W5, Response to time of sowing (TOS) & nitrogen (N)

Question 1

When is the optimum sowing date (period) for winter wheat in South Australia, and what kind of yield penalty can be expected for each week of delay?

Answer 1

First week of May.

Yield penalty for every week delay: 196 kg/ha

Question 2

How is the optimum flowering time determined?

Answer 2

It’s an environmental characteristic based on the climate and environmental constraints of the site (temperature, water availability, pest pressure, etc.)

Question 3

What is it that determines the yield response to TOS?

Answer 3

The developmental pattern (maturity type) of the variety and the timing of flowering.

Question 4

True or false?

Two crops of the same variety that are sown on different farms on the same day will have the same optimum flowering window.

Answer 4

False.

The optimum flowering window is dependant on climatic conditions and environmental constraints, which are site-specific.

Question 5

If you were advising the owner of a large cropping operation whose sowing period extended over a ~3 week period, roughly how many different varieties would you expect to grow to achieve a consistant flowering window?

Answer 5

Roughly 1 for every week of seeding. The longer the sowing period, the earlier the sowing should start, and the more winter-type varieties will be needed (stronger vernalisation and photoperiod response).

Question 6

What are some of the benefits and downfalls of sowing early?

Answer 6

BENEFITS

• quicker crop establishment
• ↑ early vigour
• planting into warm soil promotes root growth
  
  • can be important later in the season for accessing extra subsoil moisture
    
    • = ↑ water use efficiency (WUE)
• ↑ tillering and biomass production
• potential to be grazed
• requires less N up front
• ↑ WUE
  
  • The establishment phase is the period when the crop has the lowest WUE due to the high rates of soil evaporation resulting from low ground cover. By having this developmental period occur when environmental conditions are least conducive to high rates of evaporation (i.e. when the vapour pressure deficit (VPD) is low), the evaporative demand is minimised.

DOWNFALLS:

• ↑ foliar disease risk (due to denser canopy)
• ↑ lodging risk

Question 7

Why is there a difference in the response to sowing rate?

Answer 7

Early sowing = ↑ tiller production = ↑ tiller competition ↑ tiller death

The later the crop is sown, the less time there is for tiller production, and as such less ears/m2 are developed. Consequently, by increasing the sowing rate, you basically counter the reduction in tiller number by having more plants.

The earlier the sowing, the more time for tiller production, and therefore the greater the area that each plant can occupy.

Question 8

What would the effect on yield be by increasing the plant densitiy (sowing rate) of a winter wheat vs early spring wheat?

Answer 8

Winter wheat: ↓ yield from ↑ tiller competition

Spring wheat: ↑ yield from ↑ leaf area index (LAI)

Question 9

What are some typical benchmarks for the proportion of gross farm income that is allocated to a fertiliser program?

Answer 9

High rainfall: 12-14%

Medium rainfall: 10-12%

Low rainfall: 8-9%

Question 10

During what growth phase is nitrogen demand greatest for a cereal crop?

Answer 10

During stem elongation and the rest of the critical growth phase (so from ~GS30 to ~10 days after flowering).

Question 11

What is a typical relationship for how much N is fixed per kg of legume dry matter?

Answer 11

15-20 kg N fixed / t legume dry matter

Question 12

True or false?

The form of N has a considerable affect on yield response.

Answer 12

False.

N is N.

Question 13

Define and explain what is meant by partial factor productivity, and provide an example of typical ranges.

Answer 13

Basically a measure of fertiliser (N) use efficiency.

= Yield / amount of N applied

Generally 40-80 kg/kg N, but if you’re above 60 kg/kg N, you’re doing well.

Question 14

Define and explain what is meant by a partial nutrient balance, and provide an example of typical ranges.

Answer 14

Basically a measure of how effectively the N is meeting crop demand

= N removed in crop / N applied

Can be anywhere from 0.1-0.9 kg/kg N, but an average recovery is around 50% (0.5 kg/kg N). Greater than that and you’re doing well.

Question 15

True or false?

Sand dunes are more responsive to N than heavier flats/swales.

Answer 15

True.

Sand hills don’t have the capacity to retain N, so they’re very responsive to fertilisation.

Question 16

Explain what haying off is, during what season a crop is most susceptible, and what management practices can be used to prevent it.

Answer 16

Haying off is basically the crop running out of water post-anthesis (particularly during grain filling).

Spring is when the risk is greatest.

Often results from excessive N creating a large amount of biomass that can’t be supported later by the available water/nutrients. ↑ N doesn’t just ↑ water usage by ↑ biomass, it also reduces the amount of water soluble carbohydrates (WSC) which the plants rely on later in the season for maintaining growth (filling grain) under stress.

Question 17

What factors would you need to consider if you were going to apply N some time after sowing?

Answer 17

• Environmental conditions of the area
• Background level of N
• Water availability
• Likelihood of a dry spring

Question 18

True or false?

Low tillering varieties tend to have greater concentrations of water soluble carbohydrate stores.

Answer 18

True.

Question 19

How to plang growth regulators (PGRs) work, what is their purpose, and when should they be applied for the greatest benefit?

Answer 19

• PGRs inhibit the synthesis of gibberellins (GA) in plants
  
  • GA involved in cell elongation
• Purpose = increase structural regidity (stem thickness), decrease plant height
  
  • ↑ structural regidity = ↓ lodging risk & ↑ ability to store water soluble carbs
• Best applied at/during stem elongation; i.e. after GS30 (because you’re trying to reduce that elongation)