Use of a pasture growth model to estimate herbage mass at a paddock scale and assist management on dairy farms

Knowing the amount of herbage mass available on the farm (ideally measured weekly) is an important step in achieving high pasture utilization on pastoral dairy farms in New Zealand, but the information must be used in a timely manner to make efficient management decisions. However, most New Zealand dairy farmers do not measure their pastures regularly. This project aimed to develop a simple alternative, in the form of a prototype software tool (Pasture Growth Simulation Using Smalltalk, PGSUS) to predict herbage mass at an individual paddock level, which reduces (not eliminates) the requirement for physical data collection and provides more information from the measurements taken. The main data requirements are pasture herbage mass for each paddock and grazing or cutting events. A climate-driven pasture simulation model is used to predict herbage mass between intermittent pasture measurements. The pasture model contains certain empirical parameters that are fitted to the observed data for each paddock individually, using all the previous data to "train" the model. PGSUS requires daily weather data, including mean, minimum and maximum air temperature, solar radiation, rain and potential evapotranspiration. Data from the Virtual Climate Station Network (VCSN) from NIWA (National Institute of Water and Atmospheric Research Ltd., New Zealand) are used to drive the model. Preliminary testing was done on two commercial dairy farms, one in the Waikato (North Island) and the other in the Canterbury (South Island) regions of New Zealand. Up to 28 days without measurements, PGSUS estimated herbage mass with correlation of approximately 0.9 and with small bias. (C) 2010 Elsevier B.V. All rights reserved.