About this tool


Good pasture management is the key to dairy farm profitability. DairyNZ is confident that pasture growth forecasts will help farmers make better decisions on supplementary feeding, grazing rotations, silage making and nitrogen management.


The Pasture Growth Forecaster is a model that calculates pasture growth from the environmental conditions experienced at a site or region. These environmental conditions include:

  1. Climate-Rainfall: soil temperature, solar radiation and potential evapotranspiration are used to calculate the amount of water in the soil and the potential for pasture growth rate; and
  2. Soil: Soil water holding capacity and soil fertility are used to calculate how much water is available for plant growth and the productivity of the site.


The forecasts are calibrated to growth rate data measured from whole farms rather than sites. Whole farm growth rates take into account the effect of low producing areas such as swamps, sidlings, scrub, tracks, and fence lines. They are therefore lower than those measured for example in pasture cages but are more realistic for farm feed budgeting.

District forecast

Each district is divided into 5 km square grids. Each grid has the water holding capacity for 5 predominant soil types or groups. The regional forecast uses the mean value of each grid and calculates the overall mean value by averaging all grids in that region.

Historical rainfall is provided by NIWA’s Virtual Climate Network Stations (VCNS). Virtual Climate Station (VCS) data provide estimates of climate for the whole of New Zealand. These daily estimates are based on the spatial interpolation of actual data observations made at climate stations located around the country. These data are run through the Pasture Growth Forecaster to calculate the historical pasture growth rates experienced within each grid.

The VCNS also provides predicted rainfall for the next 14 day period. The regional forecast therefore displays the predicted growth and the ‘Typical’ growth. The typical growth is generated from the historical growth calculations for each grid and includes the range within which 50% of all calculated growth rates fall within. This data are presented as the average of all grids within the region.

Project farms

Project farms have been set up with an estimate of their soil water holding capacity. They may use climate data from the VCNS or actual rainfall received as recorded by the farmer. The farmers will also be recording pasture cover each month which is used by the Farmax program to calculate actual pasture growth. The Pasture Growth Forecaster uses actual growth rates to calibrate the model so that it is more accurate in its calculations of historical and 14 day predicted growth rates. The main parameter that is calibrated is the site’s fertility. The model output is used to calculate:

  1. A short forecast – this is similar to the Regional Short Forecast except the farm has a specified soil moisture holding capacity. The farmer may use actual rainfall and pasture covers to calibrate the model. This enables the model to be increasingly accurate in its calculations.
  2. Three month forecast – this displays the growth that is expected for the next three months and can be used for medium term feed budgeting. It calculates the next 14 days of growth using the VCNS predicted climate data and the soil moisture calculated each day. At the end of the 14 day forecast the model takes the calculated soil moisture as a starting point then uses all the historical years (usually 40 years) of climate data to determine the mean and range that could be expected given the sites climate history. The output shows, given the starting soil moisture the mean pasture growth rate and possible range. The range includes the bounds within which 50% of all years and 90% of all years fall within.

  3. The annual pasture growth – this displays the pasture growth rate that can be expected for each month from data produced by running the climate data for all historical years through the model. Mean growth, and the range within which 50% and 90% of all years are displayed. In so doing the annual pasture growth shows the mean growth and its variability through the year.

 

Funded by New Zealand dairy farmers through DairyNZ and the Ministry for Primary Industries through the Primary Growth Partnership.