Applying more nitrogen is not always sufficient to address dryland wheat yield gaps in Australia

Abstract

Abstract Yield gaps, which reflect the difference between the yield a grower achieves in a field, and the water-limited yield potential of that field, have been extensively discussed in the last decade. To date, most analyses have occurred at regional levels, and derived insight through surveys, remote sensing, or small scale targeted case studies. Here, we provide an analysis based on a survey of 250 fields from 2015 to 2018 inclusive. Biotic factors, nutrients and crop yields are all monitored. Crop models were calibrated locally at each site. Regression and Classification Trees (RCART) were used to determine the key drivers of the yield gap in wheat, and again used to identify attributes contributing to a yield gap for a field regardless of crop type. The mean yield gap from 697 wheat crops, collected over 4 years, was 1.03 t/ha (MSE =2.23 t/ha). This equated to a mean wheat yield gap of 20.3 % ± S.D 36.2 %. 18 % of fields had repeated yield gaps of 20 % or more, irrespective of crop type. The RCART analysis demonstrated that for wheat, the yield potential of the crop was the most important predictor, where fields with high yield potentials were most likely to have a higher yield gap. Nitrogen was the second most important predictor. Yield gaps of wheat crops grown in high yielding regions were also related to crop rotation, leaf diseases and weed populations. In low rainfall zones, wheat yield gaps were related to the presence of soil pathogens including Pratylenchus sp., Pythium sp. and Fusarium sp. In conclusion, yield potential and nitrogen are important predictors of the yield gap. Increasing nitrogen inputs would address the yield gap in 25 % of wheat crops. In 22 % of wheat crops, increasing nitrogen inputs will not correct the gap, as other biotic stresses are often present, that require sophisticated agronomic intervention. In the remaining 53 % of wheat fields yield gaps were less than 0.37 t/ha.