GRAZPLAN: Decision support systems for Australian grazing enterprises. III. Pasture growth and soil moisture submodels, and the GrassGro DSS

Abstract

This paper specifies the pasture growth module of a model for simulating grazing systems for ruminants and the soil moisture budget that drives pasture growth. Both modules operate at a daily time step. The pasture growth module is quite general in structure but recognises four functional groups of pasture plants: annual and perennial species are distinguished, as are grasses and forbs. Shoot tissue is classified as live, senescing, standing dead, or litter, and also according to its dry matter digestibility, thus enabling integration with diet selection and feed intake models. The phenological development of pasture plants is modelled, with the transitions between each stage governed by environmental variables (day length, temperature and soil moisture). Prereproductive and postreproductive phenostages of vernalisation and ‘summer dormancy’, respectively, are modelled in the appropriate cultivars. Functions predicting net primary production in response to light intercepted, mean daytime temperature, and available soil moisture, and also the process of maturation, are common to all functional groups. The model's treatment of the allocation of assimilate has a similarly general form. Seed and seedling dynamics are modelled for annual species only. GrassGro is a discrete computer package, developed for Microsoft Windows, ™ that combines the pasture growth module with a module for predicting the intake of herbage of ruminants and their productivity. This decision support system enables users to analyse simplified grazing systems in terms of pasture and animal production, gross margins, and year-to-year variability for any specified pasture cultivar, or combination of cultivars, at any specified site. The package may also be used to simulate forward from current pasture and animal conditions, for assessing the probability distribution of production outcomes, given the historical variability of weather conditions over the specified forward period.