Predicting native pasture growth in the Victoria River district of the Northern Territory
Date
2006
Authors
Cobiac, Michael D.
Editors
Advisors
Bellotti, Bill
Day, Ken
Day, Ken
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Thesis
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Abstract
Pastoralism is the major economic activity in the Victoria River District (VRD), and is
dependent on sustainable pasture use. Analysing grazing practices for sustainability
requires knowledge of annual pasture production, but little quantitative data is available. A
study was undertaken to develop the capacity for predicting native pasture growth in the
VRD using systems modelling. Twenty one field sites were studied for two years using a
standard methodology, and the Grass Production (GRASP) model was calibrated using this
field data. End of growing season total standing dry matter (TSDM) was well predicted
(mean = 2513kg/ha, r2(1:1) = 0.966, RMSE = 132kg/ha, and 98% of predictions within
measurement variance).
Developing generic parameters for common soil and pasture types allowed extrapolation of
the model. Predictive skill declined when using generic parameters (r2(1:1) = -0.265,
RMSE = 807kg/ha and 64% of predictions within measurement variance). However,
observation and prediction means were very similar, indicating that generic parameters are
suitable for broad scale applications, but site-specific parameters are necessary if a high
degree of accuracy is required. Parameters controlling plant water uptake largely determine
pasture growth in low rainfall years, while nitrogen uptake and dilution parameters limit
growth in high rainfall years. Pasture growth is constrained by nitrogen supply in 91% of
seasons in the northern VRD, and in 25% of seasons in the drier south.
Example applications of the model were demonstrated. Current and expected future levels
of pasture utilisation in the district were calculated, showing a current average of 16%,
rising to an expected 20% in the next decade. These levels are within the safe utilisation
rates recommended for the region. Economic analysis shows positive returns ($4.54
million per year) from pasture augmentation with introduced legumes if past problems with
establishment and persistence can be overcome.
Model performance would be improved by accounting for simultaneous wetting of the
entire profile in cracking clay soils, calculating growth of perennial and annual pasture
species separately, and simulating variation in nitrogen uptake and dilution between years.
Incorporation of these processes must be balanced against the increased complexity of the
model and the additional data required for calibration.
School/Discipline
School of Agriculture, Food and Wine : Agricultural and Animal Science
Dissertation Note
Thesis(PhD)-- School of Agriculture, Food and Wine, 2006
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text