Short and long-term focus needed to lift WUE

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Pre-crop management is more important than in-crop management in lifting the water use efficiency (WUE) and yield of wheat cropping systems.

This is one of the main messages of the five-year, GRDC-funded Water Use Efficiency Initiative, which will finish in June 2013.

The initiative has highlighted the critical importance of using a combination of management practices both before and during the cropping phase to improve the soil water store and WUE of cropping systems (Figure 1).

In-crop and pre-crop management

Investigations into lifting crop yields in water-limited environments often focus on the in-crop phase – the period between crop sowing and harvest. However, implementing practices that increase soil water capture and storage during the pre-crop period (years to months before the wheat crop is sown) often have much greater potential to lift crop yield.

Management strategies that improve the soil’s capacity to capture and store water are becoming more important as Australian cropping systems increasingly rely on summer fallow rainfall to grow a winter crop.

Crop row close-up

Management practices implemented months to years before a crop is even planted contribute up to two-thirds of the water use efficiency of grain systems.

Pre-crop practices such as fallow weed management, rotation choice, long-term stubble retention and minimum tillage combine to have a significant and positive impact on the WUE of wheat crops. Such pre-crop practices effectively maximise the amount of pre-season soil moisture that is captured, stored and made available to subsequent wheat crops.

In-crop practices such as sowing date and nutrient, weed and disease management can then be optimised to capitalise on the increased water storage brought about by the pre-crop agronomic practices.

Pre-crop and in-crop management practices can work together with crop genetics to lift WUE above that possible from any management practice or plant variety used in isolation.

For example, plant-breeding innovations such as the long-coleoptile wheat genotypes being developed by CSIRO could deliver additional yield benefits under water-limited conditions. Long coleoptiles can emerge from deeper in the soil profile, allowing timely sowing when effective pre-crop management such as summer weed control and stubble retention have ensured soil water is available. However, without appropriate fallow management and stubble retention, the innovative long-coleoptile genotype provides little benefit.

Relative value of management practices

FIGURE 1

The range of in-crop management options (A) that influence the productivity and water-use efficiency of cereal crops.   Critical in-crop seasonal influences at sowing, anthesis and maturity are shown. In-crop management effects are placed in the context of various pre-crop management options (B), and the continuum of overlapping influences of these various options on components of water-limited yield are shown by horizontal arrows.

Short and long term focus

Figure 2

Cumulative improvements in yield (bars) and water use efficiency (numbers) over the past 30 years in response to combined agronomic and genetic scenarios (2-6).

Cumulative improvements in yield and water use efficiency represented in a graph

Wheat yields have more than doubled in the past 30 years due to a combination of agronomic and genetic innovations. Since the 1980s (baseline scenario) yields have increased progressively with the introduction of minimum tillage and stubble retention (Scenario 2), fallow weed control (Scenario 3), legume break crops (Scenario 4), early sowing (Scenario 5) and specialised wheat cultivars (long coleoptile wheat) (Scenario 6).

The baseline yield (blue) is the yield achieved in the 1980s when full cultivation, burning and weedy fallows in continuous wheat were the norm. The red boxes represent the relatively small yield change when new innovations are added individually – with the largest individual contribution to yield improvement from fallow weed control (Scenario 3). The green bars represent the extra improvement in yields when the innovations are combined sequentially in a new system. Note the genetic innovation of long coleoptiles (Scenario 6) had no impact on yield if used alone, but improved the yield achieved from the water-saving agronomy (Scenarios 2 to 5) by 0.5t/ha (compare Scenarios 5 and 6) and increased WUE from 12.9 to 15.2.

The impact of agronomic practices and new genetics on wheat yield and WUE can be predicted using a farming system model such as the Agricultural Production Systems sIMulator (APSIM), which enables a larger range of sites and seasons to be examined than is possible with field trials.

At the start of the Water Use Efficiency Initiative, CSIRO researchers used the APSIM farm simulation model and 30 years of management and yield information from a case-study farm in Kerang, Victoria, to predict the relative contribution of a range of pre-crop and in-crop management practices, as well as a long-coleoptile wheat genotype, to the WUE of a wheat crop (Table 1).

Combined, the management practices and crop genetics resulted in a yield increase of more than 40 per cent due to improved WUE over the 30-year simulation study (Figure 2). The modelled results closely matched the wheat yield improvements on the case-study farm.

The largest single contributor to improved WUE was fallow weed management (Scenario 3, Table 1), which led to a 60 per cent increase in wheat yield by conserving valuable summer rainfall for crop growth and grain fill during spring (Figure 2).

However, to maximise the water conservation benefit of summer weed control it was necessary to combine it with stubble retention and minimum tillage, which reduced water run-off and increased water infiltration.

Introducing forage peas into the rotation (Scenario 4, Table 1) also increased water storage due to their early cutting, which in turn lifted wheat yields the following season. However, without summer weed control the water-saving benefit of the peas is diminished – reinforcing the need to use management practices in combination and not in isolation.

On the Kerang case-study farm, forage peas have become a regular rotation – particularly during the recent Millennium Drought – because forage peas are profitable even in drier years but still leave water and nitrogen deep in the soil profile for the following wheat crop.

The modelling study confirmed the yield benefits of sowing early (Scenario 5, Figure 2), but sowing wheat dry or immediately after the autumn break is only successful with good weed control during the pea break crop and fallow period and retention of surface stubble, which extends the sowing window following marginal sowing rains.  

The managers of the Kerang case-study farm learnt of the potential value of early sowing when Yield Prophet® simulations of their cropping system indicated a greater yield penalty existed for delayed sowing when water was available (such as after a pea forage) than when it was not. 

However, in some seasons sowing early resulted in lower yields, with about 20 per cent of modelled years showing lower yields compared with sowing at the official break of season. Yield reductions from early sowing can happen in years with cool, wet springs, which cause the early-sown crops to mature too early – leaving them unable to take advantage of the ideal late grain-filling conditions.

Sowing early can also increase the risk of frost damage during flowering, although the recent experience of the case-study farm managers has been that the potential frost risk has largely been offset by an increase in yield potential due to the avoidance of hot days later in spring. Nevertheless, it may be prudent to consider changing to a later-maturing wheat variety for early-sown paddocks to ensure flowering remains within the optimum window. The benefits of an early-sown, later-maturing wheat variety on WUE and yield are discussed on page 10 of this supplement.

The genotypic adaptation of a long coleoptile (Scenario 6, Figure 2) generates a large yield increase – but only when used in combination with management practices that lift soil moisture. Used properly, in drought years a long coleoptile is predicted to more than double yield potential as a result of earlier sowing that avoids grain-filling under the hostile springs experienced in such seasons.  

However, to be successful the long-coleoptile genotype must be used in combination with good fallow management to generate the extra water storage. Without good weed control, stubble retention and a suitable rotation, insufficient water is stored to germinate the long-coleoptile wheat and generate the benefits from earlier sowing.

More information:

Dr John Kirkegaard, senior principal research scientist, CSIRO Plant Industry,
john.kirkegaard@csiro.au,
02 6246 5080

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Table 1 Pre-crop and in-crop management scenarios (along with the genetic trait of practice alone and in combination on water use efficiency in wheat over 30 growing seasons (1980 to 2010)
Scenario Management practice
Tillage Stubble Summer weeds
Rotation
Sowing window
Wheat genotype
 1. Baseline (pre-1980s)
Two cultivations before sowing
 Burnt in autumn
 Uncontrolled until autumn
 Wheat/wheat  21 May to 1 August
 Short coleoptile
 2. Long term (post-1980s)
 Direct drill, minimum till
 Fully retained
 Uncontrolled until autumn  Wheat/wheat  21 May to 1 August
 Short coleoptile
 3. Fallow
 Direct drill, minimum till
 Fully retained  Full control
Wheat/wheat
 21 May to 1 August
 Short coleoptile
4. Sequence
 Direct drill, minimum till
 Fully retained  Full control
Forage peas/wheat
 21 May to 1 August
 Short coleoptile
5. In-crop
Direct drill, minimum till
Fully retained
Full control
Forage peas/wheat
25 April to 1 July
Short coleoptile
6. Genotype
 Direct drill, minimum till
 Fully retained  Full control
 Forage peas/wheat
20 April
 Long coleoptile

GRDC Project Code CSP00111

Region South, National, North, West