Evaluating greenhouse gas (GHG) emissions at farm level is an important tool to mitigate climate change. Livestock account for 80% of the total GHG emissions in Uruguay, and beef cow-calf systems are possibly the largest contributors. In cow-calf grazing systems, optimizing forage allowance and grazing intensity may increase pasture productivity, reproductive performance, beef productivity, and possibly reduce GHG emissions. This study estimated GHG emissions per kg of live weight gain (LWG) and per hectare from 20 cow-calf systems in Uruguay, with different management practices. The GHG emissions were on average 20.8 kg CO2-e.kg LWG-1, ranging from 11.4 to 32.2. Beef productivity and reproductive efficiency were the main determinants of GHG emissions. Five farm clusters were identified with different productive and environmental efficiency by numerical classification of relevant variables. Improving grazing efficiency by optimizing the stocking rate and forage production can increase beef productivity by 22% and reduce GHG emissions per kg LWG by 28% compared to “low performance” management. Further improvements in reproductive efficiency can increase productivity by 41% and reduce GHG emissions per kg LWG by 23%, resulting in a “carbon smart” strategy. However, the most intensified farms with highest stocking rate and beef productivity, did not reduce GHG emissions per kg LWG, while increased GHG emissions per ha compared to the carbon smart. This analysis showed that it is possible to simultaneously reduce carbon footprint per kg and per ha, by optimizing grazing management. This study demonstrated that there is high potential to reduce cow-calf GHG emissions through improved grazing management.