4 CONCLUSIONS
Intertemporal LP is a viable method of modelling the optimum variety structure over time for a pipfruit orchard. However, the level of detail required to model the orchard pushes software and hardware to the limit on a PC. The model can be solved but it takes a long time (2 to 4 hours). Advances in computer technology will allow faster solution times in the future.
The model as set out does give similar results to what growers are currently doing. However the model gives more information and allows sensitivities to be run to test assumptions.
The present constraints of minimum cash requirement, maximum orchard area , harvest constraints and not removing more than is planted in any one year give realistic answers. To be fully realistic, the model should include the ability to borrow and tax in the cashflows. The main effect of these would be on the minimum cashflow produced and hence on the speed at which changes would be made.
Harvest constraints are an area that requires careful adaptation to each individual situation. Growers want to see a flow of fruit evenly through the harvest season. Planting all the orchard in one variety is not an acceptable alternative.
However, the model shows that operational needs such as maintaining a steady flow of fruit can have severe effects on cashflow. The analyst using this approach should be particularly careful what harvest constraints are used and what the effect might be of relaxing them.
Many models amalgamate activities to reduce the problem to a level that is possible to solve on their available hardware and software. This practice is fraught with dangers as it can produce an optimal solution that looks satisfactory but may well be some distance from the real optimum.
Acknowledgements
The author would like to thank Dr Chris Dake for his invaluable help in formulating and testing the model structure. The author would also like to thank all the people who reviewed draft copies for their helpful comments. In particular, the comments of Tony Bywater and Robin Johnson are much appreciated.
The tolerance of John Roper-Lindsay of Ashley Software in solving the software problems posed by this model is appreciated. Any mistakes or errors remain the responsibility of the author.
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