New Zealand Journal of Agricultural Research abstracts

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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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