You can navigate to other sections of the report by folllowing these links:

  1. What Overseer is
  2. The Overseer farm file
  3. What Overseer produces
  4. How the Overseer engine works
  5. Uncertainty in model results
  6. Overseer Glossary

The Overseer science model

Overseer is a science model made up of three components: 

  • the user input fields where a farm is described,
  • the engine that models the nutrient movements for that farm through a series of calculations, and
  • the reports which display the analysis for interpretation (and are stored in the database).
All three components work together to undertake scenario analyses of different nutrient outcomes for a farm to inform management decisions.

The engine is made up of a series of sub-models that calculate the flow of seven major farm nutrients – Nitrogen (N), Phosphorus (P), Potassium (K), Sulphur (S), Calcium (Ca), Magnesium (Mg), and Sodium (Na) and greenhouse gases.


What Overseer covers

The Overseer model has been expanded over time (see development history timeline) to cover a wide range of different farm enterprises and management practices in New Zealand. It can model mixed and single enterprise farms of varying intensity and with or without irrigation.

Farm types currently modelled by Overseer include:  

Pastoral enterprises including dairy, beef, sheep, dairy goats and deer. These include farms using fodder crops (fodder beets, kale, rape, swedes, turnips), forage crops (ryegrass, barley, oats, maize, rye corn, triticale) and animal housing.

Permanent fruit crop enterprises including avocado, kiwifruit, apples, peaches and grapes.

Horticulture enterprises including:

  • Green vegetables – broccoli, brussel sprouts, cabbage, cauliflower, lettuce, spinach
  • Legume vegetables – beans, lentils, peas
  • Root vegetables – kumara, potatoes, carrots, beets, parsnips
  • Other vegetables – onions, sweetcorn, squash tomatoes.

Arable crop enterprises including:

  • Grain crops – barley, maize, oats, wheat
  • Seed crops – ryegrass, clover.

How Overseer is used

As well as providing nutrient budgets, Overseer supports farm management decisions by:

  • Identifying the maintenance fertiliser nutrient requirements for pastoral blocks, which can be used to derive fertiliser recommendations.
  • Enabling “what-if” testing of potential management changes to analysis of the impacts (scenario testing).
  • Identifying nutrient hotspots (e.g. high nutrient loss blocks).
  • dentifying optimal effluent block areas to ensure the correct amounts of N and K are being applied.

Over time, Overseer has also been developed to:

  • Provide an environmental footprint for a farm (estimating nutrient losses from the farm to air (e.g. greenhouse gases) and water (e.g. leaching, run-off).
  • Predict diffuse nutrient sources for catchment modelling.
  • Benchmark (providing a status report to be used as a baseline for comparison e.g. with future reports or against other farm reports within a catchment).
  • Establish discharge limits or targets from a farm and monitoring changes over time.
  • Inform scientific research (testing hypotheses and conducting investigations).
  • Educate agricultural science students (describing and understanding implications of nutrient management on New Zealand farms e.g. in agricultural qualifications at universities).

Software updates and version numbers

As with any software, Overseer is updated at regular intervals - to fix known problems, improve or add features and add new science. When updates are made to the science model, the model version number changes. The version number can be found on the top right of the farm analysis page in OverseerFM. 
The current version number is 6.3.0. The number format represents the scale of change

is the major version number. It only increases when there’s a major change to the design or structure of the model. The last update was in 2012, when Version 6 was released to include the urine patch model.
is the minor version number. It increases when functionality changes are made, for example, new science or sobmodels are added. The last minor version change occurred in 2015, when the irrigation module was added.
is the patch version number, and increases when minor changes are made, for example bug fixes.


Using Overseer results

Overseer adds value to farm management decisions because it predicts the outcomes of changes in farm management on nutrient loss. Because OVERSEER provides an individual analysis of a farm, it helps users identify the best ways to manage that farm and mitigate potential losses of nutrients into the environment.

The following summary identifies the farm management areas that have the most influence on nutrient loss estimates:

Category Variable Effect on calculated Ability of farmers to influence
Nitrogen Phosphorus
Animal Stocking rate × × ×
  Species ×   ×
  Gender ×   ×
  Production and Reproduction policy ×   ×
  Sheep/Beef ratio ×   ×
  Management × × ×
Fertiliser Fert N rate ×   ×
  Fert N form ×   ×
  Fert P rate   × ×
  Fert P form   × ×
  Timing × × ×
Effluent management Amount × × ×
  Timing × × ×
General Rainfall × ×  
  Irrigation rate × × ×
  Topography   ×  
Pasture Clover level ×   ×
  Pasture type ×   ×
  Pasture ME ×   ×
Soil Natural drainage × ×  
  Artificial drainage × × ×
  Anion storage   ×  
  Soil properties × ×  
  Olsen P   × ×
Supplements Farm grown supplements ×   ×
  Imported supplements ×   ×
Crops Type × × ×
  Management × × ×

Go to next section:  The Overseer farm file