Gastrointestinal parasitism in lambs is usually manifested as a sub-clinical infection and causes significant losses in performance. Its control through the use of chemoprophylaxis is no longer sustainable due to the development of parasitic resistance to anthelminthics, but also due to environmental and consumer concerns. There is thus an urgent need to develop alternative, sustainable methods of controlling gastrointestinal parasitism. These include dietary supplementation of host with protein. The aim here is to develop a mathematical simulation model for the description of the joint effects of nutrition and host performance genotype on the outcome of sub-clinical challenge by gastrointestinal parasitism for growing lambs.