Modelling the performance of the beef cow to build a herd functioning simulator

This review deals with the representation of the individual animal in herd scale models for cattle. One main problem in these models is to connect two levels of complexity: herd functioning at the farm level and functioning of the individual animal when physiological processes are considered. The text deals first with the representation of the animal and its physiological functions within existing models. Different physiological functions (i.e., growth, intake, lactation and breeding) are rarely modelled in a mechanistic way when used at the herd level, but mostly on an empirical basis, according to classic mathematical laws. The breeding function is the most important one ( since it allows for herd renewal) and is frequently modelled as a stochastic process. The body condition score is a relevant indicator of the cow's status, which requires a better representation in individual-based models. Some herd models are presented and discussed and some proposals are presented for a herd functioning model for beef cattle taking into account both animal performance and decisional processes.