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The Yield Gap

Summary

• Variability of crop yield, as shown by the wheat crop, has been greater since about 2000 than at any previous period since 1950.
• The ‘gap’ between average farm yield and estimated potential yield becomes greater as seasonal rainfall increases and there is considerable scope to increase grain yield in Western Australia.
• The yield ‘gap’ is less than 1 t/ha in the low rainfall shires, about 1 to 1.5 t/ha in the medium rainfall and from 1 to 3 t/ha in the high rainfall shires.
• Estimation of a ‘best’ case future scenario (increased yield and area) indicates that grain production could be increased by up to 60 per cent compared to a ‘worst’ case scenario (yield maintained and area decreased) of a reduction of 7 per cent.
• The major factors limiting grain yields are soil constraints, weed management, disease and pest control, and crop nutrition.
• Considering both the size of the yield ‘gap’ and the potential areas that could be brought into crop production,  the major potential to increase the value of the grain crop in Western Australia is in the southern medium and high rainfall zones.

Average farm yield of grain crops in Western Australia increased rapidly in the 1990s after a long period of more or less static performance over many decades. It is notable that the low variability of yield during the period of rapid increase has been followed by a period since about 2000 of quite extreme variability of yield. It might be inferred that an unstable yield plateau has been reached, characterised by variable yield and associated with extreme variability of seasonal rainfall.

It is also possible that the improved levels of management used by farmers since 2000 have also added to the inter-seasonal variability in grain yield in addition to the variability in seasonal rainfall. Variability of grain yield is not necessarily a problem of itself provided that water use efficiency of grain yield is maximised across a range of seasons through profitable means. However, there is considerable risk involved in committing resources during the season when the finishing rains are uncertain and this must be taken into account when estimating the value of closing the ‘gap’ between average farm yield and the water-limited potential yield.

The yield gap 

The evidence from rainfed environments indicates that when rainfall, or water supply (rainfall plus stored water), is less than about 250 mm in the growing season, rainfall itself has limited grain yield, and water use efficiency (kg grain per unit of water transpired) has been close to the theoretical maximum. However, these studies also show that when water supply is larger it is mostly management that limits productivity and examples have been given of how practices such as earlier sowing, appropriate fertilizer applications and adequate weed control can be used to lift yields closer to the potential at seasonal rainfall amounts up to about 350-400 mm. This suggests that the main scope for improving average grain yield across seasons resides in improving management to fully utilise all the rain that falls in higher rainfall years.

A technical report showing in more detail the calculated shire yield gaps for Western Australia will be released in coming weeks and uploaded onto this site.