Re-evaluating the contribution of summer fallow rain to wheat yield in southern Australia

Abstract

In southern Australia, summer fallow rain (SFR) has not traditionally been valued for winter crop production. Modern, higher yielding farming practices combined with a decade of below-average growing-season rainfall and a predicted increase in the proportion of summer rain under future climate patterns have stimulated a re-evaluation of this notion. We used a widely validated crop simulation model (APSIM-Wheat) to quantify the potential value of SFR to wheat yield under contemporary farming practices using long-term climatic data at 37 locations throughout southern Australia. The potential value of SFR was high, contributing on average 1.0 t/ha or 33% of water-limited attainable yield. Yield increases were due to both increased water use and increased water-use efficiency through higher harvest index. The contribution to yield varied significantly according to the rainfall distribution and soil type across sites. In central-west New South Wales, the equi-seasonal rainfall pattern, high soil water-holding capacity and variable spring rainfall resulted in SFR contributing up to 2.0 t/ha or 72% of mean simulated wheat yield. In contrast, in the north-western grain belt of Western Australia, SFR contributed as little as 0.1 t/ha or 3% of mean simulated yield due to strong Mediterranean rainfall pattern, low soil water-holding capacity and reliable growing-season rainfall. At all locations there was significant year-to-year variation in the simulated yield contribution of SFR. At a given site, soils with higher water-holding capacity in the surface tended to reduce summer fallow efficiency (proportion of summer rainfall stored at sowing) due to the failure of most summer rain to penetrate below the evaporation zone. Despite seasonal variability in yield contribution, interventions to preserve summer rainfall, such as strict summer weed control generated high return on investment (range 6–1328%; mean 733%). Risk of financial loss due to strict summer weed control varied across sites, with failure to achieve a return on investment occurring in 1–70% of years depending on location. The proportion of annual rain falling during the summer fallow period in some locations has increased in the last 10 years and this forecast result of climate change is likely to increase the value of SFR to wheat production in the future.