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HSC Agriculture practice questions

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Question 1Plant production

Which soil property most directly determines the amount of plant-available water a soil can hold?

  1. Soil surface crusting and seedling emergence
  2. Soil slope and its influence on runoff
  3. Soil texture and its pore-size distribution Correct answer.
  4. Soil colour and its effect on warming

Explanation

Available water is held in micropores, and the proportion of micropores depends chiefly on particle-size distribution, so texture (and structure built from it) governs water-holding capacity. Colour, slope and crusting affect temperature, drainage and emergence rather than the volume of water retained between field capacity and wilting point.

Question 2Plant production

A legume pasture is included in a crop rotation primarily because it:

  1. Breaks down soil organic matter faster
  2. Supplies nitrogen through symbiotic rhizobia Correct answer.
  3. Removes the need for phosphorus fertiliser
  4. Prevents all root diseases in the following crop

Explanation

Legumes form nodules with rhizobia that fix atmospheric nitrogen, which is released to later crops when the pasture residues break down. The rotation benefit is nitrogen supply plus disease and weed break effects, not removal of other nutrient requirements or complete disease control.

Question 3Plant production

In a photosynthesis experiment, a leaf is partly covered with foil and exposed to light. Iodine solution tests for starch. What is the expected result?

  1. Only the covered area stains blue-black
  2. No part of the leaf stains at all
  3. The whole leaf stains evenly blue-black
  4. Only the uncovered area stains blue-black Correct answer.

Explanation

Starch forms only where light reaches the leaf and photosynthesis can proceed, so the exposed tissue accumulates starch and stains blue-black with iodine. The covered area cannot photosynthesise and so does not accumulate starch, while the whole-leaf and no-stain outcomes ignore the light treatment.

Question 4Plant production

Which practice best reduces wind erosion on a bare, sandy paddock?

  1. Maintaining surface cover and roughness Correct answer.
  2. Irrigating heavily to saturate the surface
  3. Deep ploughing to bury the topsoil
  4. Removing all stubble before sowing

Explanation

Wind erosion occurs when wind at the surface exceeds the threshold needed to move loose particles; cover and surface roughness raise that threshold and trap moving grains. Burying topsoil, saturating the surface and removing stubble either expose fine material or destroy the protective layer.

Question 5Plant production

A seed lot has a germination percentage of 80% and a purity of 95%. What is its approximate pure live seed percentage?

  1. 80%
  2. 95%
  3. 88%
  4. 76% Correct answer.

Explanation

Pure live seed is calculated by multiplying purity by germination: 0.95 x 0.80 = 0.76, or 76%. This figure allows sowing rates to be adjusted so the desired number of viable seeds is established.

Question 6Plant production

Transpiration is best described as:

  1. Movement of sugars through the phloem
  2. Uptake of water by roots from the soil solution
  3. Loss of water vapour from plant surfaces, mainly through stomata Correct answer.
  4. Absorption of carbon dioxide through root hairs

Explanation

Transpiration is the evaporation of water from leaf and stem surfaces, mostly through stomatal pores, and it drives the upward pull of water in the xylem. Root uptake supplies the water but is a separate process, and phloem transport and root gas exchange are unrelated functions.

Question 7Plant production

Which macronutrient is most closely associated with root development, flowering and fruit set in crop plants?

  1. Sodium
  2. Molybdenum
  3. Phosphorus Correct answer.
  4. Calcium

Explanation

Phosphorus is involved in energy transfer and is strongly linked to early root growth, flowering and seed and fruit development. Calcium supports cell walls, molybdenum is a micronutrient used in nitrogen metabolism, and sodium is not an essential macronutrient for most crops.

Question 8Plant production

Which method of irrigation is generally the most efficient in delivering water to the root zone per unit applied?

  1. Subsurface drip irrigation Correct answer.
  2. Wild flooding of the whole paddock
  3. Overhead sprinkler irrigation
  4. Flood irrigation of furrows

Explanation

Subsurface drip places water directly in the root zone at low flow rates, minimising evaporation, runoff and deep drainage, so a high proportion of applied water is used by the crop. Flood and sprinkler methods lose more water to evaporation, runoff and uneven distribution.

Question 9Plant production

A wheat crop is sown at 100 plants per square metre and each plant produces on average 2.5 heads, with 30 grains per head. What is the approximate grain number per square metre?

  1. 7500 Correct answer.
  2. 3000
  3. 750
  4. 2500

Explanation

Grain number per square metre equals plant density multiplied by heads per plant and grains per head: 100 x 2.5 x 30 = 7500. This yield-component calculation shows how density, tillering and grain set combine to set the potential sink size.

Question 10Plant production

A soil test reports a pH of 4.8 for a paddock intended for a pH-sensitive crop. Which action is most appropriate?

  1. Apply gypsum to increase the pH
  2. Apply elemental sulfur to raise the pH
  3. Apply urea to lower the pH
  4. Apply agricultural lime and re-test before sowing Correct answer.

Explanation

A pH of 4.8 is strongly acidic, and lime (calcium carbonate) neutralises acidity and raises pH towards the target range for sensitive crops. Gypsum supplies calcium and sulfur without raising pH, elemental sulfur acidifies, and urea has only a small acidifying effect and cannot correct the problem.

Question 11Plant production

Which combination best describes the conditions that favour the development of waterlogging damage in a clay soil?

  1. Irrigation scheduled by soil moisture monitoring
  2. Heavy rainfall with poor internal drainage and a shallow water table Correct answer.
  3. Regular cultivation with high organic matter and good structure
  4. Light rainfall with deep sandy profiles and high evaporation

Explanation

Waterlogging develops when water entering the profile exceeds the rate at which it drains away, which is typical of heavy rainfall on slowly permeable clay with a shallow water table. In such conditions roots become oxygen-deficient and toxic reduced compounds accumulate.

Question 12Plant production

In a plant breeding program, a breeder crosses two pure lines and then selects within the segregating generations. Which statement best explains why selection is usually delayed until later generations?

  1. Later generations have higher mutation rates and greater variability
  2. Later generations always show higher yield than earlier ones
  3. Later generations contain more heterozygous individuals than earlier ones
  4. Later generations contain more homozygous individuals for desirable traits Correct answer.

Explanation

Self-pollination through successive generations increases homozygosity, so desirable traits become fixed and can be reliably selected and multiplied. Early generations are highly heterozygous, meaning selection there is less effective because traits are not yet stable.

Question 13Plant production

Which statement correctly distinguishes respiration from photosynthesis in a crop plant?

  1. Respiration produces oxygen and consumes carbon dioxide
  2. Respiration releases energy from organic compounds and occurs in all living cells Correct answer.
  3. Respiration requires light and occurs only in green tissue
  4. Respiration stores energy in glucose during daylight only

Explanation

Respiration breaks down organic compounds to release energy for growth and maintenance, and it proceeds in every living cell, day and night. Photosynthesis is the light-dependent process that builds glucose and releases oxygen, so the other options describe photosynthesis rather than respiration.

Question 14Plant production

A grower wants to reduce nitrate leaching from a fertilised paddock. Which practice is most effective?

  1. Splitting nitrogen applications to match crop demand Correct answer.
  2. Leaving the paddock bare between crops
  3. Applying all nitrogen in a single large dose before rain
  4. Using a highly soluble nitrate fertiliser at sowing

Explanation

Leaching risk depends on how much nitrate is present in the soil solution when drainage occurs, so matching nitrogen supply to crop uptake in split applications minimises surplus nitrate. Single large doses and highly soluble nitrate forms leave excess nitrate vulnerable to moving below the root zone.

Question 15Plant production

An insect pest is controlled by a predator that is active only within a narrow temperature range. Which monitoring approach best supports integrated management of the pest?

  1. Regular field scouting combined with temperature records for both species Correct answer.
  2. Spraying a broad-spectrum insecticide at fixed calendar intervals
  3. Counting pest numbers only once at the end of the season
  4. Relying on the pest's presence in neighbouring districts

Explanation

Integrated management depends on knowing whether pest numbers are approaching an economic threshold and whether natural enemies are active, so scouting linked to temperature data allows timely, targeted intervention. Calendar spraying ignores thresholds, single late counts give no decision point, and district reports do not describe the local field.

Question 16Plant production

Which practice best maintains soil organic carbon in a continuously cropped paddock?

  1. Retaining stubble and including a pasture phase Correct answer.
  2. Removing all stubble for hay each season
  3. Continuous clean cultivation with no residue return
  4. Burning all crop residues after harvest

Explanation

Soil organic carbon reflects the balance between carbon inputs from residues and roots and losses through decomposition and erosion. Retaining stubble and including a pasture phase increases inputs and reduces disturbance, whereas removal, burning and cultivation all deplete carbon.

Question 17Plant production

A pasture seed mix is being chosen for a paddock that is periodically waterlogged in winter and dries out in summer. Which selection criterion is most important?

  1. Maximum growth rate under ideal, well-watered conditions
  2. Lowest seed cost per kilogram of seed purchased
  3. Highest possible seed germination percentage in the laboratory
  4. Adaptation of species to both waterlogging and summer moisture stress Correct answer.

Explanation

Species persistence depends on surviving the limiting conditions of the site, so a mix must be adapted to both winter waterlogging and summer drought. Ideal-condition performance, laboratory germination and cheap seed do not predict survival in the target environment and may lead to pasture failure.

Question 18Plant production

Which explanation best accounts for the yield benefit of sowing a crop at the optimum time rather than later in the season?

  1. Earlier sowing guarantees freedom from insect pests
  2. Later sowing always increases the risk of frost damage at flowering
  3. Later sowing increases the number of grains set per head
  4. The crop can intercept more solar radiation during the period when temperature and moisture suit growth Correct answer.

Explanation

Yield is driven by the amount of radiation intercepted and converted to dry matter, so sowing at the optimum time aligns the crop's growth with favourable temperature and moisture and a longer growing period. Later sowing shortens the vegetative phase and reduces intercepted radiation, while pest and frost risks are not guaranteed outcomes.

Question 19Plant production

Which statement correctly describes the role of stomata in regulating water loss?

  1. Guard cells close the pore when light intensity increases
  2. Guard cells change shape to open or close the pore in response to water status and light Correct answer.
  3. Stomata are found only on root surfaces where they absorb water
  4. Stomata remain permanently open and cannot regulate water loss

Explanation

Guard cells take up water and become turgid to open the stomatal pore and lose turgor to close it, allowing the plant to balance carbon dioxide uptake against water loss. This regulation responds to light, carbon dioxide and plant water status, and stomata occur mainly on leaves and stems.

Question 20Plant production

A soil has a bulk density of 1.5 g/cm3 and a particle density of 2.5 g/cm3. What is its approximate total porosity?

  1. 60%
  2. 67%
  3. 40% Correct answer.
  4. 25%

Explanation

Total porosity equals one minus the ratio of bulk density to particle density: 1 - (1.5/2.5) = 0.40, or 40%. Porosity indicates the pore space available for air and water, and compaction raises bulk density and lowers this value.

Question 21Plant production

In a crop rotation, a disease is caused by a pathogen that survives in soil for only one season and infects one crop family. Which rotation design gives the greatest reduction in disease carry-over?

  1. A single-season break with a different cultivar of the same crop
  2. Continuous cropping of the susceptible crop with fungicide seed treatment
  3. A break of at least two seasons with non-host crops before returning to the susceptible crop Correct answer.
  4. Alternating the susceptible crop with a related crop in the same family

Explanation

The pathogen's survival is limited to one season, so a break of at least two seasons with non-host crops ensures the inoculum has declined before a susceptible host returns. Same-family crops and different cultivars of the same crop still host the pathogen and allow it to multiply.

Question 22Plant production

A grower applies 150 kg/ha of a fertiliser containing 20% nitrogen, 10% phosphorus and 0% potassium. How much phosphorus is applied per hectare?

  1. 15 kg/ha Correct answer.
  2. 20 kg/ha
  3. 10 kg/ha
  4. 30 kg/ha

Explanation

The phosphorus applied equals the product rate multiplied by the nutrient concentration: 150 x 0.10 = 15 kg/ha. The nitrogen applied would be 150 x 0.20 = 30 kg/ha, which is a different nutrient and must not be confused with the phosphorus figure.

Question 23Plant production

A field trial compares two wheat varieties across four replicated blocks, and variety A out-yields variety B in every block. Which conclusion is most defensible?

  1. The yield difference is due to random chance because only four blocks were used
  2. Variety A will always out-yield variety B in all environments
  3. The result cannot be interpreted without measuring soil nitrogen
  4. The yield difference is consistent across the blocks and is likely due to variety Correct answer.

Explanation

When one variety exceeds the other in every replicate, the difference is consistent and unlikely to be explained by random variation within the trial. However, the result applies to the conditions tested, so it cannot be generalised to all environments without further testing.

Question 24Plant production

A plant shows interveinal chlorosis on young leaves while older leaves remain green. Which nutrient deficiency is most consistent with this pattern?

  1. Iron Correct answer.
  2. Nitrogen
  3. Phosphorus
  4. Potassium

Explanation

Iron is poorly mobile within the plant, so deficiency appears first in young leaves as interveinal chlorosis while veins stay green. Nitrogen, potassium and phosphorus are mobile, so their deficiency symptoms typically appear first on older leaves.

Question 25Plant production

Which reasoning best explains why a legume-based pasture can increase the yield of a following cereal crop even when soil nitrogen tests show adequate levels?

  1. The pasture removes all weeds so the cereal has no competition
  2. The pasture reduces the cereal's requirement for phosphorus
  3. The pasture improves soil structure and breaks disease cycles in addition to supplying nitrogen Correct answer.
  4. The pasture increases soil pH to a level that suits the cereal

Explanation

Rotation benefits are not limited to nitrogen; a well-managed pasture improves aggregation and infiltration and interrupts the build-up of soil-borne pathogens and weeds. Weed removal, pH change and reduced phosphorus demand are not general consequences of a legume pasture phase.

Question 26Plant production

Which term describes the process by which a seed absorbs water and resumes metabolic activity before the radicle emerges?

  1. Vernalisation
  2. Imbibition Correct answer.
  3. Photosynthate partitioning
  4. Senescence

Explanation

Imbibition is the initial uptake of water by dry seed tissues, driven by the low water potential of proteins and carbohydrates in the seed. It triggers enzyme activation and respiration that precede radicle emergence. Vernalisation is a cold treatment affecting flowering, not germination.

Question 27Plant production

Which plant hormone is chiefly responsible for promoting cell elongation in shoots and is produced mainly in the growing apices?

  1. Cytokinin
  2. Auxin Correct answer.
  3. Abscisic acid
  4. Ethylene

Explanation

Auxin, principally indole-3-acetic acid, is synthesised in shoot and root apices and drives cell wall loosening and elongation. It also mediates phototropism and apical dominance. The other hormones named regulate dormancy, ripening and cell division respectively.

Question 28Plant production

Which soil structure type generally provides the best combination of drainage, aeration and root penetration for most crops?

  1. Single-grained sand
  2. Layered platy structure
  3. Granular crumb structure Correct answer.
  4. Massive blocky clay with no visible aggregation

Explanation

A granular or crumb structure consists of small, stable aggregates with pores of many sizes, allowing both drainage and water retention while permitting roots to penetrate. It is typical of well-managed topsoil. Massive, single-grained and platy structures give poor pore continuity.

Question 29Plant production

Which group of organisms is most directly responsible for converting ammonium to nitrate in the soil?

  1. Rhizobia in root nodules
  2. Denitrifying bacteria
  3. Mycorrhizal fungi
  4. Nitrifying bacteria Correct answer.

Explanation

Nitrification is a two-step microbial process in which ammonia-oxidising organisms convert ammonium to nitrite and nitrite-oxidising bacteria convert it to nitrate. It is carried out by specialised soil bacteria, not by nodule bacteria or fungi. Denitrification removes nitrate rather than forming it.

Question 30Plant production

Which process in a crop plant converts light energy into chemical energy stored in carbohydrate?

  1. Transpiration
  2. Respiration
  3. Photosynthesis Correct answer.
  4. Translocation

Explanation

Photosynthesis uses light energy in chloroplasts to fix carbon dioxide into sugars, converting radiant energy into chemical energy. Translocation moves the products, transpiration loses water vapour, and respiration releases energy from carbohydrate rather than storing it.

Question 31Plant production

Which practice is most appropriate for controlling a wind erosion risk on a bare, sandy paddock over summer?

  1. Deep ploughing to bring moist subsoil to the surface
  2. Maintaining surface cover with stubble or a cover crop Correct answer.
  3. Increasing the stocking rate to compact the surface
  4. Removing all crop residue to leave a smooth surface

Explanation

Wind erosion occurs when the wind speed at the soil surface exceeds the threshold for particle detachment. Keeping stubble, mulch or a cover crop reduces wind velocity at the surface and binds particles, so it is the standard control measure. Exposing or smoothing the surface increases risk.

Question 32Plant production

A grower wants to propagate a uniform cultivar of a perennial fruit crop that must remain genetically identical to the parent. Which method is most suitable?

  1. Hybrid seed production
  2. Vegetative propagation by cuttings Correct answer.
  3. Random cross-pollination
  4. Open-pollinated seed collection

Explanation

Vegetative propagation uses somatic tissue, so offspring are clones of the parent and retain the desired genotype. Seed production involves recombination and segregation, which produces variable progeny. Uniformity is therefore achieved asexually.

Question 33Plant production

Which term describes the removal of a shoot tip to encourage lateral branching in a nursery plant?

  1. Pinching Correct answer.
  2. Stratification
  3. Scarification
  4. Girdling

Explanation

Pinching removes the apical meristem, breaking apical dominance so that lateral buds grow and the plant becomes bushier. Scarification and stratification are seed pre-treatments, and girdling removes a ring of bark to interrupt phloem transport.

Question 34Plant production

A paddock has a long-term average growing-season rainfall of 500 mm and a stored soil water reserve of 80 mm. Assuming a crop water use of 400 mm and ignoring losses, which estimate of available water is most reasonable?

  1. 500 mm
  2. 80 mm
  3. 400 mm
  4. 580 mm Correct answer.

Explanation

Available water for the crop is the sum of growing-season rainfall and stored soil water that the roots can reach, giving 500 + 80 = 580 mm. The crop water use figure describes demand, not supply. Losses such as runoff and evaporation would reduce the effective figure, but ignoring them the supply estimate is 580 mm.

Question 35Plant production

A soil test shows a paddock has a high phosphorus buffer index, meaning much applied phosphorus is rapidly fixed. Which fertiliser strategy is most appropriate?

  1. Apply a large single application of soluble phosphorus at sowing
  2. Apply phosphorus in bands near the seed rather than broadcasting Correct answer.
  3. Apply phosphorus only as surface broadcast at the end of the season
  4. Withhold phosphorus until deficiency symptoms appear

Explanation

In strongly fixing soils, concentrating phosphorus in bands reduces the volume of soil reacting with the fertiliser, so less is fixed and more remains available to roots. Banding also places phosphorus where root density is highest. Large broadcast applications expose more phosphorus to fixation.

Question 36Plant production

A grower plants a cultivar that is resistant to a foliar rust, but after several seasons the disease becomes severe in that cultivar. Which explanation best accounts for this outcome?

  1. The pathogen population shifted towards virulence that overcomes the resistance Correct answer.
  2. The fungicide used previously lost its residual activity
  3. The cultivar reverted genetically to a susceptible form
  4. The soil became deficient in a nutrient required for resistance

Explanation

Pathogen populations are genetically variable, and resistant cultivars impose selection that favours virulence variants able to infect them. Over repeated exposure these variants increase in frequency and the resistance appears to fail. This is a population-level change in the pathogen, not a change in the plant.

Question 37Plant production

Which measurement is the most direct indicator of the proportion of pore space filled with water in a field soil sample?

  1. Volumetric water content Correct answer.
  2. Electrical conductivity
  3. Bulk density
  4. Gravimetric water content

Explanation

Volumetric water content expresses the volume of water as a fraction of total soil volume, so it directly reflects the fraction of pore space that is water-filled when compared with porosity. Gravimetric content is a mass ratio and requires bulk density to convert to a volume basis. Bulk density and conductivity measure other properties.

Question 38Plant production

A pasture paddock is being spelled after heavy grazing. Which outcome best explains why spelling benefits subsequent pasture production?

  1. It allows carbohydrate reserves in roots and stubble to recharge and tillers to regrow Correct answer.
  2. It permanently reduces the pasture's water requirement
  3. It raises soil pH through increased root exudation
  4. It eliminates all soil-borne pathogens from the paddock

Explanation

Grazing removes leaf area and draws on stored non-structural carbohydrates. Spelling allows photosynthesis to exceed consumption so reserves rebuild and new tillers establish, improving persistence and later growth. It does not sterilise soil or change pH.

Question 39Plant production

A crop is irrigated with water having an electrical conductivity of 2.5 dS/m and the soil drains poorly. Which management response is most appropriate?

  1. Reduce irrigation to prevent any further salt input
  2. Apply a leaching fraction with adequate drainage and monitor soil salinity Correct answer.
  3. Apply gypsum regardless of the soil's sodicity status
  4. Increase irrigation frequency to dilute salts in the root zone

Explanation

Salts accumulate in the root zone when evaporation exceeds leaching, so the standard response is to apply extra water to leach salts below the roots, which requires adequate drainage. Monitoring confirms whether salinity is being held below crop thresholds. Simply changing frequency or withholding water does not remove salts.

Question 40Plant production

In a crop canopy, which change would most increase the proportion of incident light intercepted during early vegetative growth?

  1. Higher plant density with a more even spatial arrangement Correct answer.
  2. Sowing a later-maturing cultivar at the same density
  3. Wider row spacing with the same plant density
  4. Reducing leaf area by defoliation

Explanation

Light interception depends on the leaf area index and how completely the canopy covers the ground. Increasing density and distributing plants evenly closes the canopy sooner and reduces gaps that transmit light to the soil. Wider rows and defoliation increase those gaps.

Question 41Plant production

Which statement best explains why a crop grown after a long fallow can show a nitrogen response even without recent nitrogen fertiliser?

  1. Mineralisation of soil organic matter during the fallow accumulates nitrate in the profile Correct answer.
  2. Fallow prevents all leaching, so applied nitrogen from previous years is conserved
  3. Fallowing fixes atmospheric nitrogen through free-living bacteria at a rate matching crop demand
  4. Fallow increases soil pH, which converts phosphorus into available nitrogen

Explanation

During a fallow, microbial mineralisation of organic matter releases ammonium that nitrifies to nitrate, and without a crop to take it up, nitrate accumulates in the soil profile. The following crop can then use this stored nitrogen. Fallow does not fix nitrogen or convert phosphorus to nitrogen.

Question 42Plant production

A grower notices that a windbreak reduces yield loss in adjacent crops during hot, dry winds. Which mechanism best explains this effect?

  1. The windbreak increases rainfall over the protected area
  2. The windbreak reduces wind speed, lowering evapotranspiration and mechanical damage Correct answer.
  3. The windbreak shades the crop completely for most of the day
  4. The windbreak raises soil temperature in the protected zone

Explanation

Windbreaks reduce wind speed in a sheltered zone, which lowers the aerodynamic component of evapotranspiration and reduces physical damage such as lodging and abrasion. The effect depends on porosity and distance from the barrier. Windbreaks do not create rainfall or fully shade the crop.

Question 43Plant production

Which factor is the most important reason to test seed for germination percentage before sowing a field crop?

  1. It allows the sowing rate to be adjusted to achieve the target plant population Correct answer.
  2. It identifies the cultivar purity of the seed lot
  3. It measures the seed's resistance to storage fungi
  4. It determines the market price of the harvested grain

Explanation

Germination percentage indicates how many viable seeds will produce seedlings, so the sowing rate can be corrected to reach the target plant density. Cultivar purity and disease status require separate tests. Grain price is unrelated to the germination test.

Question 44Plant production

A soil has a cation exchange capacity of 20 cmol(+)/kg and is dominated by calcium and magnesium. Which consequence is most likely compared with a soil of similar texture dominated by sodium?

  1. The calcium-dominated soil will disperse more readily on wetting
  2. The calcium-dominated soil will have a lower pH than the sodium-dominated soil
  3. The calcium-dominated soil will have more stable aggregates and better infiltration Correct answer.
  4. The calcium-dominated soil will hold less water at field capacity

Explanation

Calcium and magnesium flocculate clay particles, producing stable aggregates and good pore structure, whereas sodium causes dispersion and surface sealing. Better aggregation improves infiltration and drainage. Water holding at field capacity depends mainly on texture and organic matter.

Question 45Plant production

In an integrated pest management program for a caterpillar pest in a vegetable crop, which sequence best reflects the principle of IPM?

  1. Monitor pest and beneficial populations, use thresholds, and combine cultural, biological and selective chemical tactics Correct answer.
  2. Apply the most persistent insecticide available at the first sign of any insect
  3. Apply a broad-spectrum insecticide on a calendar schedule regardless of pest numbers
  4. Stop all control measures and rely on natural enemies alone

Explanation

IPM integrates monitoring, economic thresholds and multiple control tactics, with chemicals used selectively and only when justified. This preserves natural enemies and reduces selection for resistance. Calendar spraying and reliance on a single tactic are contrary to the approach.

Question 46Plant production

A cereal crop shows poor establishment with seedlings that are stunted, have brown lesions on the roots and fail to respond to nitrogen. Which diagnosis is most consistent with these symptoms?

  1. Root disease caused by a soil-borne pathogen Correct answer.
  2. Nitrogen deficiency alone
  3. Phosphorus toxicity
  4. Herbicide drift from a neighbouring paddock

Explanation

Root lesions combined with stunting and failure to respond to fertiliser point to a root disease impairing water and nutrient uptake. A pure nutrient deficiency would usually respond to fertiliser and would not produce discrete root lesions. Herbicide drift and phosphorus toxicity have different symptom patterns.

Question 47Plant production

A grower is choosing between two cultivars with the same yield potential, one with a shorter growing season and one with a longer one, for a region with a short reliable growing season and a high frost risk at maturity. Which choice is better supported?

  1. The longer-season cultivar, because it can use more of the available radiation
  2. The shorter-season cultivar, because it is more likely to mature before the frost risk Correct answer.
  3. The longer-season cultivar, because it will use more stored soil water
  4. Either cultivar, because yield potential is the only relevant criterion

Explanation

In a short growing season with a frost risk at maturity, the key risk is failing to fill grain before frost. A shorter-season cultivar is more likely to reach maturity within the safe window, so it is the better-supported choice even with equal yield potential. Longer-season types expose the crop to frost and water stress.

Question 48Plant production

Which combination of practices is most likely to reduce the development of herbicide resistance in a weed population?

  1. Spraying only when resistant weeds are already widespread
  2. Applying the same herbicide at higher rates more frequently
  3. Using the same herbicide group repeatedly at reduced rates
  4. Rotating herbicide groups and integrating non-chemical weed control Correct answer.

Explanation

Resistance develops when the same mode of action repeatedly selects survivors. Rotating herbicide groups, using full label rates and integrating cultivation, grazing or competition reduces selection pressure and removes survivors by other means. Repeated or reduced-rate use of one group accelerates resistance.

Question 49Plant production

A trial measures the response of a crop to four nitrogen rates and the results show a strong linear increase up to the highest rate tested. Which conclusion is best supported by this evidence?

  1. The highest rate is necessarily the most profitable
  2. The response curve is still rising within the tested range, so the optimum may lie beyond the highest rate Correct answer.
  3. The crop is not responsive to nitrogen
  4. The economic optimum nitrogen rate has been identified

Explanation

A linear response across the tested range means the plateau or economic optimum has not yet been reached, so rates beyond the highest tested should be examined. The most profitable rate also depends on grain price and fertiliser cost, not yield alone. A flat response would indicate no nitrogen response.

Question 50Plant production

In a crop rotation, a legume is included before a cereal. Which combination of mechanisms best explains the rotational benefit when the legume is terminated and the cereal sown?

  1. Reduced water use by the cereal because the legume leaves a dry profile
  2. Increased soil phosphorus availability from legume root exudates only
  3. Nitrogen contribution from fixed nitrogen and a break in cereal disease and weed cycles Correct answer.
  4. Increased soil pH and reduced bulk density from legume roots

Explanation

A legume in rotation contributes nitrogen through fixation and residues, and as a different species it interrupts the life cycles of cereal-specific pathogens and weeds. These two mechanisms together explain much of the rotational benefit. Soil pH and bulk density changes are minor, and a dry profile would be a disadvantage.