W3, Crop development & canopy management

Question 1

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

Answer 1

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 2

What is one of the major risks associated with too much early vigour?

Answer 2

That the crop will run out of water, meaning that tiller survival will be low, grain set and size will be poor, risk of high screenings, HI will be low, and WUE will be low.

Question 3

What is the difference between leaf area index and green area index, and at what point is the critical leaf and green area index determined?

Answer 3

• leaf area index = m² leaves / m² ground area
  
  • LAI 4 = 4 m² leaves / m² ground
  • LAI_crit ≈ LAI 3-4
    
    • If you walk up to a crop and see very little ground, you’re probably close to the LAI_crit
• green area index = m² total green area / m² ground area
  
  • GAI_crit = ~90% light interception

Question 4

Which is the best development phase for maintaining pasture productivity?

Answer 4

Phase 2 ≈ 1200-2500 kg/ha food on offer (FOO)

• = max pasture growth rate
• = 70-90% ground cover (LAI ≈ LAI_crit)
• = limited self shading
• = high feed quality

Question 5

What number of plants/m² should you aim to achieve in a cereal cropping system?

Answer 5

80-200 plants/m²

Question 6

What number of heads/m² should you target in cereal production?

Answer 6

≈ 300-400 heads/m² is ideal, even in high rainfall + high production zones it’s enough to produce 7 t/ha yields.

Question 7

At what stage in the development of a cereal crop do you want to maximise the LAI relative to the available moisture?

Answer 7

Around the critical period (GS37 - 10 days after flowering) where the yield is largely being determined.

Question 8

What is the effect of applying N late (say at GS30 and GS37) rather than up front on tiller and ear number?

Answer 8

Tiller number (per m²) is reduced slightly, but the number of heads/m² increases substantially (by almost 50%).

Question 9

If your aim is to maximise the number of heads/m2, when would you apply N?

Answer 9

At GS30 and GS37.

Question 10

Describe the effect of residual LAI on regrowth from defoliation.

Answer 10

The higher the LAI and the greater the reserves of stored carbohydrates in the crown, the faster the recovery.

Question 11

List some important practices that affect canopy density and leaf area

Answer 11

• sowing date
• sowing rate
• row spacing
• timing and amount of N application
• level of defoliation (grazing)
• weed management

Question 12

True or false?

In wheat, once the terminal spikelets have been formed, the size of the ear (#spikelets) is fixed.

Answer 12

True.

Question 13

True or false?

In cereals, temperature remains a major developmental control throughout the lifecycle of the plant.

Answer 13

True.

Question 14

What are the major factors controlling the floral initiation stage of wheat?

Answer 14

• vernalisation temperature
• vernalisation time
• daylength (photoperiod)
• temperature

Question 15

Define thermal time and explain why it is important to cereal development.

Answer 15

Thermal time is the accumulation of heat over a period of time (measured as degree days - similar to with insect development).

As the temperature increases, the developmental rate (of cereals) increases too (up to an optimum temp - around 25-30°C, afterwhich developmental rate decreases)

Thermal time = ∑ ((T_max - T_min / 2) - T_b)

T_max - T_min = average daily temp (T_av)

T_b = base temperature (min temp required for development). In cereals it’s ~0°C.

e.g. (T_av1 - Tb) + (T_av2 - Tb) + (T_av3 - Tb)…

Question 16

How are thermal time and leaf development related, and what use is knowing this?

Answer 16

The rate of leaf appearance is directly proportional to thermal time. Knowing this can help plan for when to apply chemical controls that are suited to a certain amount of leaves being present.

• 1 leaf appears (roughly) every 100 degree days (°Cd)
• Rule of thumb: 1 leaf / 7-10 days
  
  • (using degree days is more accurate)

Question 17

True or false?

A cereal variety with high vernalisation sensitivity is likely to be a spring type.

Answer 17

False.

Varieties with vernalisation sensitivity require a period of cold temperatures, and are therefore planted before winter (meaning they’re long-season, winter types).

Question 18

Vernalisation

Answer 18

A period of cold temperatures (generally < 10 °C) 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.

Percieved at the shoot apex.

Question 19

Where are vernalisation and photoperiod detected within a plant (cereals)?

Answer 19

Vernalisation: shoot apex

Photoperiod: leaves

Question 20

What happens if a plant is sensitive to both photoperiod and vernalisation?

Answer 20

Only once the vernalisation requirement has been satisfied does the plant become receptive to photoperiod.

Question 21

True or false?

The longer the vernalisation requirement of a variety, the earlier it can be sown.

Answer 21

True (the earlier the better).

Question 22

Why has there been renewed interest in winter wheat?

Answer 22

• An increase in the size of farms and sowing areas has promoted calendar-style sowing programs where sowing goes over a number of weeks, requiring varieties that have a wide maturity range.
• Provides benefits
  
  • More effective weed management
  • Grazing
    
    • Fills the winter feed gap

Question 23

Describe examples of the interactions between wheat maturity and management.

Answer 23

The effect of sowing time on the development of wheat is determined by the variety chosen, due to:

• vernalisation requirement
• photoperiod sensitivity

Question 24

How do wheat and barley differ in how you’d manage their canopies?

Answer 24

Barley, especially 2-row barley, has smaller heads than wheat and is less able to compensate for low #heads/m² by increasing #seed/head or seed size.

Barley is also often grown after another cereal, so [soil N] is often low. This is beneficial for growing low protein (<12%) malting barley, but makes barley more reliant on N applications at sowing to ensure an adequate number of heads/m².

Question 25

When can a crop start being grazed?

Answer 25

At around the 3 leaf stage, or when the crop is anchored strongly enough in the ground to avoid being ripped out by livestock.

To test, use the pinch, twist and pull method (grab the leaves and give them a pull and a twist to simulate being grazed and see if the leaves break off or the plant gets pulled up (in which case the crop shouldn’t be grazed yet)).

Question 26

True or false?

Grazing delays cereal development

Answer 26

True (R² = 0.95).

For every 5 days grazing, flowering is delayed by ~1 day.

Question 27

Why might a grazed crop yield more than an ungrazed crop?

Answer 27

Grazing is just another form of canopy (i.e. resource) management. Grazing reduces the leaf area, and therefore the water usage during the pre-anthesis (vegetative) period, saving that water for use in the post-anthesis (reproductive) period where it is crucial for achieving the yield potential determined pre-anthesis. By reducing the size of the canopy, humidity within the canopy is also reduced, thus reducing the likelihood of foliar diseases (like powdery mildew).

Question 28

True or false?

Grazed canola is less susceptible to blackleg.

Answer 28

False.