Among many otherwise laudable suggestions, Mark Eisler and colleagues propose limiting feedstuffs for livestock to fibrous fodder, such as grass and silage (see Nature 507, 32–34; 2014). However, we believe that any attempt to meet the rapid growth in world demand for meat and dairy products by focusing on ruminant grazing systems would be damaging for biodiversity and for the global climate.
Although ruminants convert grass and silage into animal protein, they do so inefficiently; they therefore require much more land to produce a given amount of meat or milk than ruminants fed on diets that include grain. Growing enough fodder to satisfy demand would require the large-scale expansion of grazing lands (see go.nature.com/7mf63y) — a leading cause of biodiversity loss, tropical deforestation and carbon dioxide emissions.
The environmental impacts of meat and dairy production should instead be addressed by stringent efforts to decrease consumption, halt the expansion of grazing, and increase yields on land that is already used for livestock. Promoting extensive grazing without tackling demand would do more harm than good.