Model Development for a Deferred-Grazing System

A model of the early growth of an annual pasture and liveweight response of grazing sheep was built using first order differential equations to study the practice of deferred grazing. The dynamic behavior of the system over time was simulated by solving the equations on a computer. The model is an interpretative representation of a subterranean clover pasture in Western Australia and relates to a specific site and set of seasonal conditions. Use was made of the literature and a recent grazing experiment to develop the model. Herbage growth is estimated from known relationships with radiation received, leaf area exposed, soil moisture, and herbage removed by grazing. Change in soil moisture is estimated from rainfall and pan evaporation data. Defoliation is based on stocking rate, pasture weight, and pasture height to account for the effects of animal numbers and availability of pasture. Liveweight change of the consuming animal is calculated as a function of intake, digestibility, and the partitioning of metabolizable energy between maintenance and weight change. Validation of the model by results observed in the grazing experiment is presented.