Global society is dependent on ruminant livestock to convert human indigestible plant matter into high quality dairy and meat food sources. This reliance on ruminants is a product of the symbiotic relationship between the ruminant and the rumen microbiome, which allows nutrition to be gained from forage. Pasture has long been utilised as a feed source for ruminants due to its economic benefits. Indeed, variation exists in the effects on animal performance amongst different forage species. However, the fermentation of forage by ruminants is associated with the production of GHGs, which negatively effects the environment and is associated with reductions to animal performance. Therefore, this chapter aims to examine the relationship between the rumen microbiome, host feed efficiency and environmental outputs in pasture based productions systems. In addition, the impact of different forages and grassland management practices have on the rumen microbes and the previously mentioned phenotypes will be reviewed.