A discrete events simulation of flock dynamics: a management application to three lambings in two years

Sheep farmers are interested in models able to shed light on the consequences of new flock management decisions on the production level of ewes, and on the distribution of production within the annual calendar. In order to model the links between decisions and animal responses that lead to flock production, we conceived a simulator using the discrete events technique. The data came from the INRA Limousin flock, which used a three lambings in two years lambing management between 1974 and 1982. The process of the simulator design is presented. We formalised the flock as a system with two actors: the ewes and human decisions. The latter included strategic and oper- ational steering, which are related to physical (i.e., animal and flock) and functional (i.e., groups, batch breeding cycles, ewe lambs stock) management entities. The ewe was defined by its productive trajectory and its biological responses (i.e., fertility, conception date, number of lambs born, and func- tional longevity) that were modelled statistically. The discrete events modelling technique is well suited to characterising the system since it includes various levels of abstraction and management of biological time and a decision calendar. The functional validation process was based on a plan of 50 computer experiments designed to test the correct functioning of the simulator. It shows a relative stability of the annual production level to moderate changes in the three lambings in two years man- agement rules and in the fertility characteristics. We illustrate the regulation properties of the flock system by analysing the animal flows between the batch breeding cycles, entry/exit from the flock and the diversity of the productive trajectories of the ewes.