GCE A/L Agricultural Science flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

C4 photosynthesis advantage

C4 plants such as maize, sorghum and sugarcane use PEP carboxylase in mesophyll cells and decarboxylation in bundle-sheath cells, minimising photorespiration and raising water-use efficiency in hot, bright conditions.

Leaf area index (LAI)

LAI is total leaf area per unit ground area. Crop light interception increases with LAI until self-shading sets in, so cereals often perform best around LAI 4-6.

Net assimilation rate (NAR)

NAR is crop growth rate divided by leaf area index, expressing dry matter produced per unit leaf area per unit time. It indicates photosynthetic efficiency of the canopy.

Photoperiodism in crops

Flowering response to night length. Short-day crops such as rice and soybean flower when night exceeds a critical length; long-day crops such as wheat flower when night is shorter; day-neutral crops are insensitive.

Cation exchange capacity (CEC)

CEC is the quantity of exchangeable cations a soil can hold, expressed in milliequivalents per 100 g. Clay and organic matter raise CEC, improving nutrient retention and buffering.

Soil bulk density

Bulk density is oven-dry soil mass divided by total soil volume, including pore space. High bulk density indicates compaction, which restricts root growth and reduces water infiltration.

Available water capacity

The water held between field capacity and permanent wilting point that plants can extract. Sandy soils hold little available water; loams and clay loams hold more.

C:N ratio in organic matter

The ratio of carbon to nitrogen in residues. Near 25:1 decomposition proceeds steadily; wider ratios such as straw immobilise soil nitrogen temporarily, while narrow ratios release it quickly.

Rumen microbial protein synthesis

Rumen microbes convert dietary fibre and non-protein nitrogen such as urea into microbial protein, which the ruminant digests. Urea must be introduced gradually and is unsafe for monogastrics.

Feed conversion ratio (FCR)

FCR equals feed consumed divided by liveweight gain over the same period. A lower FCR means the animal converts feed into body mass more efficiently, improving enterprise profitability.

Colostrum and passive immunity

The first milk, rich in antibodies, must be fed within hours of birth because ungulate newborns receive no antibodies across the placenta. Without it, neonatal disease risk rises sharply.

Gestation lengths of livestock

Approximate pregnancy periods: cattle about 283 days, sheep about 147 days, pigs about 114 days, goats about 150 days. These guide breeding calendars and dry-period planning.

Gross margin calculation

Gross margin equals gross output (yield multiplied by price) minus variable costs for an enterprise. Fixed costs are excluded and deducted later to give net farm income.

Break-even yield

Break-even yield equals total cost divided by product price per unit. Producing below this yield means the enterprise makes a loss at that price.

Price elasticity of demand

Elasticity is the percentage change in quantity demanded divided by the percentage change in price. Staple foods are inelastic, so price changes cause large swings in farm revenue.

Opportunity cost

The value of the next best alternative forgone when a resource is used in one enterprise rather than another. Economic analysis includes it; accounting profit does not.

Wheel slip percentage

Slip equals loaded travel distance minus unloaded travel distance, divided by unloaded distance, times 100. Excessive slip wastes fuel, reduces work rate and damages soil structure.

Field capacity of machinery

Theoretical field capacity in hectares per hour equals implement width in metres multiplied by forward speed in km/h, divided by ten. Multiplying by field efficiency gives effective capacity.

PTO shaft speeds

Tractor power take-off shafts transmit engine power to implements at standardised speeds, usually 540 rpm or 1000 rpm depending on the system, allowing compatible implement design.

Drip irrigation efficiency

Drip delivers water directly to the root zone through emitters, achieving application efficiency often 85-95%, conserving water and reducing foliar disease compared with overhead sprinklers.

Law of segregation

Mendel's principle that the two alleles of a gene separate during gamete formation, so each gamete carries only one allele. Fusion at fertilisation restores the pair in offspring.

Monohybrid cross ratio

Crossing two heterozygotes (Aa x Aa) yields offspring in a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio when one gene with complete dominance is involved.

Heterosis (hybrid vigour)

The superior performance of an F1 hybrid over its parents in traits such as yield, vigour and stress tolerance. It is exploited in maize and many vegetable crops.

Integrated pest management (IPM)

A control strategy combining cultural, biological, mechanical and chemical methods, using chemicals only as a last resort to reduce costs, resistance and environmental harm.

Climacteric versus non-climacteric fruit

Climacteric fruits such as banana, tomato and mango ripen with a respiration and ethylene burst and can ripen after harvest; non-climacteric fruits such as citrus and grape cannot.

Controlled atmosphere storage

Storage in reduced oxygen (about 2-5%) and raised carbon dioxide to slow respiration and extend shelf life. Excessively low oxygen causes fermentation and off-flavours.

Safe grain storage moisture

Cereals store safely at roughly 12-14% moisture on a wet basis, depending on species and climate. Higher moisture allows mould growth and mycotoxin formation.

Post-harvest loss causes

Losses after harvest arise from mechanical damage, moisture loss, microbial decay, insect and rodent attack, and poor handling, sorting, packaging or storage conditions.

Conservation agriculture principles

Three principles: minimum soil disturbance, permanent soil cover with mulch or cover crops, and diverse crop rotation. Together they protect soil structure, moisture and organic matter.

Eutrophication from farm runoff

Excess nitrogen and phosphorus entering water bodies cause algal blooms, oxygen depletion and fish kills. Buffer strips and balanced fertiliser use reduce nutrient runoff.

Agroforestry

Deliberate integration of trees with crops or livestock on the same land. It improves soil fertility, microclimate, biodiversity and income diversification.

Agricultural greenhouse gas sources

Methane arises from ruminant digestion and flooded rice paddies; nitrous oxide arises from fertiliser and manure. Improved feeding, water management and precise fertiliser use reduce emissions.