The procedures adopted for incorporating breeding decisions in a livestock market model have implications for the simulation of the model. Specifically, there is a need to consider the nature of the biological lags between livestock breeding decisions and outputs, and how these are incorporated in the breeding inventory used in such a model. In this paper, five alternative breeding inventory specifications are used in a structural econometric model of the Australian prime lamb market. The predictions of lamb slaughterings and of price responses of lamb producers are affected by the specification of the lamb breeding inventory used. The extent to which the modelling results approximated reality was substantially improved when disaggregated and seasonally allocated data were used to estimate lamb breeding inventories.