Simulations were run to test the sensitivity of OZCOT to varietal differences in developmental characteristics known to influence fruiting dynamics in cotton. Thirty one years of historical weather data from Myall Vale were used. The results indicate that the model is responsive to varietal differences over the actual range measured in two field experiments; both in terms of yield and time to 60% bolls open. Simulated changes in time to 60% bolls open were consistent with experimental evidence. It is therefore necessary to allow for these differences as inputs to the model and to measure the developmental characteristics for new cultivars to keep the model relevant to current production practices.