Technical Abstract: Beef production is recognized as a source of greenhouse gas (GHG) emissions; however, little information exists on the net emission from production systems. A life cycle assessment (LCA) was conducted using the Integrated Farm System Model (IFSM) to estimate whole-farm GHG emissions from representative beef production systems in California. The IFSM is a process-level farm model that simulates crop production, feed production and use, animal production, and the return of manure nutrients back to the land to predict the environmental impacts and economics of production systems. The carbon footprint of major production systems was determined as the net exchange of all GHG in carbon dioxide (CO2) equivalent units per unit of hot carcass weight (HCW) produced. The calculation of net emissions determined the relative contributions of the cow-calf, stocker, and feedlot phases of beef production to the overall carbon footprint of the consumable product. The IFSM was used to predict all important sources and sinks of methane, nitrous oxide, and carbon dioxide from primary and secondary sources. Primary emission sources included enteric fermentation, manure, cropland used in feed production, and fuel combustion in handling manure and producing feed. Secondary emissions were those produced during the production of resources used on the farm, which included fuel, electricity, machinery, fertilizer, pesticides, and replacement animals. Simulated beef production systems included the cow-calf, stocker, and feedlot phases for the traditional British beef breeds and calf ranch and feedlot phases for Holstein steers. An evaluation of differing production management strategies produced carbon footprint values ranging from 5.4 to 19.3 kg CO2 equivalent per kg of (HCW) produced. Within the British beef production cycle, the cow-calf phase was responsible for approximately 70% of total GHG emissions with 12% from feedlot sources. Holstein steers that entered the beef production system as a by-product of milk production had the lowest carbon footprint because the emissions associated with their mothers were attributed to milk rather than meat production.