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The general public is increasingly critical of extensive, ruminant-dominated systems for their attributed high greenhouse gas emissions. However, advocates of low input, grass-fed systems present them as paradigmatic sustainable production systems because of their biodiversity, land use, rural development and animal welfare benefits. We reconcile both analyses by proposing to assess baseline emissions in grazed ecosystems. We show that policies aiming at transitioning grass-fed systems towards fodder-based (concentrate- or grain-based) systems can be ineffective at reducing emissions because wild ruminants or termites fill livestock's ecological niche. Climate change policies targeting livestock should carefully evaluate derived emissions scenarios.