Development of nitrogen dilution curves for current Australian wheat varieties

Accurate nitrogen (N) application rates are of major importance in agriculture. While fertiliser guidelines have improved, the relationship between yield and N rates remain the main reference. However, this relationship is highly variable with soil, variety and season. A better reference is a ‘nitrogen dilution curve’ which relates the critical N concentration of wheat shoots (i.e. the lowest concentration of N needed to achieve maximum growth and (theoretically) maximum yield), with biomass. Crops can be benchmarked against this curve to determine their N status, i.e. assess if N fertiliser is applied at the correct rate, or if the crop is over- or under-fertilised. Nitrogen dilution curves exist for winter wheat, but these are developed for irrigated (non-drought stressed) crops and for older (European and Asian) wheat varieties. In South Australia, winter wheat is grown under rain-fed conditions where water and N may co-limit biomass production. In addition, breeding for grain yield has shifted the biomass-nitrogen balance over the past 20 years and the original dilution curve needs to be updated. Here we introduce the GRDC funded project “Benchmarking wheat yield against nitrogen use” (2014-2017). During this project N dilution curves will be produced for current wheat varieties under irrigated and rain-fed conditions. We will assess how drought stress may change the parameters of the curves and how this affects yield. The curves can be used to benchmark wheat in both growers’ fields and National Variety Trials. Here we present preliminary results from the first year (2014) of the project and outline the trials for the next two years.