Opinion: It's time to recognise the positive aspects of Overseer to make sure we don't throw the baby out with the bath water, Jeff Morton warns.
I write as someone who has had a 30-year association with Overseer as a scientist involved in the early development, as a user helpingfarmers and as a teacher with fertiliser sales representatives.
Most of the debate is around Overseer as a predictor of nitrogen (N) leaching losses although it has many other functions such as estimating phosphorous (P) loss and greenhouse gas emissions.
It was developed as a decision support programme so that it assists land users in making changes in management so to reduce N leaching losses.
As such, it is a world-class programme that has not been duplicated in any other country.
Like any model or results from an experiment there is variability around any single estimate of N leaching loss which is normal in a biological system such as farming.
Critics say that it was not designed nor is useful for regulation. I agree with the first point, but would argue that it can be useful if the N leaching losses are interpreted as relative values as part of a trend rather than absolute values.
So if the Regional Council has a N leaching loss target for a particular farming system on a specific soil type, and the N loss values are trending downwards towards this target in response to changes in management on-farm then Overseer has a role to play.
"If you cannot measure, you cannot manage," they rightly say, and for many other practices, such as grazing management or improving genetic merit, farmers correctly measure pasture cover or DNA markers.
Determining N leaching losses from on-farm measurements are too costly and no more accurate than Overseer anyway, as shown by the Otago Regional Council exercise several years ago.
And what are the alternatives to the current use of Overseer?
Input controls are clumsy and often ineffective as seen with our first attempt with the recent limit on fertiliser N use.
There is nothing to stop dairy farmers substituting for the extra feed from N by applying it on runoffs right up to the limit and importing extra feed into the milking platform. Or just buying in more supplement. Also input controls are very expensive to administer.
Despite recent statements, the science behind the N leaching losses in Overseer is very sound and based on peer-reviewed publications of calibration trials in most environments.
Sure, high rainfall areas - like on the West Coast - lack validation, but you are only talking about a very small proportion of dairy farms.
It is true that Overseer has been mainly calibrated for farms on flat land.
So in the short term, until more research has been carried out, why not just use it for this environment. As is generally the case at present.
Anyway, flat land is where most of the intensive farming with cattle that causes N leaching is carried out.
In hill country, P loss on sediment is much more of a problem and there are other programmes such as LUCI or MITIGATOR that can be used for modelling them.
Arable farming where each paddock has a different management history is much more difficult to model for N leaching loss and there has been less research carried out.
It makes sense to only use Overseer with caution in these farming systems. But I still cannot see that trends over time do not have some relevance.
It has also been stated by reviewers that we need a model that predicts real-time N leaching losses, rather than long-term average losses as in Overseer. But to what advantage in terms of practical farm management?
Say we have a high rainfall event in early spring causing excess drainage and a spike in the N leaching rate.
The N lost from the root zone is a result of the deposition of urine from previous grazings so it is all retrospective and little can be done to reduce it in the short term during or after the event.
