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Analysis of Cropping Systems in Northern NSW using Simulation Models

Marcellos, H., Dr; Ph: (02) 6763 1168; Fax: (02) 6763 1222;

Freebairn, D., Dr; Ph: (07) 4688 1391; Mobile: 019 482 798; Fax: (07) 4688 1193;

Research organisations: Tamworth Centre for Crop Improvement, New South Wales Agriculture, PMB 944, Tamworth NSW 2340; APSRU, PO Box 102, Toowoomba Qld 4350

Sponsor: GRDC, Grains Research and Development Corporation


1. Evaluate the APSIM model for its capacity to predict farming systems research data from experiments in northern NSW, and its validation as an accurate tool for simulating cropping system outcomes based on coupling crop models, climatic risk and seasonal sequences.

2. Evaluate specific crop models for wheat, barley and chickpea, and contribute to the development of the chickpea model.

3. Analyse and compare various water and nutrient use strategies for optimal long term productivity and profitability.


The scientist at Tamworth would:

  • assemble, audit and enter data sets (crop, soil, environment)
  • test APSIM (validate, optimise, new data)
  • apply the model to analyse scenarios, link with extension officers and growers


A primary aim of the project was to validate the computer simulation crop systems model APSIM developed by APSRU in Queensland for use in northern NSW. To do this, a number of data sets from several cropping systems experiments in northern NSW comprising crop productivity, soil water and nitrogen, and environmental variables were collated and APSIM used to predict the results observed. Secondary aims of the project were to provide input to facilitate further development of models within APSIM, in particular the chickpea model, and to use those parts of APSIM found to be valid in northern NSW to analyse a number of nutrient input strategies. All milestones relating to these objectives were met.

The work involved four primary tasks: data collation, model development, model testing and model application. Data-sets from various sources (primarily NSW Agriculture) were collated compatible with model input and output requirements. Development of the APSIM chickpea module involved conducting field trials to obtain the parameters needed. Model validation required that the component models (wheat, chickpea, soil water and nitrogen routines) be tested individually, as well as the overall integrated performance of the system model. Following evaluation of the strengths and weakness of APSIM, the model was applied to a number of nutrient management issues.

Issues addressed in improving the chickpea model were the response of the crop to water stress during grain filling, its phenology, biomass partitioning (to improve yield prediction), critical tissue nitrogen concentrations, and nitrogen fixation. As a result, some code changes were indicated for the chickpea model, to enable it to better predict biomass, and discriminate between fertiliser and soil nitrogen.

Three wheat models were available in APSIM, and two were tested, NWHEAT the one in most common use and IWHEAT the most recently developed. These models differed significantly in their ability to predict experimental outcomes in northern NSW:

NWHEAT was unable to adequately predict wheat grain N%, and needs further development. IWHEAT predicted grain N% in relation to soil N quite adequately. It was found in testing NWHEAT that in some circumstances APSIM did not satisfactorily predict soil nitrogen mineralisation following a wheat crop. This was thought to be due to the effects of wheat root growth and C:N ratios. Adjustments to the code of NWHEAT improved its ability to simulate wheat growth in northern NSW.

The performance of the systems model was assessed for its ability to simulate an unfertilised wheat and chickpea rotation. All system components remained un-reset though-out the seven year run to test the feed-on effects from one component to the next; e.g. that the residues from the crops have sensible impacts on the soil water and nitrogen simulations in the following fallow. Predictions of soil water and soil nitrogen were reasonably good but relatively small errors in these predictions caused significant errors in the predictions of plant biomass and yield. Simulated soil water, nitrate and production components behaved very like the ‘real world’ in terms of their range and variability. Particularly encouraging was the good simulation of the effects of the two chickpea crops on subsequent nitrate mineralisation.

The last activity in the project was that of applying the model to analyse and predict various management outcomes. Five applications were completed during the life of the project. The first addressed whether farmers should consider paddock history and soil tests in relation to nitrogen fertilisation. The second and third addressed the consequences of application of N fertiliser early in the fallow, and the values of soil N deep in the profile. In another application, predictions were made of monthly soil N mineralisation for 12 sites in the northern grains region, and used as reference material in the NSW Agriculture N Management Workshop program, and in the TOPCROP handbook for Queensland. The last application was to analyse the relative risks associated with planting chickpea and wheat under different starting moisture conditions.

In conclusion, some components of APSIM may be validly used in northern NSW, whereas others require further development. This is to be expected for a relatively new, and continuously improving, technology. The long term potential for the application of APSIM outside the frame of reference within which it was developed appears to be very good.

The progress made should be continued with further development of APSIM and its component crop and systems models, and their testing against diverse data sets, to enhance its capacity to accurately simulate crop systems over a wide range of environments and cropping scenarios (rotations, agronomic management). Indeed, steps have been taken in this direction with the redeployment of Dr Jillianne Turpin to a new project based at Tamworth, in which her brief is to refine the models for growth and yield chickpea and faba bean, and for N fixation.

Period: starting date 1994-07; completion date 1997-12

Status: completed

Keywords: Crop systems simulation, wheat, chickpea, soil nitrate, soil water


Hayman, P. T. and. Turpin J. E (1998). Nitrogen fertiliser decisions for wheat on the Liverpool Plains, NSW. II. Should farmers consider stored soil water and climate forecasts? Proceedings of the 9th Australian Agronomy Conference, Wagga Wagga, pp. 653-656.

Probert, M.,E., Carberry, P. S., McCown, R. L. and Turpin J.E. (1998). Simulation of legume-cereal systems using APSIM. Australian Journal of Agricultural Research 49, 7-27.

Turpin, J. E., Probert, M. E., and Foale, M. A. (1998). Predicted consequences of early application of nitrogen fertiliser. Proceedings of the 9th Australian Agronomy Conference, Wagga Wagga, pp. 863-864.

Turpin, J.E., Hayman, P.,T., Marcellos H., and Freebairn, D.F. (1998). Nitrogen fertiliser decisions for wheat on the Liverpool Plains, NSW. I. Should farmers consider paddock history and soil tests? Proceedings of the 9th Australian Agronomy Conference, Wagga Wagga, pp. 761-764.


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