One of the architects of the much-criticised nutrient budgeting program Overseer has defended the software, saying it might not be perfect but it is a world-leading model.
Dr Ants Roberts says Overseer, developed in the early 1990s, is up to version six, incorporating new technology.
It helps farmers see what is happening to the nutrients they apply to their farms, such as fertiliser. Controversially, it is being used by Environment Canterbury (ECan) to assess how much nitrogen is leaching beyond a crop's root zone, and into groundwater and waterways.
Farmers say the software should not be used as a regulatory tool and is not able to accurately handle some cropping and irrigation situations. The Foundation for Arable Research is reviewing the program.
Dr Roberts says Overseer is a mathematical interpretation of a biological system, and is a tool to manage nutrients and report on nutrient losses.
"There are a lot of people who have never opened an Overseer program who have been commenting on it," he told the Ashburton Water Zone Committee recently. "There is a lot of misunderstanding about it."
He says the latest version has been tested at the Lincoln University dairy farm, where Overseer-estimated nitrogen losses closely matched actual losses recorded by lysimeters.
Dr Roberts, chief science officer for fertiliser company Ravensdown, was on the team that brought Overseer into the world two decades ago. The program was developed by AgResearch and part-funded by the fertiliser industry and government departments to the tune of $15 million.
The software is free to farmers, who developers say can stop wasting money on applying fertiliser that is not required and halt environmental damage from overuse of fertiliser.
Dr Roberts says Overseer can handle complex cropping rotations and stock mixes. More money is needed to improve it to cope with some irrigated scenarios. The software requires annual rainfall figures as it takes a long-term view - the amount of irrigation needs to match rainfall inputs, otherwise Overseer may overestimate drainage and nitrogen loss.
In his presentation to the zone committee, he outlined the main driving factors for nitrogen leaching: soil texture, soil drainage, rainfall and irrigation, pasture development, animal type and winter management.
The fertiliser industry expects to be busy helping farmers adjust to nutrient limits through the development of nutrient-management plans, an essential part of farm-management plans. Ravensdown has carried out nearly 3000 nutrient-management plans nationwide for dairy farmers.
In Canterbury alone there are about 16,000 farmers - from big corporate to lifestyle-block owners - and all are required to have a nutrient budget. Will the industry have them compliant by 2017, as ECan's Land and Water Regional Plan requires?
Dr Roberts says the workload is worrying but Ravensdown management has said its farmer shareholders would meet the deadline.
The fertiliser industry said Overseer would be a valuable tool for farming within environment limits. Use of the program has enabled nutrient-budgeting to be rolled out to virtually all dairy farmers, and there has been a good start made with sheep and beef farmers as well, says Dr Philip Mladenov, chief executive of the Fertiliser Association of New Zealand.
"We have to be careful that we manage its development so that it continues to add value by supporting profitable farming within environmental limits."
Dr Mladenov says expectations of what Overseer can do have risen while new technology is continually becoming available.
One of the key intentions of an August upgrade was to improve modelling of particular farming types or local conditions.
"As a result of requirements in the Canterbury region, for example, the model's drainage component has been improved to achieve more reliable estimates on shallow and stony soils, which drain quickly and where there is growing use of irrigation."
DairyNZ's strategy and investment leader for productivity, Dr Bruce Thorrold, says the dairy industry supports the development of Overseer as a useful tool: "We support Overseer for farm-management decisions when it is used by people who are trained and competent in its use," he says.
Dr Mladenov says Overseer is an important guidance tool but warns that it has to be seen for what it is. "Overseer was designed to model nutrient movements and losses in a variety of farming systems - not specify exact amounts in every situation. It is an important distinction."
Meanwhile, advancements in the support around Overseer are under way.
These will include improved training in its use and certification of those involved in providing advice to farmers.