Nitrogen management to optimise canola production in Australia

Abstract

The expansion of canola production in Australia coincided with an increase in cropping intensity and a reduction in pastures and tillage. These changes mean that nitrogen (N) is often recognised as the most limiting nutrient in canola production, and is the largest single input cost for many growers. Canola responds to added N by producing larger plants that results in a longer leaf area duration, building a larger photosynthetic canopy for seed filling. Although the crop can compensate for poor early growth, a larger canopy is able to compete more effectively against weeds and helps reserve water for crop transpiration rather than soil evaporation. Nitrogen uptake is most rapid during stem elongation, and the N acquired can be remobilised to developing pods and then to seeds. Unlike wheat, N uptake can continue until drought or high temperatures prevent further assimilate supply to the reproductive apex. Data from Australian experiments that measured N uptake over the whole growth period showed that each tonne of seed required ~80 kg N to be taken up, and this forms the basis of a budgeting approach for determining N supply. Typically, added N reduces seed oil concentration at a rate of between –0.03 and –0.13%/kg N. Despite this decline due to added N, oil yield usually increases and the overall value of the crop also increases. Nitrogen has little impact on oil quality or seed glucosinate concentration. The efficiency and effectiveness of N management depends first on selecting a rate appropriate to the water-limited yield potential. Most growers estimate the N rate required using an N budget based on supplying 80 kg N/t less indigenous N supply. The budgeted N can be split over two, three or even more applications with little loss in agronomic efficiency. Splitting application enables growers to make decisions about N when there is more certainty about seasonal conditions. Urea is the most common N source used, and unless there are particular loss processes that are likely to occur, it is cheap and effective. Suggested areas for future N research on canola are to develop tools that can assess in-crop N status, an evaluation of late season N product rate and timing particularly on seed oil concentration, N management for grazed canola, and the development of guidelines to identify, and then address, particular N loss pathways using enhanced efficiency fertilisers.