Quantifying the spatial pattern of the yield gap within a farm in a low rainfall Mediterranean climate

Quantification of the gap between water-limited yield and actual crop yields at a sub-field scale across grain farms are now becoming possible in industrialised agriculture because of routine yield mapping and readily available methods to estimate potential yield. Analyses at sub-field scale can be used to target intervention strategies such as matching fertiliser inputs to yield variation or amelioration of soil constraints. As such they complement the predominant "top-down" approaches to the quantification of yield gaps and are relevant to individual farm decision making. In this study we bring together yield maps, farm soil surveys, and rainfall records to generate whole-farm maps of actual yield, water-limited potential yield, the yield gap, the contribution of soil constraints to the yield gap, and a list of the feasible interventions to close that gap. We apply the approach in a case study of a 4500 ha rainfed farm, located in the low-rainfall cropping zone of the Western Australian grainbelt We found a substantial gap between water-limited potential yield and actual yield, in an environment where wheat yields typically range between 0.5 and 2.5 t/ha. On a whole-of-farm basis, mean wheat yield was 50-60% of the water-limited yield and the yield gap varied from 0.6 t/ha in a dry year to 1.5 t/ha in wet year. The size of the yield gap varied among and within soil types due to associated soil constraints, and the analysis suggested that addressing the predominant constraint of acidity on the average performing areas on the farm could generate the greatest gains, rather than focussing on the below-average areas that have large yield gaps, which are afflicted by intractable soil constraints. The approach can be used by advisors and farmers to assess the proportion of the farm that reaches attainable or water-limited yield, and a tool to target regions within the farm that fail to reach attainable yield. This style of analysis lends itself to landscapes where there is a strong association between crop yield and soil type and environments where yield is strongly determined by rainfall and hence water-limited yield can be easily calculated. With an increase in the routine use of yield monitors by grain growers we anticipate an increase in yield map coverage across farms that will eventually make our method readily applicable. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.