Wednesday, July 6, 2022

Crop model needs growth

Improvements need to be made to the Overseer model before it can be effectively used by regional authorities to manage nutrient losses from arable crops. That’s the bottom line from a report commissioned by The Foundation for Arable Research (FAR). FAR research development director Roger Williams says they commissioned the report because a new cropping model had recently been integrated into Overseer and several regional councils indicated in their policy plans that they will be using Overseer to manage nutrient losses from all farming systems, not just pastoral.

“We’ve got the prospects of regional councils around the country including policy-setting around water quality,” Williams said.

“It indicated they would be using Overseer to estimate nutrient losses from all farming systems, which would include arable.”

The Overseer model, which was developed by owners AgResearch, the Fertiliser Association and Primary Industries Ministry, is already an established tool used by several regional authorities for nutrient management in pastoral farming.

However, the approach of the Overseer cropping model is vastly different to current arable nutrient management tools, he said.

In recent years nutrient management in arable crops has been based around seasonal real-time tailoring of fertiliser inputs according to soil fertility, crop needs and weather using tools such as Wheat Calculator and AmaizeN.

The cropping model which has been integrated into Overseer however, predicts long-term average nutrient flows and losses instead of looking at the real-time crop management decisions. It uses long term average annual climatic data, a monthly time step and a single soil layer whereas previous cropping models have been based on short-term climatic data, daily time steps and multiple soil layers.

“The key thing in cropping is we’ve had a suite of crop calculators which are designed to provide real time, in season, tactical guidance on nutrient management.

“Overseer looks at long-term average trends, so it’s trying to do something quite different to the conventional cropping tools that we’ve been using.”

Because of these differences, the report states that Overseer might not be able to model the impacts of crop management interventions which occur on a scale of days, weeks and months, for example timing of fertiliser to meet crop needs.

More testing is therefore needed to determine whether these simplifications impair Overseer’s capability for predicting long-term average nitrate leaching in arable systems.

“The two key issues that were found was doing some more testing to make sure we’re really confident in the estimates that it generates for arable systems – because a lot of the work has been done around pastoral systems.

“The second thing is the user interface and usability – because it’s pretty hard to model cropping rotations, it’s quite time consuming and we think there is more to be done there.

“I think Overseer is a really useful tool for pastoral farming systems but because of the nature of annual crops it’s less immediately useful, which is why we’ve got these other crop calculators.

“So its main purpose in the future could be in this water quality policy arena rather than being useful, in itself, for cropping farmers,” Williams said.

The report also looked at the expectations on Overseer from a wide range of stakeholders and the scrutiny that it is going to come under when using it as a tool for nutrient management. These stakeholders include farmers, policy agencies, nutrient management advisors, the owners of Overseer and the research community.

With many eyes on the performance of Overseer, the review found “there needs to be an open and independent review of the science underpinning the model, as well as the assumptions made in its application and the operational aspects of the model’”

The view stated that transparency will help to build confidence in the Overseer model. 

The report’s recommendations:

  • Compare Overseer estimates of leaching to measured values
  • Compare Overseer estimates of leaching to estimates of established research models
  • Continue to improve Overseer based on findings from recommendations 1 and 2
  • Establish a process for peer review of Overseer
  • Implement operational protocols
  • Facilitate wider stakeholder engagement in strategic development.
  • Improve the user interface of the crop model.
  • Inform and train Overseer stakeholders.
  • Review its governance.
  • Review its resources.
  • Review its risks.
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