Nitrogen (N) plays a crucial role in crop production but causes substantial economomic and environemtnal costs. Farmers must understand whether they are optimising their N fertilisation. Grain yield hardly changes as N supplies become superoptimal, but grain protein (or N%) continues to respond, so is the best indicator of whether a crop’s N supply was optimal. Farmers can use grain protein retrospectively to understand whether their crop took up enough N. Growers should target a protein level of approximately 11% (or 1.9% N in grain DM) for feed wheat, or variety specific protein levels may be published for the region. Grain protein levels below target suggest that N availability or uptake was insufficient. Conversely, high protein content suggests over-application of N fertiliser. A protein response of 1% relates to a change in total N applied of 50 kg/ha, and indicates the change to N application necessary to achieve optimal N use. Farmers should analyse samples from barometer fields over several years, to draw robust conclusions. However, even results from grain stores can indicate the success of N management on the farm.