Beef production is the most important farm enterprise on Northern Ireland farms, accounting for 32.5% of Gross Agricultural Output. In beef production the end saleable product is carcass rather than live weight. When undertaking nutrition studies with beef cattle, it is essential to evaluate effects on carcass characteristics, as improvement in live weight may not transfer to improvements in carcass weight and characteristics due to change in gut fill effects. Undertaking carcass assessments in beef production studies increases experimental costs. To determine carcass gains it is essential to slaughter a representative batch of cattle pre-experimentally in order to develop a relationship between initial live weight and initial carcass weight. Slaughtering store cattle further adds to the costs of beef experimentation. The present study was undertaken to develop a relationship between live weight and carcass weight of beef cattle offered grass silage-based diets to facilitate the determination of initial carcass weight of store beef cattle at the point of initiation of nutritional studies.