Which Is the Most Important Crop When Assessing Nitrogen Applications the next or the Last?

A review of nitrogen (N) fertiliser management in the Australian sugar industry was undertaken to identify new strategies to improve N fertiliser use efficiency and to fill gaps in knowledge relevant to N fertiliser management. The Australian sugar industry has a history of high N fertiliser usage, with applications increasing from the 1960s to the late 1990s. However, industry average sugarcane production has not kept pace with N fertiliser applications, resulting in a steady increase in N fertiliser applied per ton of sugarcane harvested. Historical and recently developed N management strategies rely on matching N applications to the predicted/expected yield of the forthcoming crop. High applications of N fertiliser by farmers are a rational reaction to (i) the uncertainty of these predictions/expectations, and (ii) the yield reductions caused by long-term under-fertilising with N. While sustained under-application of N undoubtedly reduces cane yields, there is considerable evidence in the Australian industry that greatly reducing N fertiliser applications for a single crop does not significantly reduce sugarcane production. Consequently, a new philosophy of N fertiliser management can be envisaged, that removes the uncertainty driving farmers’ decisions to over-apply N. Rather than aiming at fertilising the coming crop, one could aim at replacing the amount of N lost from the previous crop, the majority of which is in harvested cane and can be easily estimated. To better examine this new ‘replacement’ N management strategy, long-term simulations were undertaken of sugarcane production in four regions in Australia and one in South Africa. The replacement strategy had similar productivity, greater profitability and lower environmental N losses than conventional N fertiliser management. The choice of soils and the inclusion of a South African climate in the simulations ensured that the results are biophysically relevant to the South African industry. However, before this system can be considered as a practical N fertiliser management system in any country, it needs extensive field testing. A testing programme is commencing in Australia.