Evaporative protection in wheat

By BCG Staff and Contributors
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Take home messages

  • Protecting crop inter-row in wheat from evaporation using PVC cover significantly increased
    yield by up to 50 per cent (%), which demonstrates the benefits of water conservation
    and/or concentration of water shed to the crop row in a decile 2 growing season.
  • Applying a spray-on polymer or adding 5t/ha of stubble as a mat did not provide the same
    yield benefit as using PVC cover.
  • Given the promising results for the PVC cover, testing other inter-row control measures
    including alternative polymer formulations, stubble load and arrangement or canopy
    designs to limit evaporative losses of soil water, in-season would be beneficial.

Background

In dryland cropping regions, where rainfall during the growing season is low and unreliable, water storage and conservation within the soil for crop use is vital for buffering dry periods (O’Leary and Connor, 1997). Consequently, management practices which limit evaporative water losses, in-season, could improve yield stability of crops.

If an economic method of using polymers or other cover material for protecting against evaporative losses within crops could be applied, this would increase the proportion of water used by the crop compared with evaporative losses. Such interventions could increase crop water use and provide the industry with tools to increase yield within semi-arid cropping environments.

To determine the impact of inter-row evaporative water loss on crop yield, we tested the concept of protecting the inter-row region of wheat crops from evaporative loss using polyvinyl chloride (PVC) covers. We also tested the comparative effectiveness of an alginate-based spray-on polymer and a 5t/ha stubble mat on wheat growth.

Aim

To test the benefit of reducing inter-row evaporative losses in-crop on wheat growth using PVC covers and comparing the relative effectiveness of possible agronomic management options such as a spray‑on polymer and stubble mats.

Paddock details

Location: Longerenong
Crop year rainfall (Nov-Oct): 284mm
GSR (Apr-Oct): 187mm
Soil type: Grey vertosol
Starting soil N (0-1.2m): 101kg/ha
Paddock history: Barley (1.1t/ha standing stubble)

Trial details

Crop type/s: Scepter wheat
Treatments: Refer to methods
Seed rate: 89kg/ha
Target plant density: 140 plants/m²
Seeding equipment: Knife points, press wheels, 30cm row spacing, cross sown into a 1.1t/ha standing stubble
Sowing date: 19 June 2018
Harvest date: 4 December 2018
Replicates: Five

Trial inputs

Fertiliser – sowing: Granulock® Z @ 60kg/ha + with flutriafol @ 200mL/100 kg
Chemical inputs: Trifluralin @ 1.5L/ha + Glyphosphate @ 2L/ha +
Avadex® @ 2 L/ha + Sakura® @ 118g/ha
Irrigation: To create two different backdrops of water supply and yield
potential, a single irrigation event of 20mm (24 August) was
applied at mid-tillering (high water (HW) wheat) and was
compared with low water (LW) crops which were rainfed alone.

Method

A field trial was sown using an unbalanced randomised block trial design to assess the effect of a range of inter-row evaporative protection methods, timing of protection and growing season water.

Treatments included a factorial combination of:

  • Protection method (nil; PVC – 225mm half pipe cover; spray-on polymer (alginate based) and
    stubble mat at 5t/ha) (Figure 1)
  • Protection timing (mid-tillering to heading; mid-tillering to maturity and heading to maturity)
  • Growing season water (low water (LW), rainfed; high water (HW), rainfed + 20mm irrigation at mid‑tillering)

Evaporative protection in wheat figure 1 edited

Figure 1. Inter-row evaporative protection methods, PVC (a, b & c) and spray-on polymer (d, e & f), for Scepter wheat at mid-tillering, (a & d); heading, (b & e) and maturity (c & f).

The nil and PVC cover treatments were in a factorial combination with protection window and growing season water treatments, whereas the spray on polymer and straw treatments were applied across both water regimes, but to a restricted set of protection windows.

Results and interpretation

Where inter-row PVC cover was imposed at mid-tillering for LW crops, anthesis biomass was equivalent to where crops had an additional 20mm of irrigation applied at mid-tillering and were uncovered (Figure 2). Moreover, for crops with inter-row cover, growth was equivalent across water treatments (4.3t/ha), whereas for those crops with no cover, irrigation increased anthesis biomass by 26%. Evidently greatest benefit of inter-row cover on crop growth occurred under drier conditions.

Evaporative protection in wheat figure 2 edited

Figure 2. Comparison of Scepter wheat growth (t/ha) up to anthesis for crops where a
factorial combination of inter-row cover (no cover and PVC) and external water supply
(rainfed and rainfed + 20mm) were tested. LSD is for the interactive effect of method of cover and irrigation.

Applying PVC inter-row cover from mid-tillering to both heading and maturity significantly increased yield by 42 and 50% respectively compared with where there was no inter-row cover (Table 1). This increase in yield was linked with a 50 and 53% increase in grain set respectively. The effect of removing the PVC inter-row cover at heading (mid-tillering to heading treatment) was a significant reduction in kernel size compared with all other treatments (data not shown), indicating that the absence of post‑flowering protection increased late water stress and limited grain-filling. When PVC inter-row cover was restricted to the heading to maturity growth window, there was no significant increase in yield compared with the uncovered control.

Table 1. Comparison of Scepter wheat yield for a range of inter-row cover (protection)
methods applied across three different windows during the growing season. Protection windows were mid-tillering to maturity (Mt to Mat), heading to maturity (Hd to Mat) and mid‑tillering to heading (Mt to Hd). LSD is for the interactive effect of these two treatments.

Evaporative protection in wheat table 1 edited

The pattern of response for inter-row cover effects remained the same across the LW and HW treatments. The main effect of an additional 20mm applied at mid-tillering was to increase yield from 1.1 to 1.4t/ha (pooled across protection method and window treatments).

For the spray on polymer, applied at both mid-tillering and heading, there was no yield advantage compared with crops where there was no inter-row cover (Table 1). The inconsistency between the response of PVC and the spray-on polymer may be due to the formulation being semi-permeable to water and/or the longevity of cover before breakdown (Figure 1 d-f ), which did not adequately protect against surface evaporation of soil water. For the stubble mat imposed at mid-tillering (straw horizontal and aligned), at 5t/ha within the inter-row, this provided no yield benefit compared with the control.

Commercial practice

Overall, imposing the inter-row PVC protection demonstrates the benefits of water conservation and/or concentration of water shed to the crop row region on yield (up to 50% increase), applied in a decile 2 growing season. Both the spray-on polymer and manipulation of stubble presentation to limit inter-row surface evaporation offer potential management options that could be imposed by industry to protect against evaporative losses in crop. For the approaches tested, we could not demonstrate a yield benefit of these options, and next steps could include testing other polymer formulations, stubble presentation methods or canopy designs to limit the impact on evaporative losses of soil water, in-season, like that observed when using the PVC cover.

References

O’Leary GJ., Connor DJ., 1997. Stubble retention and tillage in a semi-arid environment: 1 Soil water accumulation during fallow. Field Crops Research 52, 209-219.

Acknowledgements

This work was funded by the ‘Improving practices and adoption through strengthening D&E capability and delivery in the southern region’, Regional Research Agronomists program (DAV00143) as part of the Grains Research and Development Corporation (GRDC) and Agriculture Victoria – Department of Jobs, Precincts and Regions (DJPR), Bilateral Research Agreement. The authors also wish to acknowledge the assistance of BCG in delivery of the experimental program.

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