Grass-fed cows gain weight more slowly, so they produce more methane per unit of meat produced. However, it can be argued that grass fed cattle farming is a net positive for Greenhouse emissions, since the carbon captured by the larger pastures can offset the effect of the methane emissions from the cattle. In some cases the entire farm's operational emmisions could be offset. Note that this is climate dependent. [1]

It is important also to note that if the land were instead used for growing crop for human consumption, then the carbon capture effect would be far higher, and that generally grass fed beef in the US are fed grain in the final months before slaughter. Other factors like water usage of cattle farming per calorie produced compared to human consumed crop can put even grass fed cattle farming in a bad light.

[1]: https://www.npr.org/sections/thesalt/2019/08/13/746576239/is...