- Summer and autumn rainfall events of more than 25mm will generally provide good subsoil moisture at seeding. Good levels of stored moisture can improve crop yields.
- Weed control is vital to conserve soil moisture.
- Residue management has less impact on stored subsoil water availability than summer weed control.
Good summer and autumn rainfall across parts of the WA wheatbelt and south and west coastal areas may boost subsoil water availability for this season’s crops.
Moisture from rainfall events of 25-30mm between November and April is often still available in the soil at seeding.
Water from events of less than 25mm generally evaporates before sowing starts, especially on sandy soils.
These are key insights from an analysis by CSIRO and Department of Agriculture and Food WA (DAFWA) researchers using two different models - the Simultaneous Heat and Water (SHAW) model and APSIM.
According to CSIRO research scientist Phil Ward, who headed the GRDC-funded analysis in 2012, the average amount of summer and autumn rainfall retained in WA soils is usually about 20-30 per cent.
This means if total summer rainfall was 100mm, about 20-30mm could still be in the soil profile on May 1.
An extra 30mm of soil moisture at seeding is estimated to increase yield by up to 0.6 tonnes/hectare, especially in dry conditions when moisture is the limiting factor. (See Table 1.)
Table1: Simulated impact of summer rainfall on crop yields for locations in WA (after Hunt et al 2011). SOURCE: Phil Ward, CSIRO
|| Grain yield (t/ha)
Total PAWC (mm)
|PAW in May (mm)
||With summer rain
| Without summer rain
Modelling of sandy soils, where there is good weed control, shows that most of the moisture received in rain falls of 50mm and 100mm from November to April is stored in the soil, with only 20-30mm lost to evaporation.
Heavier soils retain less from each fall, with about 20mm stored from a summer rainfall event of 50mm and about 60mm from an event of 100mm.
Timing of rain is more important on heavy soils, with later falls providing greater benefit.
Economic modelling indicates that summer weed control is profitable in 30-40 per cent of years in lower-rainfall areas and 50-60 per cent of years in wetter areas with higher yield potential.
Dr Ward and his team have found that, in WA’s northern wheatbelt, uncontrolled weeds reduce the amount of soil water available at seeding by 10mm.
And in one in every 10 years, there is 15mm less moisture in the soil at May 1 where weeds are not controlled than in paddocks with good weed control.
This was reinforced by results from a Merredin trial in which summer weed control increased crop yields by up to 10 per cent.
Summer weeds also reduce the amount of nitrogen available to crops because they use nitrogen mineralised after summer and autumn rainfall.
In other trials at Merredin, summer weeds reduced mineral nitrogen at sowing by up to 24kg/ha, depending on summer rainfall and weed biomass.
Residue management is unlikely to have a great impact on the level of stored soil water at sowing, according to the 2012 SHAW analysis, but can reduce wind and water erosion risk and extend the sowing window.
Modelling of conditions in the northern WA wheatbelt shows only a 4mm difference in stored soil water between areas with 3t/ha residue and areas with 0.5t/ha residue.
However, higher residue levels or standing stubbles encourage infiltration and can reduce evaporation for up to a week after a rainfall event. This may be useful in extending the sowing period after the break.
PHOTO CAPTION: Phil Ward, CSIRO
For more information:
Dr Phil Ward, CSIRO
08 9333 6616
Melissa Williams, Senior Consultant, Cox Inall Communications
042 888 4414
See 2013 Agribusiness Crop Updates Paper at: www.grdc.com.au/UpdatePapers
GRDC Fact Sheet: Time of Sowing, Western Region, When to Sow Wheat to Minimise Risk: www.grdc.com.au/GRDC-FS-TimeOfSowingWest
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