New Zealand Journal of Agricultural Research abstracts
A climate-driven, soil fertility dependent, pasture production model
J. L. MOIR
D. R. SCOTTER
M. J. HEDLEY
Soil and Earth Sciences
Institute of Natural Resources
Massey University
Private Bag 11222
Palmerston North, New Zealand
A. D. MACKAY
AgResearch Land Management
Private Bag 11008
Palmerston North, New Zealand
Abstract Field trial data relating pasture growth to measures
of soil fertility are confounded by many site-specific environmental factors,
particularly the weather. One approach to accommodate this is to express
fertiliser responses in terms of relative rather than absolute yields, but this
approach places constraints on trial design and is unhelpful when attempting to
extrapolate data to estimate actual yields at other sites or in other years. We
suggest an alternative approach that includes the soil moisture, and so the
effect of climate, as it influences evapotranspiration. The model assumes that
pasture growth is proportional to evapotranspiration, and that the
proportionality constant (k) depends on soil fertility.
Evapotranspiration is calculated from a simple daily soil water balance. Values
for k varied from 11 to 19 kg DM ha-1 mm-1 of
evaporation. The greatest divergence between the measured and modelled
production occurred during a prolonged dry period. Possible reasons for this
are discussed. With simulated weather data, the model can be used to generate
probability-density functions of pasture production. The advantage of the
approach is that prediction of "actual" yield is a very helpful measurement for
producers, and more valuable to scientists than relative yield when modelling
nutrient cycling. This modelling approach also has potential applications in
farm risk management and feed budgeting.
Keywords pasture growth model; soil fertility; climate;
evapotranspiration; Olsen P; relative yield; water balance model
New Zealand Journal of Agricultural Research, 2000, Vol. 43: 491-500
0028-8233/00/4304-0491 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (723K); (scanned from paper original: notes about this process)
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