Deriving seasonally optimal nitrogen fertiliser rates for a ryegrass pasture based on APSIM modelling with a refined AgPasture model

Productive pasture systems require a regular supply of mineral nitrogen (N) to replace the N removed through the export of animal products and other loss pathways such as leaching and emissions. High spatial and temporal variability of both N supply by the soil and plant demand means that synchronising these is challenging, and early indicators are lacking. A simulation study, using the Agricultural Production Systems Simulator with a refined version of APSIM’s pasture module (AgPasture), was done to determine optimum fertilisation rates for an irrigated ryegrass pasture in Canterbury, New Zealand. Simulations comprised 20 fertilisation rates. Improvements in AgPasture included ideal N concentrations for tissues of different ages (growing, mature, senescent and dead) and allowing N remobilisation to occur from all tissue stages. The critical N concentration curve was tested as an early indicator for guiding N fertilisation, which maximizes plant growth while minimising environmental impacts. Highest pasture yields were achieved with application rates of 20, 30, 140 and 160 kg N/ha for winter, autumn, summer and spring. Pasture N contents corresponding to the standing dry matter of the pastures in the different seasons were similar to the critical N curve when constant fertilisation rates were applied throughout the year, but lower critical N concentrations were obtained for autumn and winter. Before the critical N reference curve can be used to guide fertilisation for pastures further experimental studies,model testing and parameterisation are required. It is also possible that season specific critical N curves need to be established for pastures.