EXAM | Repeated exam questions

Question 1

DEFINE

Water use efficiency

Answer 1

WUE = yield / water use

= yield / (Δ soil moisture - growing season rainfall)

units should be t/ha/mm

Question 2

DEFINE

Harvest index (HI)

Answer 2

HI = yield / total dry matter

Typical in Aus: 0.3-0.45

Typical in EU/NZ (long, cool finishes): 0.5 or more

Question 3

DEFINE

Vernalisation

Answer 3

A period of cold temperatures required by some cultivars in order to transition from vegetative to reproductive growth.

In wheat, vernalisation accumulates most rapidly between 3-10°C, but can accumulate at a slower rate up to 17°C.

Question 4

DEFINE

Determinate vs indeterminate crops

Answer 4

Determinate crops like cereals have distinct vegetative and reproductive phases that do not overlap, whereas indeterminate crops like canola and most grain legumes exibit overlapping vegetative and reproductive phases, as well as overlapping reproductive and grain filling phases.

Question 5

DEFINE

Critical growth stage for wheat yield

Answer 5

The critical growth stage is the time when the yield is most sensitive to the growing conditions. There is generally two, including the establishment phase, which determines the starting plant density and impacts weed pressure. In wheat, the other critical growth stage is from flag leaf emergence (~GS37) to about 10 days after flowering, where a lot of the yield potential is determined and the sensitivity of the yield to the growing conditions is greatest.

Question 6

Define and explain:

Vapour pressure deficit (VPD)

Answer 6

The drying power (evaporative demand) of the atmosphere, which affects the efficiency with which a crop or pasture can utilise water.

• Saturated vapour pressure = 100% relative humidity = total amount of water that can be held
• VPD = actual VP of air vs. saturated VP
• ↑ temp (T) = ↑ ability of air to hold water
• ↑ VPD = ↑ aridity = ↑ evaporation
• VPD drives evaporation and crop water demand
  
  • its almost as important as rainfall (in terms of correlation with yield)
• Transpiration efficiency proportional to 1/VPD
  
  • In other words, ↑ VPD = ↓ WUE

In winter, the VPD is low because the temp is low and humidity is high, whereas in summer the VPD is high because the temp is high and humidity is low.

The cooler and wetter an environment, the lower the VPD, and therefore the lower the water demand by crops.

Question 7

Define and explain:

Plant available water (PAW)

Answer 7

PAW = drained upper limit (DUL) - lower limit of extraction (LLOE) / rooting depth

• DUL = field capacity
  
  • > DUL = waterlogged
• LLOE = the driest that the crop can make the soil
  
  • reflects the ability of the crop to extract water
• PAW = aka bucket size

Unlike the permenant wilting point (PWP = 1.5 MPa), the LLOE is affected by soil properties

• e.g. If there’s B toxicity or salinity, the plant’s ability to extract water is going to be different than the PWP.
• different crops have different LLOE (different root and soil properties = different LLOEs)

Question 8

Define radiation use efficiency (RUE) and how it’s calculated.

Answer 8

RUE = the proportion of photosynthetically active radiation (PAR) that is used by the crop to generate the harvestable biomass.

RUE = Δ biomass / PAR intercepted (MJ/m²)

PAR intercepted = PAR * (1-I₁/I₀) / 100

PAR = ~50% total solar radiation

I₁ = light intensity at the bottom of the crop (near the ground)

I₀ = light intensity just above the crop

Question 9

CANOPY MANAGEMENT

What is meant by the term ‘canopy management’?

(Be able to discuss the physiological principles)

Question 10

CANOPY MANAGEMENT

What is haying-off in cereal crops, in which growing seasons are they most susceptible, and how can its risk be minimised through good canopy management?

Support your answer with examples, and be sure to discuss the role of nitrogen management.

Question 11

CANOPY MANAGEMENT

How do seasonal conditions influence a wheat crop’s reliance on stored stem carbohydrates for grain filling?

Question 12

GRAZING

Discuss the impact of grazing crops in mixed farming systems on crop development

Question 13

GRAZING

Discuss the impact of grazing crops in mixed farming systems on crop water use

Question 14

GRAZING

What management practices can be used to maximise available feed for grazing animals and minimise yield reduction due to grazing?

Question 15

GRAZING

How can farmers use strategic grazing management to improve the competitive ability of pasture legumes and improve pasture composition?

Answer 15

By resting (i.e. not grazing) the pasture over winter, the height of reproductive structures in grass species is raised, meaning they have a larger probability of being grazed off, thus reducing seed set and the amount of competition with pasture legumes (i.e. pasture composition improves).

Planting bitter-tasting forages is another way of using grazing to manage weeds, because livestock will preferentially graze the sweeter-tasting weeds before moving on to the forage (thus reducing weed seed set and maintaining good pasture composition).

Question 16

GRAZING

How does the change in grazing duration affect crop development and water use, and how it affect grain yields in seasons with above-average and below-average rainfall?

Question 17

SOIL COMPACTION

Which soil types are more prone to compaction and why?

Answer 17

Sandy soils are more prone to (deep) compaction which is more difficult to remediate.

Clays compact closer to the surface because they have a higher particle surface area, meaning the downward force is being spread over a greater area.

Question 18

SOIL COMPACTION

What level of soil strength (resistance) can prevent root penetration?

Answer 18

2 MPa will restrict root growth.

> 3 MPa and most roots can’t really penetrate at all.

Question 19

SOIL COMPACTION

What management practices can be used in broad-acre crop production systems to remediate soil compaction (e.g. hard pans) and minimise the occurance of it in the future?

Question 20

TIME OF SOWING

Discuss some of the issues and management decisions that you would need to consider when recommending early sowing (e.g. mid-April) of wheat crops in the dryland farming systems of southern Australia.

Question 21

TIME OF SOWING

“The optimum sowing time of a wheat variety reflects the optimum time of flowering.”

Discuss this statement in relation to the selection of varieties for a sowing program that may extend from mid-April to early June.

Question 22

TIME OF SOWING

What factors influence the optimum time of flowering in crops?

(Consider both biotic and abiotic factors)

Question 23

COMPETITION

Briefly discuss important plant traits associated with improved weed competitive ability of cereal crops.

Which traits could improve crop competitive ability without reducing its yield potential?

Answer 23

• Early vigour
• Height
• Stress tolerance

Question 24

SPATIAL VARIATION

What are the most common sources of spatial variation (in soil) that affect grain yield?

Answer 24

• pH
• EC (particularly ESP)
• Soil texture & organic matter content
  
  • Strength (resistance)
  • Cation exchange capacity
  • Bufferring capacity
  • Water-holding capacity
  • Infiltration rate
• Nutrient availability (deficiency, toxicity)
• Biology
  
  • Diseases

Question 25

SPATIAL VARIATION

How can precision agriculture techniques be used to diagnose production constraints and improve the efficiency and economic returns in paddocks with large spatial variation in grain yield?

Question 26

AGROCHEMICALS

Metazachlor is a pre-emergent herbicide with a water solubility of 450 mg/L and an organic carbon sorption constant (K_OC) of 54mL/g.

Why is it important to know about these characteristics of pre-emergent herbicides and what do the values suggest about the behaviour of the herbicide and its potential for intended consequences?

Answer 26

The solubility and organic carbon sorption constant (or octinol-water coefficient, K_OW) determine the fate of the compound both within the plant and within the environment, including:

• how the compound is likely to be absorbed,
• how translocatable within the plant it is,
• whether the compound is likely to adsorb to soil particles, and
• how great the potential is for the compound to move with soil water (i.e. be leached or included in runoff).

INCREASING SOLUBILITY VALUE =

• ↑ solubility (in water)
• ↑ likelihood of being absorbed through roots
• ↑ translocation throughout plants
• ↑ risk of movement within the environment (water cycle)
  
  • leaching
  • runoff

INCREASING K_OC or K_OW VALUE =

• ↑ sorptivity
• ↑ likelihood of being absorbed through leaves/membranes
• ↓ risk of leaching
• ↑ risk of loss through erosion
• ↑ risk of bioaccumulation

Metazachlor is highly soluble, with a low-med sorptivity, meaning it is unlikely to be strongly bound to crop residues/stubble and soil, is likely to be absorbed through the roots rather than leaves, and has a high likelihood of being leached or becoming runoff with significant rain events.

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