Depends on your metrics. If you consider land and water use as a factor they are very inefficient. In terms of nitrogen (for fertilizer etc.) we get most of from the air these days (via the Haber-Bosh process). Manure also has a cost as a source of water pollution and food contaminate. The CO2/NOX emissions are a downstream cost. We are paying for these as externalities and subsidizing the industry via our taxes in turn.