Food supply and demand, a simulation model of the functional response of grazing ruminants

A dynamic model of the functional response is a first prerequisite to be able to bridge the gap between local feeding ecology and grazing rules that pertain to larger scales. A mechanistic model is presented that simulates the functional response, growth and grazing time of ruminants. It is based on results of studies on voluntary food intake and animal production of ruminants. Physiological requirements and food quality are key factors that determine the intake. The model is also based on the results of studies on the functional response of grazers. Quantitative aspects of vegetations i.e. bulk density and height, and the animals’ mouth dimensions are the main factors that determine how much a grazer can swallow. The structure of the model applies to ruminants under tropical conditions. The model parameters are set and validated for the African buffalo, Syncerus caffer. Three variants of the model are elaborated: a non-reproducing female, a reproducing female and a male ruminant. The similarity between the results of the simulation runs and field data are considered to be satisfactory. The results suggest that the quality of the vegetation i.e. the crude protein content, together with the animal's energy requirements, govern the daily intake of food. However, the quantitative characteristics of the vegetation, especially height, can be critical. The model aims to be a tool in the interpretation of empirical field data and in the development of grazing theory. Transparency, concision and a robust empirical basis were therefore striven after in the construction of the model.