Models that predict phenotypic responses from the interaction between genotypic descriptors and the environment are desirable both in the context of both animal production systems and evolutionary ecology. Nutrient availability is often related to the ability of a host to control an invading parasitic population and mounting an immune response is often associated with a nutritional cost. This cost is shown by the peripartuient break down of immunity (Houdijk et al., 2003) and reports of negative correlations between production and resistance traits (Rauw et al., 1998). The aim was to develop a model which is able to make predictions relating to the effects of resource allocation as determined by nutrition and genotype, on the course of microparasitic infection in farm animal hosts.