Optimal N fertiliser management based on a seasonal forecast

Achievable grain yields can vary widely between seasons in rain-fed agriculture. Adjusting N fertiliser inputs according to achievable grain yields could reduce over-fertilisation in low-yielding seasons and allow increasing gross margins in potential high-yielding seasons. Seasonal rainfall forecasts from the coupled ocean-atmosphere global circulation model POAMA were skill tested and employed for N fertiliser decision making in the Western Australian wheat-belt. The POAMA seasonal rainfall forecast showed significant skill in forecasting rainfall season types in southern regions of the Western Australian wheat-belt. This skill resulted in about A$50ha ^-1 of additional benefits when used in N management decisions in wheat cropping. However, such a forecast should not be used without considering other systems knowledge available to farmers. Combining a forecast with systems information such as initial soil water conditions can be crucial in obtaining value from a forecast. Another important factor to consider is the risk behaviour of farmers, where the gross margin from additional fertiliser is expected to exceed the cost by a factor of two or more. Finally, variations in fertiliser cost and wheat prices are critical in determining the benefits from using a forecast system for management decisions in agriculture. Using a forecast for only the wet season-type can further increase a forecast value because the additional gains in wet seasons are often higher than the savings from reduced fertiliser in dry seasons. It is expected that skilful seasonal forecasting systems will become increasingly valuable in regions where rainfall is decreasing because they help to capture benefits in the declining number of potentially high-yielding seasons and minimise the losses in the increasing number of low-yielding seasons.