Optimal crop sequences to control cephalosporium stripe in winter wheat
Danielson, Joan Gay
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Cephalosporium gramineum, a soil borne fungus, causes a stripe disease in winter wheat. The fungus restricts the flow of water and nutrients to the plant head resulting in significant yield losses. The disease is passed from winter wheat crop to winter wheat crop through Cephalosporium stripe infested straw. Rotating to non-host spring crops or fallow, allowing time for decomposition of infested residue, is the primary means of controlling Cephalosporium stripe. The objective of this thesis was to determine economically optimal land use sequences to control Cephalosporium stripe in winter wheat. Control of the disease is a stochastic dynamic problem and as such was formulated within a stochastic dynamic programming framework. The economic criterion used was maximization of expected present value of returns over variable cost. The model was applied to a representative dryland grain farm in the Judith Basin of central Montana. The decision alternatives were winter wheat, barley, and fallow. The state variables included in the model were previous land use, years of control, level of Cephalosporium stripe infection in the last winter wheat crop, barley price, and winter wheat price. Transformation functions were derived for all of the state variables. Based on statistically estimated transformation functions, transition probability functions were developed for the stochastic state variables: past Cephalosporium stripe infection level, winter wheat price, and barley price. The relationship between Cephalosporium stripe infection level and winter wheat yield was also estimated. The optimal policy is dominated by fallow and barley decisions when there are less than four years of control unless the past infection level is very low. Once three years of control has been exceeded, winter wheat decisions become optimal at higher levels of past infection and increase as winter wheat prices increase. Finally, it is evident that consideration of at least a three-year control sequence would increase annual returns significantly regardless of the past Cephalosporium stripe infection level.