Herbicides used on alfalfa pasture in northwest China can increase greenhouse gas emissions. Two herbicides, Quizalofop-p-ethyl and Bentazone, were studied in 2017 and 2018. Both herbicides led to higher methane and carbon dioxide emissions from the soil. Nitrous oxide emissions decreased, but overall greenhouse gas emissions increased significantly. Quizalofop-p-ethyl had a greater impact on emissions compared to Bentazone. This research shows that herbicide use on alfalfa can contribute to higher greenhouse gas levels, which can affect global warming in the future.