Model Explanation

Modified April 11, 2012

This spreadsheet encodes a modified version of the model developed by Burt and Ferris (1996) for determining the optimum length of rotation to non-host crops before returning to sugar beets in fields infested with the sugar beet cyst nematode.

The current model is adapted specifically for determining optimum length of rotation to non-host or poor-host crops before returning to potatoes in the organic soils of the Tulelake basin of California.

The model includes the option of using a nematicide prior to the potato crop and for specifying the level of efficacy and cost of the nematicide.  Note that certain nematicides have not been very effective in organic soils.

Use of the model requires user input of the estimated market value and production costs of potatoes with various levels of surface blemish due to the Columbia root-knot nematode, Meloidogyne chitwoodi.  It also requires market value and production costs of the rotation crop and barley is suggested.

The parameter values for nematode reproduction on potatoes, damage to potatoes, and population decline of the poor host are derived from a 7-year crop rotation experiment designed and implemented by Dr. Harry Carlson commencing in the late 1980s with nematode components of the study provided by Ferris and Westerdahl (Ferris et al., 1994).

Parameter values for barley are used in the study.  Barley is listed in the literature as a moderate to poor host to the Columbia root-knot nematode (Ferris et al., 1993).  In the crop rotation experiment, the nematode population declined at a greater rate under barley than in fallow soil.

Starting with the population level of M. chitwoodi measured in the fall (juveniles/1000cc soil) and then the option of applying a nematicide of specified efficacy prior to a potato crop the following spring.  The potato crop can then be grown annually, or after rotation to the non-host crop.  The optimum number of years of crop rotation is determined by maximizing the annual returns from the rotation sequence averaged over 50 years.

Adjustment of the market values at various blemish levels and production costs of the primary and rotation crops, and the efficacy and cost of the nematicide treatment provides insights into the behavior and sensitivities of the system in relation to average annual economic returns, expected nematode damage at the optimum rotation length, and examples of cycling of the nematode population at various rotation lengths.

Only the boxes highlighted in yellow may be adjusted by the user to protect the integrity of the population and crop damage parameters.

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