David Gobbett

Recent changes in southern Australian frost occurrence: implications for wheat production risk

Abstract. Frost damage remains a major problem for broadacre cropping, viticulture, horticulture and other agricultural industries in Australia. Annual losses from frost events in Australian broadacre agriculture are estimated at between

Yield gap analysis of rainfed wheat demonstrates local to global relevance

120 million and

Regional scale application of the precision agriculture thought process to promote improved fertilizer management in the Australian sugar industry

700 million each year for this sector. Understanding the changing nature of frost risk, and the drivers responsible, are important steps in helping many producers manage climate variability and change. Our analysis, using Stevenson screen temperature thresholds of 2°C or below as an indicator of frost at ground level, demonstrates that across southern Australia, despite a warming trend of 0.17°C per decade since 1960, ‘frost season’ length has increased, on average, by 26 days across the whole southern portion of Australia compared with the 1960–1990 long-term mean. Some areas of south-eastern Australia now experience their last frost an average 4 weeks later than during the 1960s. The intersection of frost and wheat production risk was quantified at 60 sites across the Australian wheatbelt, with a more in-depth analysis undertaken for 15 locations across Victoria (i.e. eight sites common to both the National and Victorian assessments and seven sites exclusive to the Victorian analysis). The results of the national assessment highlight how frost-related production risk has increased by as much as 30% across much of the Australian wheatbelt, for a range of wheat maturity types, over the last two decades, in response to an increase in later frost events. Across 15 Victorian sites, sowing dates to achieve anthesis during a period with only a 10% chance of a 0°C night occurring shifted by 23 days (6 June) for the short-season variety, 20 days (17 May) for the medium-season variety and 36 days later (9 May) for the long-season variety assessed.

Quantifying yield gaps in rainfed cropping systems: A case study of wheat in Australia

Australia has a role to play in future global food security as it contributes 0·12 of global wheat exports. How much more can it contribute with current technology and varieties? The present paper seeks to quantify the gap between water-limited yield potential (Yw) and farmer yields (Ya) for wheat in Australia by implementing a new protocol developed by the Global Yield Gap and Water Productivity Atlas (GYGA) project. Results of past Australian yield gap studies are difficult to compare with studies in other countries because they were conducted using a variety of methods and at a range of scales. The GYGA project protocols were designed to facilitate comparisons among countries through the application of a consistent yet flexible methodology. This is the first implementation of GYGA protocols in a country with the high spatial and temporal climatic variability that exists in Australia. The present paper describes the application of the GYGA protocol to the whole Australian grain zone to derive estimates of rainfed wheat yield gap. The Australian grain zone was partitioned into six key agro-climatic zones (CZs) defined by the GYGA Extrapolation Domain (GYGA-ED) zonation scheme. A total of 22 Reference Weather Stations (RWS) were selected, distributed among the CZs to represent the entire Australian grain zone. The Agricultural Production Systems sIMulator (APSIM) Wheat crop model was used to simulate Yw of wheat crops for major soil types at each RWS from 1996 to 2010. Wheat varieties, agronomy and distribution of wheat cropping were held constant over the 15-year period. Locally representative dominant soils were selected for each RWS and generic sowing rules were specified based on local expertise. Actual yield (Ya) data were sourced from national agricultural data sets. To upscale Ya and Yw values from RWS to CZs and then to national scale, values were weighted according to the area of winter cereal cropping within RWS buffer zones. The national yield gap (Yg = Yw-Ya) and relative yield (Y% = 100 × Ya/Yw) were then calculated from the weighted values. The present study found that the national Yg was 2·0 tonnes (t)/ha and Y% was 47%. The analysis was extended to consider factors contributing to the yield gap. It was revealed that the RWS 15-year average Ya and Yw were strongly correlated (R = 0·76) and that RWS with higher Yw had higher Yg. Despite variable seasonal conditions, Y% was relatively stable over the 15 years. For the 22 RWS, average Yg correlated positively and strongly with average annual rainfall amount, but surprisingly it correlated poorly with RWS rainfall variability. Similarly, Y% correlated negatively but less strongly (R = 0·33) with RWS average annual rainfall, and correlated poorly with RWS rainfall variability, which raises questions about how Australian farmers manage climate risk. Interestingly a negative relationship was found between Yg and variability of Yw for the 22 RWS (R = 0·66), and a positive relationship between Y% and Yw variability (R = 0·23), which suggests that farmers in lower yielding, more variable sites are achieving yields closer to Yw. The Yg estimates appear to be quite robust in the context of estimates from other Australian studies, adding confidence to the validity of the GYGA protocol. Closing the national yield gap so that Ya is 0·80 of Yw, which is the level of Yg closure achieved consistently by the most progressive Australian farmers, would increase the average annual wheat production (20·9 million t in 1996/07 to 2010/11) by an estimated 15·3 million t, which is a 72% increase. This indicates substantial potential for Australia to increase wheat production on existing farmland areas using currently available crop varieties and farming practices and thus make a substantial contribution to achieving future global food security.

Climate trends account for stalled wheat yields in Australia since 1990

Nitrogen (N) fertilizer management in the Australian sugar industry is guided by the ‘SIX EASY STEPS’ (6ES) advisory program, for which the potential yield and amount of N that is potentially mineralizable from the soil are key input parameters; the latter is estimated from soil carbon (C) content. Whilst 6ES is not prescriptive about the scale at which it is used to deliver advice to sugarcane growers, common practice is to use the ‘district yield potential’ (DYP) to guide N fertilizer recommendations at the farm and block scales. Analysis of yield variation at the block scale, using sugar mill records over 7 seasons (2009–2015) from the Herbert River district, showed yield to be markedly spatially variable, with the patterns of this variation stable across seasons and crop class. Accordingly, DYP is sub-optimal as an input to 6ES. A block yield potential (BYP), derived from a map of the estimated maximum block-scale yield of first ratoon cane achieved over the 7 seasons, is suggested as a better alternative which can be readily updated as more data become available. Further refinement of the application of 6ES is possible with access to soil C data, derived from either regional soil survey or local soil testing. The present study suggests that use of BYP rather than DYP could lead to a total annual reduction in N applied of approximately 1700 t N over the Herbert River district without negatively impacting yield. Whilst the value of this to growers (A

Reprint of “Quantifying yield gaps in rainfed cropping systems: A case study of wheat in Australia”

23/ha) is a minimal proportion of the costs of production, a reduction in the risk of N loss to receiving waters of this magnitude could be of significant benefit to the protection of the Great Barrier Reef. Since data similar to those used here are collected by all sugar mills, similar analyses could be conducted in other sugarcane growing areas. The approach may also be of value in other cropping systems which use central points of delivery (e.g., grain silos).