Continued genetic improvement in meat sheep breeds has resulted in increased carcass weights at younger ages. This has been achieved in direct response to selection criteria within the terminal sire index [weight gain (60%), lean (20%) and fat (-20%)] with an increasing movement to selection on weaning weight at 100 days of age. Such selection will potentially lead to increased mature size. In light of producer observations that some lambs had growth rates in the order to 400-500 g/day, this project was commissioned to explore the capacity of present day production systems to provide the nutrients required to exploit such superior genetics. This project quantified the realised changes in growth, fat and lean measures that have occurred over the past 20 years in the Australian lamb industry. It used a modelling approach to determine the extent to which change in animal phenotypes has been influenced by nutrient supply.