GRASIM is a comprehensive grazing model that links all components of the pasture system. It predicts standing biomass, herbage nutritional quality, and nutrient leaching under pasture. GRASIM can be used to obtain a better understanding of the pasture system and determine management strategies which yield more efficient utilization of pastures. It can generate information suitable for estimating the financial and environmental consequences of alternative dairy management strategies including, partial mechanical harvest in the context of the year round feed needs of the dairy herd, storage/harvest needs and year to year variability, and stocking rate effect on supplementation and amount of harvested feed.
GRASIM is designed as a tool to simulate intensive rotational grazing management. It accounts for carbon, nitrogen and water budgets in the pasture environment. GRASIM data requirements include minimum and maximum daily temperatures, daily rainfall, average daily solar radiation, soil physical properties, grass growth parameters, soil nitrogen transformation coefficients, and initial levels of soil water and soil nitrogen. The model output includes daily biomass production, and water and nitrogen levels in different pools. GRASIM state variables are the grass biomass, grass growth rate, leaf area index, soil moisture content, soil nitrogen content, plant residues, and organic matter content.
GRASIM operates with a daily time step. It models multiple paddocks, each paddock in the pasture system is budgeted separately. the paddocks share some soil and weather information but can have a different crop. GRASIM has four main components, the grass component of the model contains two main carbon compartments, storage and structure./ It accounts for the following processes: root growth and maintenance, shoot growth (partitioned into leaf and stem), shoot respiration, senescence, and recycling. The soil matrix is partitioned into two zones. The first zone is where all water and nitrogen additions, surface evaporation, plant uptake, and nitrogen transformations are taking place. The second zone activities include plant uptake of water and nitrate-N. These activities include: nitrification, mineralization, uptake, volatilization, denitrification, and leaching. Soil water is accounted for using a simplified water balance that considers runoff after a heavy rainfall, evapotranspiration, soil matrix water dynamics, and leaching. The harvest component controls the grazing events and is where all pasture management input are handled.
GRASIM field evaluations require the following data: