A computer simulation model which describes the spatial and temporal variation in the extent of listerial contamination within a damaged silage bale is presented. The silage bale is assumed to be split into a number of distinct sites and these sites are represented by a two dimensional lattice structure. Each site is classified in relation to its listerial composition. This classification results in three states which are dormant, active and unpopulated. Sites change state as a result of the movement of oxygen through the bale. This movement is initiated when a hole is punched in the plastic covering of the bale. The model is stochastic in nature and at any time following damage, the proportion of the bale which is contaminated is calculated. Furthermore, the spatial distribution of contaminated sites is predicted. The models are a first attempt at introducing structure into the selection process for feeding silage. We highlight areas of future research which will be invaluable for validation and practical use of the model.
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