Purpose A life cycle assessment was performed for the production of a total mixed ration (TMR) fed to finishing feedlot cattle in California, USA. The goal was to determine the climate change impact of the feed supply chain associated with the production of 1 kg finishing TMR (kg CO2e/kg TMR). A secondary goal was to compare the climate change impact of feed versus finished beef (kg CO2e/kg live weight). Methods The TMR was based on feeds commonly fed to finishing cattle in California. The Livestock Environmental Assessment and Performance Partnership (LEAP) guidelines were followed for inventory data collection. System boundaries included the production of crops and feed additives, transportation of TMR components, and compound feed production. Data were sourced from national databases and Ecoinvent™ unit process data. Three scenarios were assessed as a result of allocation at the transportation step: Scenario A (100% empty return load); Scenario B (50% empty return load): and Scenario C (0% empty return load). Energy, mass, and economic allocation, and system expansion of dried distillers grain solubles (DDGS) were assessed for sensitivity analysis. Total feedlot emission data from Stackhouse-Lawson et al. (2012) were used to compare to impacts of TMR production. Results Total emissions were determined to be 0.630 kg CO2e/kg TMR for Scenario A, 0.576 kg CO2e/kg TMR for Scenario B, and 0.521 kg CO2e/kg TMR for Scenario C. Corn production, transportation, and liquid premix production were primary contributors to the life cycle impacts of TMR production. Mass-based allocation of DDGS was found to have the most significant effect on overall impacts of the finishing TMR, with a 42% increase in life cycle emissions compared to other allocation methods. For Scenario A, feed used in Angus feedlot production contributed to 76% of total Angus feedlot emissions. Additionally, feed used in Holstein feedlot production contributed to 58% of total Holstein feedlot emissions. Conclusions and recommendations The present study demonstrates a need to better assess the feed supply chain of feedlot beef production in order to accurately identify areas that have the most significant impacts on overall emissions. This may aid in minimizing impacts associated with feed production and, by extension, beef production. The present study may also serve to inform future decisions for improvements or alterations of the LEAP guidelines.