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It has long been recognized that pasture production in New Zealands hill country, beef and sheep sector, is highly variable. Annual pasture production on individual farms typically varies between 4,000 to 11,000 kg DM ha-1. The terrain is often difficult to access and travel on with wheeled vehicles, this lead to the use of topdressing aircraft for fertilizer application some 50 years ago. Over the last decade the industry has seen the uptake of GPS technology for guidance assistance for the pilots. This project has taken it one step further by implementing variable rate technology for fertilizer application.

The criteria for deciding on the fertilizer application rate are presently the slope and aspect of the terrain. The properties are mapped using existing imagery and a digital elevation model created. Other areas such as native bush and recognized waterways are filtered out in order to avoid fertilizer contamination. The system will shut off flow before the water body is reached and start applying once the water body is crossed. The system does not restrict the jettisoning of fertilizer or lime in an emergency.

The system has been tested and meets Civil Aviation Authority (CAA) requirements. The shut off distances and distances to change flow rate have been tested, shut off is achieved within 0.5 of a second, the distance this translates to, in terms of fertilizer landing on the ground, depends on aircraft height, speed, material and rate being spread. The system is presently being used in the North Island of New Zealand to evaluate the technology.