Chao Chen1, Noboru Ota1,
The size of the yield gap, defined as the difference between water-limited potential yield and farm yield, is often a result of a combination of management activities. A small yield gap implies that management is near optimum. Nitrogen (N) deficiency often limits crop yields and it is essential to understand its effect of N fertiliser on yield gap. This study assessed whether N fertiliser was deficient and limited dryland wheat yield in Western Australia in 2015, using the APSIM model and agronomic practices surveyed from 47 paddocks. The farm survey indicated that the N fertiliser applications ranged from 0 to 98 kg/ha, with 80% of surveyed paddocks applying less than 45 kg/ha. The simulated yield with N fertiliser applied on farm (farm yield) ranged from 1.9 to 5.7 t/ha with an average of 3.2 t/ha. The yield gaps due to N fertiliser deficiency ranged from 0 to 1.6 t/ha, with an interquartile range of just 0.6 t/ha. The range of the relative yield gap (expressed as yield gap % of water-limited potential yield) was from 0 to 34%, with only 2 out 47 paddocks having a relative yield gap > 30%. We concluded that wheat yield was not limited by N deficiency in the majority of surveyed farms in Western Australia in 2015 due high soil mineral N at sowing and mineralised N. However, there is still a potential to increase yield for paddocks that had higher potential yield through improved N fertiliser management.