The cropping model’s capability falls short as a management tool to improve water quality, with one industry leader far from convinced that the model will ever be any good.
The review suggests all users of the model must recognise its limitations, as it is increasingly being adopted as a regulatory tool by regional councils.
The Foundation for Arable Research’s (FAR) review of Overseer has resulted in some wide-ranging recommendations to improve the management planning tool in respect to arable farming.
The review panel found that Overseer, while currently the best tool available for estimating long-term, average nitrate-leaching losses from the root zone across the diversity and complexity of farming systems in New Zealand, needs further work on the cropping model to enhance confidence in estimates of nitrate leaching from arable farms.
The review was initiated by FAR in August last year and was carried out by science and industry specialists from NZ, Australia and the United Kingdom after concerns Overseer was becoming a major element of many regional authority plans for improving water quality.
“Although this computer model has a well-established role in managing nutrients in pastoral systems, its use in estimating nutrient losses from cropping farms is a new and relatively untested application,” FAR director of research development Dr Roger Williams said.
“FAR decided it was time to lift the bonnet on Overseer and take a look at the science underpinning the modelling of cropping systems.”
There were two key problems – the practicalities of inputting cropping data and confidence in the accuracy of the modelling.
While the report was welcome, Federated Farmers national grain and seed chairman and spokesman for water and the environment Ian Mackenzie was far from convinced Overseer can do the job.
“The report is good in that it highlights two important factors: one, that regional councils were using Overseer as a regulatory tool prematurely; and two, that some regional councils are being somewhat lazy on addressing water quality and aggressively attacking nitrates when there are also other causes in respect to water quality,” the Mid Canterbury cropping farmer said.
“We all want to see the best water quality and while the report suggests Overseer is the right tool to use for what it is designed for, it has no way or means to measure nitrate losses.”
There were better ways to manage water quality than trying to further develop Overseer. Good management practice, as outlined through the Land and Water Forum, was preferable, Mackenzie said.
“Overseer is not the right tool to measure nitrates and I don’t believe it ever will be. There is an awful lot of work to be done to get it near right to measure nitrates. By the time they get it right we all will be dead.”
Mackenzie acknowledged the “intelligent” regional councils were listening, but there were those that were clearly not understanding of the limitations of the model, including the Environment Court.
“It (Overseer) is designed for predicting the difference in nitrate levels but not to specifically measure actual nitrate losses. It will only ever be good at making assumptions,” Mackenzie said.
The independent expert panel had made a range of recommendations to Overseer’s owners, AgResearch Ltd, the Fertiliser Association of New Zealand and the Ministry for Primary Industries.
These recommendations include further validation of the cropping model through field experiments, comparison with other crop model predictions of nutrient losses, and an overhaul of the way that users enter data about crop rotations.
The panel recommended introducing independent peer review into the process of Overseer development, with information and training so that all users, including regional authorities, understood the strengths and limitations of Overseer.
An increase in funding was recommended for further work on the crop model and other aspects of Overseer improvement.
“We considered alternative crop models for estimating long-term leaching from arable farms but we are confident that building on the existing strengths of Overseer is the way to go,” Williams said.
“FAR is already talking to the owners about implementation of the panel’s recommendations and we look forward to a joined-up approach to making sure Overseer will meet the future needs of the arable sector.”
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