agriculture weeks 9-11

What is sustainable intensification?

Increasing production on existing agricultural land without destroying natural ecosystems, without negative off-site environmental effects, and without degrading natural resources.

What is agronomy?

Using scientific understanding to improve agricultural production, profitability and sustainability.

What makes agronomists unique?

They are broad generalists with shallow but wide knowledge across many disciplines.

What is the difference between commercial and research agronomists?

Commercial agronomists advise farmers on inputs; research agronomists conduct experiments to improve understanding and management.

What is farm yield (FY)?

The yield achieved by farmers in their fields.

What is potential yield (PY)?

Yield of the best cultivar with optimal agronomy and no manageable stresses.

What is water-limited potential yield (PYw)?

Yield limited only by water supply, with all other stresses removed.

What is economic yield (EY)?

Yield achieved using economically optimal inputs, recognising diminishing returns. Typically 75-85% of PYw.

What is the yield gap (YG)?

Difference between FY and PYw.

What is the exploitable yield gap (EYG)?

Difference between FY and EY.

What determines a farmer's yield gap?

Management skill, investment in inputs, incentives, and capacity to achieve higher yields.

What causes yield gaps in developing countries?

Economic, infrastructural, legislative and logistical barriers — not lack of skill.

What proportion of PYw do Australian wheat farmers achieve?

55% on average. Leading farmers have closed the exploitable yield gap.

What determines potential yield?

Solar radiation available for photosynthesis and the time available to capture it during the crop life cycle.

What is growth?

Accumulation of dry matter via photosynthesis minus respiration.

What is development?

Speed at which a crop moves through its life cycle.

Why is the ratio of growth to development important?

If development is too fast, growth is insufficient for high yield.

What is the critical period for yield determination?

The life-cycle window where seed number is determined; usually around flowering.

When is the critical period in wheat?

Approximately 20 days before to 10 days after flowering.

What reduces yield during the critical period?

Any stress (water, temperature, nutrient, biotic).

What is the photothermal quotient?

Ratio of solar radiation to temperature; used to estimate potential yield.

How does latitude affect potential yield?

Higher latitudes → cooler temps + long days → higher photothermal quotient → higher potential yield.

How can potential yield increase over time?

Through plant breeding and improved management. Wheat breeding increases yield ~0.5% per year.

What determines water-limited potential yield?

Crop water balance: rainfall + stored soil water - evaporation - drainage.

What environmental factors determine potential yield?

Solar radiation and temperature, and how they align with the crop's critical period.

What physiological factors determine potential yield?

Growth rate, development rate, and seed number formation during the critical period.

What happens if a crop develops too quickly?

Growth is insufficient and yield is reduced.

What happens if radiation is low during the critical period?

Seed number and yield decrease.

How do high temperatures affect yield?

They speed development, shortening the critical period and reducing seed number.

What is the critical period for yield determination?

The life-cycle window where seed number is determined, usually around flowering.

When is the critical period in wheat?

Approximately 20 days before to 10 days after flowering.

Do all crops have the same critical period timing?

No — peas, lupins and chickpeas have critical periods after flowering.

What is the photothermal quotient?

The ratio of solar radiation to temperature.

How does photothermal quotient affect potential yield?

Higher quotient → higher potential yield.

Why does New Zealand have very high potential wheat yields?

Long, cool summer days with high radiation.

How do agronomists estimate potential yield?

Using relationships between radiation, temperature, and yield from well-managed experiments.

Why can potential yield increase over time?

Improved cultivars and improved management.

How fast has wheat yield potential increased through breeding?

About 0.5% per year.

What is water-limited potential yield?

Yield limited only by water supply, with all other stresses removed.

What determines PYw?

Crop water balance: rainfall + stored soil water - evaporation - drainage.

Why is PYw more useful in Australia?

Most Australian cropping is rainfed/dryland.

How did French & Schultz estimate water-limited potential yield?

By relating grain yield to seasonal water supply (rainfall + stored soil water).

What is the key idea behind their model?

Each mm of water contributes a predictable amount of biomass and grain yield.

What environmental factors determine PYw?

Rainfall, evaporation, soil water storage, drainage losses.

What physiological factors determine PYw?

Water-use efficiency, rooting depth, transpiration efficiency.

What are the four ways to increase yield in water-limited environments?

Increase water supply, reduce evaporation losses, increase transpiration efficiency, increase harvest index.

What are the four timescales over which agronomic management influences yield?

Long‑term, crop sequence, fallow, in‑crop.

Why does management influence decline after sowing?

Because once genotype is selected and planted, management options reduce.

What are the four mechanisms by which yield can be increased in water‑limited environments?

Increase crop water use; increase proportion of water transpired; increase transpiration efficiency; increase harvest index.

Why is long‑term management important?

It affects soil's ability to capture, store and supply water and nutrients.

What determines soil infiltration rate?

Porosity and surface residue cover.

What determines soil porosity?

Texture, mineral composition, soil organic matter (especially humus).

What is humus and why is it important?

Decayed microbial material that glues soil particles into aggregates, increasing porosity.

How does residue cover affect infiltration?

Protects soil from raindrop impact and slows water flow, increasing infiltration.

What happens when rainfall rate exceeds infiltration rate?

Water ponds, then runs off, causing water loss.

What determines a soil's ability to store water?

Texture mainly; structure and organic matter also influence.

What is drained upper limit (field capacity)?

Water content after soil is wetted and allowed to drain.

What is crop lower limit?

Water content when a healthy crop has extracted as much water as possible.

What is permanent wilting point?

Crop lower limit measured under 1500 kPa suction in a lab.

What is plant available water capacity (PAWC)?

Difference between drained upper limit and crop lower limit.

Which soils store the most water?

Clay soils (up to 300 mm PAWC).

Which soils store the least water?

Sandy soils (as little as 60 mm PAWC).

How does cultivation affect infiltration?

Breaks aggregates, reduces porosity, buries residues, increases organic matter mineralisation.

What are the consequences of reduced infiltration?

Run‑off, erosion, nutrient loss.

Why are crop residues important?

Protect soil from raindrop impact and wind erosion; maintain organic matter.

Why might farmers remove residues?

Livestock feed value; interference with machinery.

How does residue retention affect soil organic matter?

Residues feed microbes that form humus.

How does soil organic matter affect fertility?

Stores nutrients (especially nitrogen) and releases them as humus decomposes.

What happens if nutrient export exceeds nutrient input?

Soil organic matter declines.

Why is long‑term nutrient balance important?

It maintains soil's ability to supply water and nutrients.

What are the three principles of conservation agriculture?

Minimum soil disturbance; permanent soil cover; diverse crop sequences.

What fourth principle was proposed by Giller et al. (2015)?

Maintenance of neutral nutrient balance.

How does long‑term management affect yield?

Through effects on infiltration, storage, organic matter, and nutrient balance.

Which Passioura mechanism does long‑term management mainly influence?

Mechanism 2: proportion of water transpired.

What is crop rotation?

A fixed, repeating order of different crop species grown over time.

What is a crop sequence?

A non‑fixed order of crops that does not necessarily repeat.

Why do crop sequences influence yield?

Because different crops host different pests, diseases, weeds, and use water/nutrients differently.

What is a break crop?

A crop grown to break the life cycle of diseases affecting the main crop.

What is the break crop effect?

The yield increase in a crop grown after a break crop compared to the same crop grown continuously.

How is the break crop effect measured?

By growing wheat and break crops in year 1, then wheat in year 2 and comparing yields.

How do break crops reduce soil‑borne disease?

By removing the host plant, reducing pathogen inoculum, and sometimes releasing root toxins.

What is biofumigation?

Brassica roots release isothiocyanates that suppress pathogens.

Why are break crops important for wheat disease control?

Few wheat cultivars have resistance to major soil‑borne diseases; fungicides give only small yield gains.

How do legumes increase nitrogen for following crops?

They fix atmospheric nitrogen through rhizobia; unused nitrogen remains for the next crop.

What determines how much nitrogen legumes fix?

Biomass production, species differences, and stresses like drought or disease.

Which legumes fix the most nitrogen?

Faba beans and lupins.

How do break crops help reduce grass weeds?

They allow different herbicides to be used that are more effective than those used in wheat.

Why are break crops used for weed control?

Grass weeds often develop resistance to herbicides used in wheat.

How does hay production reduce grass weeds?

Cutting hay prevents grass weeds from setting seed.

How do crop species differ in water extraction?

Some crops extract water poorly, leaving more for the next crop.

How does hay cutting affect soil water?

Early termination leaves more water for the next crop.

When does water carryover increase yield?

In low‑rainfall regions where waterlogging is not a problem.

How does disease reduction increase yield?

Healthy roots extract more water.

How does legume nitrogen increase yield?

Through mechanisms that enhance water use, evaporation reduction, and transpiration efficiency.

How does weed reduction increase yield?

Less water transpired by weeds allows more for the crop.

How does leftover water increase yield?

More water available to the next crop.

Why is summer fallow weed control important?

Weeds use deep soil water and nitrogen that should be saved for the next crop.

How do fallow weeds affect yield?

They reduce water and nitrogen available, leading to lower yield.

Can fallow weeds host diseases?

Yes, they can host root and foliar diseases.

How does residue cover affect fallow water capture?

Residues protect soil, maintain infiltration, and reduce run‑off.