If Overseer is to continue to be used by policy writers as a means of controlling land use, then it is important farmers ensure their own nutrient budget is completed as accurately as possible.
Regional councils are generally only interested in the figure a nutrient budget gives for nitrogen (N) loss to water, which is estimated in kilograms/N/ha/year. In some catchments phosphorus loss is also a focus.
Overseer is a model and as such will only ever provide estimates rather than infallible accuracy. The following areas should be considered in order to achieve the best accuracy possible with Overseer.
Professional advice
The primary users of Overseer are fertiliser representatives, with an increasing number of primary sector organisation staff, farm and environmental consultants, and regional council staff operating the model.
The fertiliser association has established a nutrient management accreditation regime to recognise competent advisers.
The bulk of this accreditation involves intermediate and advanced courses on sustainable nutrient management at Massey University. While not every fertiliser representative is accredited yet, most will have completed the Massey courses.
The intermediate course involves some reading in preparation and a brief familiarisation with Overseer for those who don’t use it regularly, then three days at Massey, ending with an exam.
The advanced course involves more detailed Overseer use and four assignments focused on nutrient budgeting and management plans for different farm types, as well as a three-day stint at Massey and an exam to finish.
Both these courses are often referred to as Overseer training but in reality their focus is wider and looks at responsible nutrient management based on productive and environmental objectives.
The advanced course provides a stronger grounding in Overseer use than the intermediate course.
Many consultants using Overseer regularly have done only the intermediate course, while some have done neither. It is worth checking what level of qualification an adviser has in using Overseer before employing them, given the cost of their time.
Consistency
With Overseer’s accuracy for nitrogen leaching estimated at 20-30%, the consistency of the model’s use is worth careful consideration.
In order for a farm business to be capped or limited according to its Overseer Nitrogen leaching figure, there needs to be a high level of confidence in the results.
The computer model is incapable of perfectly quantifying nutrient balances in what is a biological system so expecting perfect figures is naïve. What then becomes important is consistent use of the model within a business over time and between farms under the same catchment or regional policy regime.
If all farmers facing the same rules model their systems on the same basis then results at least become fairer in terms of benchmarking their Nitrogen loss number.
It does not give their nitrogen loss numbers guaranteed accuracy but it should put them in the right place relative to each other.
Understanding limitations
Three fundamental assumptions underpin Overseer.
1 – The use of annual average inputs and annual average outputs over a number of years
2 – Near equilibrium conditions with minimal change each year
3 – Actual and reasonable inputs. Users need to understand how input changes alter outputs. Overseer also assumes best management practices are used on farm.
These principles are enforced strongly during the Massey University sustainable nutrient management courses but are not always given due recognition in policy making and consenting arenas where Overseer is being relied on for crucial business decisions.
Understanding these assumptions helps an adviser to understand Overseer’s limitations and use it with realistic expectations.
Record keeping
With many farmers around the country facing the prospect of providing several years of nutrient budgets in order to satisfy regional councils, industry resources will be stretched.
Dairy farmers have faced the requirement to use Overseer for several years but for most arable and sheep and beef farmers this is a new issue.
Many farmers are not using Overseer in their business but will have to soon. If the model is to be used retrospectively for them then record keeping becomes vitally important.
Important Overseer inputs which should be recorded where possible, include:
- fertiliser inputs (rate, timing, areas, and product)
- supplements made and their destination
- supplements bought in and where they are fed
- stock movements
- soil tests, and
- irrigation use.
Much of the required information can be taken from a farm’s annual accounts if not recorded elsewhere.
The prospect of heightened Overseer use is already testing industry resources and some previously free services are now costing farmers.
The fewer hours it takes a consultant or fertiliser representative to complete Overseer, the less it will cost. An efficient record-keeping system is likely to save time and money.
Blocks
For nutrient budgeting with Overseer a property is split into blocks based on differences in factors such as management, soil type, irrigation and productive capability.
This can be time-consuming for complex farms which can have upwards of 10 blocks, but careful block set-up helps make the results as meaningful as possible. Coarsely aggregating blocks together limits the ability to interpret any differences in nutrient flows within the farm system.
Once one nutrient budget has been done, if there is minimal change in the system, subsequent modelling will be quicker.
Real data versus defaults
Drainage through the soil profile influences nitrogen loss in Overseer so the amount of irrigation and rainfall are crucial factors.
Irrigation is one area of particular controversy where using real water use data can result in wildly different Nitrogen loss numbers to the defaults Overseer assumes if data is not provided.
People without real data are reliant on the defaults. Regional councils generally accept real data use if it is possible but without robust information, defaults will have to be used.
Drainage through the soil profile greatly influences nitrogen loss in Overseer so the amount of irrigation and rainfall are crucial factors to consider.
Both these parameters drive drainage, which in turn affects leaching, in the model. Measuring water use, soil moisture monitoring and accurate weather records will help increase accuracy.
Soil type is another area where various levels of detail are available. Some farmers have invested in detailed soil maps which help ensure accurate nutrient budgeting.
Without this level of detail the default is generally Smap, an online tool from Landcare Research – go to smap.landcareresearch.co.nz/home.
SMap coverage is limited by funding so in some parts of the country the best information available may be old soil maps, which are not on a detailed scale, and local knowledge. Regional councils will expect the best information available to be used for nutrient budgeting.
New versions
One concern often voiced by farmers facing nutrient regulation is that their nitrogen loss numbers are changed by updated versions of Overseer.
The reality is that policy-makers will allow for this providing Overseer is used consistently.
So providing a farm system has remained stable but the leaching estimate has increased solely because of an upgrade to the model, this shouldn’t penalise the farmer. As Overseer continues to improve, changes are inevitable.
