The Decisions...

Pre-irrigation is the first decision of the season and is not without its own risks. Get it right means timely sowing, excellent establishment and enough soil moisture to build sufficient crop biomass through winter until the irrigation system is able to supply water in the spring.

With surface irrigation, it is an ‘all or nothing’ decision and timing depends on your level of risk you want to take – too early and you risk losing surface moisture that delays establishment, too late and additional rainfall delays sowing, waterlogging may occur, or autumn rainfall was sufficient meaning the pre-irrigation was an unnecessary expense.

Overhead irrigators have more options in that the amount of water applied can be tuned to the autumn or water price, but still have to decide on the level of soil moisture they want to have going into the winter where not enough will impact on crop growth if they cannot access irrigation water through the winter.

The broadleaf crops need to accumulate more biomass and nutrients as they rapidly grow in spring to attain their potential. The cereals need moisture to maintain tillers and keep the number of flowers (and hence grains) alive to keep their potential. Failure to do so will result in lost yield that cannot be recovered by a late irrigation.

For the cereals, the period from stem elongation through to flowering is a period where moisture stress can reduce yield potential. During stem elongation, plants will respond to moisture stress by allowing tillers to die, which cannot be compensated for by a spring irrigation. In late stem elongation and spike (head) development, moisture stress will see the plant reduce the number of florets (flowers) developed, limiting the potential number of grains. The critical period to ensure there is no moisture stress is three weeks before anthesis (flowering), or about booting stage. Moisture stress post-flowering will reduce grain size, hence yield.

Pre-irrigation - pros and cons

The 2022 season exposed all the decision-making surrounding pre-irrigation; paddock layouts with poor drainage, spring water logging, winter/early spring drought scenarios resulting irrigation applications in July where water was available. In many cases in 2022 the only irrigation applications made to paddocks were in the autumn or winter as the spring was so wet.

Three years of GRDC’s Optimising Irrigated Grains (OIG), on top of research conducted under the ‘Smarter Irrigation for Profit’ project, have highlighted the irrigation decisions that need to be made by irrigators on how and when to use their irrigation water to set up their irrigated crops to be the most profitable.

The changing irrigation environment has seen irrigation water become an input where the price can be highly variable based on seasonal conditions and allocations. Efforts to make irrigation more efficient has seen investment in improved layouts and infrastructure such as overhead sprinklers or fast flow surface irrigation, giving irrigators flexibility in the amount of water applied and the choice of crops.

Pre-irrigation (where fallow paddocks are irrigated prior to the sowing of a crop) has always been a judgment call by irrigators, based on timing to enable timely sowing and adequate moisture for the crop to develop over winter. Using surface irrigation, could mean using anywhere between 0.75 to 2.0 Mega litres/ha (75-200mm/ha) to wet up the soil profile. The timing of pre-irrigation must be considered in order to allow the paddock to dry sufficiently to enable sowing on time, but not to dry too much and then be at the mercy of ‘the autumn break’ for sowing similar to a dryland grower. Many irrigators have a story about the pre-irrigation that went badly – where it rained, and sowing couldn’t proceed, or winter waterlogging was detrimental to the crop as the soil profile was full going into winter. However, pre-irrigation does provide soil moisture over winter as some irrigation regions do not have access to water between 15 May and 15 August to allow the water authorities to service and repair the water delivery network.

Irrigators have installed overhead irrigation as a means to be able to have more control over the amount of water applied. Instead of the large volume of water applied via surface irrigation as a pre-irrigation, irrigators can apply enough water to ensure timely establishment of their crop. This can be a considerable saving of water but does then run the risk of a ‘winter drought’ if the winter period is dry and winter rainfall is inadequate to meet the needs of the crop. In these cases, yield potential is lost before the irrigation water becomes available in the spring. In shorter season crops or in warmer regions where spring growth occurs earlier (before mid-August) yield potential starts to be reduced since crops are stem elongating but without the water reserve to sustain this period of rapid development.

The OIG project, with its geographically diverse project partners, has illustrated the different thinking that drives irrigators decision making on irrigation. Higher rainfall regions are unlikely to pre-irrigate due to the risk of autumn irrigation leading to waterlogging if they go into winter with a full profile. Similarly, those in the east of the Murray and Murrumbidgee valleys are more confident of a timely break for sowing and follow-up winter rainfall to get the crop through to the spring when irrigation can commence. Those to the west who have soils (e.g. grey clays) that require more water to fill the profile, are less confident of the break being in late April/early May and have lower winter rainfall to tide them over until the irrigation season opens in the spring. Depending on the crop type, restoration of yield potential with spring irrigation following a winter drought can be more limited with early maturing wheat, since it has already started developing rapidly whilst the crop is under spring drought conditions. In some cases, the restoration of yield potential is adequate (e.g. faba beans) but this does depend on whether heat stress was additional to the lack of soil moisture and becomes part of the yield equation. These geographical differences also manifest themselves in the responses to disease management where irrigation does not appear to favour conditions that promote the fungal diseases compared to the naturally more disease prone high rainfall zones.

Drainage and paddock aspect has also been noted as a key part of decision making with some growers avoiding pre irrigation using surface flow systems on paddocks prone to water logging or which have poor drainage.