Will the future continue to repeat the past?
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Humans↗
Biomedical subjects
S J Woodward
Publications and source records attributed to S J Woodward.
Validating a model that predicts daily growth and feed quality of New Zealand dairy pastures.
The Pasture Quality (PQ) model is a simple, mechanistic, dynamical system model that was designed to capture the essential biological processes in grazed grass-clover pasture, and to be optimised to derive improved grazing strategies for New Zealand dairy farms. While the individual processes represented in the model (photosynthesis, tissue growth, flowering, leaf death, decomposition, worms) were based on experimental data, this did not guarantee that the assembled model would accurately predict the behaviour of the system as a whole (i.e., pasture growth and quality). Validation of the whole model was thus a priority, since any strategy derived from the model could impact a farm business in the order of thousands of dollars per annum if adopted. This paper describes the process of defining performance criteria for the model, obtaining suitable data to test the model, and carrying out the validation analysis. The validation process highlighted a number of weaknesses in the model, which will lead to the model being improved. As a result, the model's utility will be enhanced. Furthermore, validation was found to have an unexpected additional benefit, in that despite the model's poor initial performance, support was generated for the model among field scientists involved in the wider project.
Agriculture↗
A differential-delay model of pasture accumulation and loss in controlled grazing systems.
A grazing population dynamics model is proposed where organisms in a grazed population have a fixed life span. The motivating context is that of ruminants grazing grass-dominant pasture. The model takes the form of a differential-delay equation in which the rate of loss of pasture due to senescence at some time depends on the rate at which leaves are reaching maturity at that time. Comparisons are made with data from a continuous grazing experiment due to Bircham and Hodgson (Grass and Forage Science, 38:323-331, 1985), leading to a prediction of 21.9 days for herbage life span. Predictions of herbage utilization are consistent with measured data. The model predicts lower senescence in swards in regrowth than in grazed swards at the same herbage mass. Solutions and equilibria are obtained for the linear form of the model with continuous grazing pressure. Solutions and bounds are obtained for the linear model with intermittent grazing pressure, and its usefulness in modeling grazed pastures is discussed. A delay model is a simple but powerful means of including the concept of fixed herbage life span in grazing modeling. Questions of herbage life span and percentage utilization are naturally contained in the mechanism of a differential-delay model. There are not so well handled by models that treat senescence of herbage empirically.
Agriculture↗