We present a global agricultural greenhouse gas model that assesses emissions from land-use change. In addition to evaluating shifts in and out of crop production, we develop a pasture model to assess extensification and intensification of global livestock production based on herd size and stocking rate. We apply the model to a scenario that introduces a tax on methane emissions from cattle in the United States. The resulting expansion of pasture in the rest of the world leads to substantially higher emissions than without the tax. The yearly average emissions from the tax are 260 metric tons of CO2-equivalent.