root reserves.
3. Reduces parasite buildup: Moving livestock regularly breaks the life cycles of internal parasites, as larvae on rested paddocks die before animals return.

Award 1 mark for each valid advantage suggested (up to 3 marks):
- Prevents overgrazing / selective grazing of favorite species [1]
- Allows rested pasture sections to regrow/recover [1]
- Breaks parasite life cycles / reduces worm burden [1]
- Better utilization of available forage / higher carrying capacity [1]
- Allows for easier pasture maintenance (e.g., fertilizing, weeding) of unoccupied paddocks [1]

(a) In strip grazing, cattle are confined to a small strip, which forces them to eat all the grass rather than selectively grazing. It also prevents them from trampling and fouling the rest of the pasture. (b) A major disadvantage is the high labor requirement because the electric fence must be moved manually every day.

(a) Max 2 marks: [1] for forcing even/non-selective grazing or confining animals to a small area; [1] for preventing trampling or fouling of uneaten grass; [1] for allowing the rested pasture to recover. (b) Max 1 mark: [1] for high labor requirement or daily fence moving; [1] for high cost of temporary fencing equipment; [1] for difficulty providing movable water supply.

(a) Crop rotation helps maintain soil fertility through several mechanisms:
- Growing legumes (e.g., beans, peas) fixes atmospheric nitrogen into the soil via root nodules, increasing nitrogen availability for subsequent nitrogen-hungry crops like maize.
- Different crops have different rooting depths (deep-rooting vs. shallow-rooting), which prevents the depletion of nutrients from a single soil layer and helps recycle nutrients from deeper levels.
- Different crops have different nutrient requirements, preventing the rapid exhaustion of specific minerals from the soil.
- Alternating crops disrupts the life cycles of soil-borne pests and diseases, reducing the need for chemical soil sterilants.

(b) Incorporating green manure improves clay soil:
- Physical properties:
- Organic matter binds fine clay particles together to form stable soil crumbs (improving structure).
- It increases the macro-pore space, which improves aeration and drainage, reducing waterlogging.
- It enhances the soil's workability (tilth), making it less sticky when wet and less hard when dry.
- Chemical properties:
- Decomposing organic matter releases essential plant nutrients (N, P, K) back into the soil as it decays.
- It increases the cation exchange capacity (CEC), allowing the soil to retain nutrients more effectively against leaching.
- It acts as a buffer to stabilize soil pH.

(c) Contour farming and strip cropping on sloping land:
- Contour farming involves ploughing and planting across the slope (along the contour lines) rather than up and down. This creates ridges and furrows that act as mini-barriers, slowing down runoff water and allowing more time for water infiltration, thus reducing soil wash.
- Strip cropping involves growing alternating strips of different crops (e.g., a row crop like maize alternated with a close-growing cover crop like grass or clover) across the slope. The dense cover crop traps soil particles washed from the row crop strip and slows down surface runoff, effectively filtering sediment and preventing gullies from forming.

**Part (a) [4 marks]**
Award 1 mark for each valid point up to 4:
- Legumes fix atmospheric nitrogen, replenishing soil N levels. [1]
- Varying root depths extract nutrients from different soil zones / recycles deep nutrients. [1]
- Different nutrient demands prevent selective depletion of specific minerals. [1]
- Pest/disease cycles are broken, preserving soil health / structure. [1]
- Cover crops in rotation protect soil from direct rain impact and leaching. [1]

**Part (b) [5 marks]**
Award up to 3 marks for physical improvements and up to 2 marks for chemical improvements:
*Physical (Max 3):*
- Organic matter binds clay particles to form soil crumbs/aggregates. [1]
- Improves soil structure/aeration / increases macro-pore space. [1]
- Enhances drainage / reduces waterlogging. [1]
- Improves ease of tillage / tilth / makes soil less sticky. [1]
*Chemical (Max 2):*
- Adds/releases essential nutrients (N, P, K) upon decomposition. [1]
- Increases Cation Exchange Capacity (CEC) / nutrient-holding capacity. [1]
- Buffers/stabilizes soil pH. [1]

**Part (c) [6 marks]**
Award up to 3 marks for contour farming and up to 3 marks for strip cropping:
*Contour farming:*
- Ploughing/planting across the slope / along contour lines. [1]
- Creates physical barriers (ridges/furrows). [1]
- Slows down surface water runoff / increases water infiltration / prevents soil wash. [1]
*Strip cropping:*
- Alternating strips of cultivated crops with dense ground cover crops across the slope. [1]
- Dense grass/cover strips act as a filter to catch suspended soil particles. [1]
- Reduces velocity of wind/water runoff across the slope. [1]

(a) Difference between roughage and concentrate:
- Roughage: High in fiber (bulk), low in digestible nutrients/energy. Examples include hay, silage, straw, or fresh pasture grass.
- Concentrate: Low in fiber, high in digestible nutrients, energy, and protein. Examples include maize meal, soybean meal, groundnut cake, or commercial dairy meal.

(b) Paddock grazing and prevention of overgrazing:
- Paddock grazing involves dividing a large pasture into several smaller, fenced sections called paddocks.
- Animals are placed in one paddock to graze for a short, controlled period (e.g., a few days) before being moved (rotated) to the next paddock.
- This prevents overgrazing because:
- It restricts the animals from continuously eating the selective, palatable species, forcing even grazing.
- The grazed paddocks are left empty (rested), which gives the grass adequate time to regrow, photosynthesize, and replenish root reserves before animals return.
- It prevents soil compaction and physical destruction of plant crowns from constant trampling.

(c) Changing nutritional requirements of a breeding female mammal:
- Early pregnancy: Requirements are relatively low, close to maintenance levels, as the fetus is small and growing slowly. Only a small amount of extra energy and minerals is needed.
- Late pregnancy: Rapid fetal growth occurs. The female requires a significant increase in energy, high-quality protein, and minerals (especially calcium and phosphorus) to support skeletal development of the fetus and prepare the mammary glands for colostrum production.
- Lactation (Peak): This is the most demanding phase nutritionally. Huge amounts of energy, protein, water, and minerals (especially calcium) are required daily to synthesize milk. If requirements are not met, the female will mobilize her own body tissues, leading to severe weight loss and metabolic diseases like milk fever.

**Part (a) [4 marks]**
- Definition of roughage (high fiber, low energy/nutrients). [1]
- Definition of concentrate (low fiber, high energy/protein/digestibility). [1]
- Correct example of roughage (e.g., hay, straw, fresh grass). [1]
- Correct example of concentrate (e.g., barley, maize meal, oilseed cake). [1]

**Part (b) [5 marks]**
Award 1 mark for description of paddock grazing and up to 4 marks for explaining how it prevents overgrazing:
*Description:*
- Dividing pasture into smaller fenced plots (paddocks) and rotating animals sequentially. [1]
*How it prevents overgrazing:*
- Limits the duration of grazing in a single area, preventing animals from grazing the grass down to root level. [1]
- Provides a rest period for grazed paddocks to recover and regrow. [1]
- Encourages uniform grazing rather than selective grazing of preferred species. [1]
- Reduces trampling and soil compaction, protecting plant root health. [1]
- Disrupts parasite lifecycles as a secondary benefit to plant recovery. [1]

**Part (c) [6 marks]**
Award up to 2 marks for each stage of the breeding cycle:
*Early pregnancy:*
- Nutrient needs are low/near maintenance levels. [1]
- Excessive feeding should be avoided to prevent obesity, which causes birth difficulties. [1]
*Late pregnancy:*
- Fetus grows rapidly, requiring high-energy and high-protein intake. [1]
- Minerals like calcium and phosphorus are critical for fetal bone development. [1]
*Peak lactation:*
- Highest nutritional demand for milk synthesis (very high energy, protein, and water). [1]
- Calcium demand peaks; deficiency leads to milk fever/metabolic disorders. [1]