Are urea-based enhanced-efficiency fertilisers widely appropriate for reducing nitrogen application rates in sugarcane production;

Possible reductions in nitrogen (N) application rates using urea-based enhanced-efficiency fertilisers (EEFs) have recently been highlighted within the Queensland sugar industry. It was suggested that these commercially available products provide growers with an ability to reduce their N inputs and improve N-use efficiency (NUE) without negatively affecting on-farm productivity and/or profitability. This paper reports on the results of an investigation conducted in the Herbert district during the period 2015?2020. The aim was to assess the efficacy of two EEFs and standard urea on N-uptake, and cane and sugar yields over a crop cycle. The plant crop was harvested in 2016 and the four ensuing ratoons were harvested in 2017?2020. The EEF formulations were urea coated with a nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP-urea) and poly-coated urea (PC-urea). The trial site was located on a clay soil with a soil organic carbon (Org C) content of 2.15% in a poorly-drained position subject to seasonally high water-tables and/or persistently wet conditions. Such conditions had previously been identified as particularly appropriate for EEFs to enhance NUE and productivity when denitrification losses could be expected. Seasonal climatic conditions varied markedly during the trial period contributing to overall yield differences among the ratoons. In the relatively dry 2017/18 season, the highest mean yield was 113 t cane/ha. In contrast only 63 t cane/ha (determined as biomass) was achieved in 2018/19 which was a particularly wet season. The SIX EASY STEPS N guideline (110 kg N/ha) for ratoon cane grown on a soil with 2.0-2.4% Org C was confirmed as generally applicable per season. Despite significant N-uptake and yield responses to applied N in the ratoon crops, use of EEFs did not contribute to improved yields compared to each other or standard urea at the same rate. At the site, reducing N rates below the SIX EASY STEPS guideline, irrespective of the N-formulation used, would adversely affect yield within seasons and across the crop cycle. Although EEFs may provide an opportunity for improved environmental stewardship through improved NUE in some circumstances, they should not be considered an appropriate mechanism for systematically reducing the amount of N applied. Such uses of EEFs could put the industry at risk of possible losses in on-farm productivity and/or profitability, cane supply, and mill throughput and viability.