The conundrum of Overseer

COMPLICATED: Much of the drive to use Overseer outputs within a regulatory framework has been an attempt to address water quality.

Most farmers understand budgets and the importance of balancing them.

Whether you are managing your cash flow or your soil and plant nutrients, having the best available data upon which to make decisions for future management is a critical component of success.

The Overseer model has been developed by New Zealand scientists over the past 25 years to assist farmers and farm advisors to understand nutrient use and movement within the farm system so production is optimised and environmental impacts are minimised.

Because Overseer is a model, it provides estimates of nutrient use and movement, not absolute and exact measures. It's a model driven by production primarily because production is a straightforward parameter for farmers and farm advisors to either predict or to measure.

Overseer contributes significant value to New Zealand's primary sector by adding to the pool of knowledge that farmers and farm advisors have to assist in good decision making. This allows optimised sustainable production systems and minimises environmental impacts.

So we know that Overseer provides an estimate of nutrient use and movement. We also know that as Overseer evolves and information improves those estimates of nutrient use and movement also change.

The need to continue to refine Overseer and recognise the uncertainty inherent in an evolving model must be accommodated in any application of Overseer output. For example, continued work to better calibrate Overseer outputs under low and high rainfall systems and under complex cropping regimes must be a high priority to improve the estimates of nutrient use and movement by Overseer.

Much of the drive to use Overseer outputs within a regulatory framework has been an attempt to address water quality concerns. The assumption has been that if you know what is leaching beyond the grasp of roots then by reducing these nutrient losses water quality will be improved.

However, what happens between the bottom of the root zone and the water body of interest is poorly understood. Currently an estimated attenuation value is nominated which has very little refinement or recognition of landscape variables, hydrological features or even distance from the base of the root zone to the water body.

Attenuation has in the past been assumed to be 50 per cent, that is 50 per cent of the nutrient loss from the base of the root zone does not appear in the water body of interest. Poor understanding of the dynamics of nutrient attenuation is a significant knowledge gap and an area where increased research must be supported.

If we can improve our knowledge of attenuation mechanisms, putting in place measures which increase attenuation as well as minimise the level of nutrient loss from the farm system, then significant reductions in nutrient loading to water is likely to result.

Other recent initiatives which add to the robustness and consistency of Overseer outputs include the development of best practice data input standards for Overseer users and the nutrient management advisor certification programme administered by Massey University.

If Overseer is to be used within a regulatory framework, regulators must acknowledge that outputs of nutrient use and movement are indeed estimates which have an inherent margin of error and that as Overseer evolves different estimates from Overseer may be produced which don't reflect an actual change in farm nutrient losses but rather a change in the way those estimates are made and therefore quantified.

Ideally, Overseer is applied as part of a suite of tools which represent good management practice through the estimate of multi-year average nutrient losses which demonstrate a direction of change towards reduced nutrient loss over the long-term.

Use of Overseer assists farmers to maximise resource use efficiency and minimise environmental impacts of farming activity. This is what Overseer was designed to do and it does this very well.

¦Footnote: The National Farming Review invited agribusiness consultant Alison Dewes to discuss Overseer. The Headlands' lead consultant did initially make comments which were subsequently checked and approved by her. However, when asked for further clarification during a second interview the consultant requested that all comments be retracted as she longer wanted her views published.