The paper in Nature Ecology & Evolution is here: http://go.nature.com/2inNH09

With numerous colleagues, Ron Milo and I have been working on the environmental impacts of diets in general and beef in particular for several years now. This work follows my even older work with other collaborators, comparing agricultural resource use by plant vs. animal based whole diets. And if the cumulative knowledge this work has delivered is to be reduced to a sound bite, it is “want an environmentally sound diet but change averse? Avoid beef. The rest is secondary.” This truncated wisdom stems straightforwardly and unambiguously from our 2014-2017 papers. In them, we show that growing the feed needed to produce a kg of beef protein requires 10-20 times the high-quality land needed for growing the feed necessary for producing a kg of poultry protein.  Similarly vast disparities also characterize beef’s irrigation water, synthetic N fertilizer, and greenhouse gas emission needs. We also show that the 38 billion kg feed protein fed to U.S. beef only deliver 1 billion kg of beef protein, a 3% conversion efficiency. With poultry’s conversion efficiency of 21%, we then show that reallocating the agricultural resources currently used for production of beef feed to producing poultry feed would nearly quadruple the amount of animal protein delivered to the U.S diet, enough extra protein to meet the full protein needs of over 140 million people.

Far larger than their estimated uncertainty, these stark liabilities of beef relative to most any other alternative plant or animal protein source have captured the attention of and persuaded large audiences comprising fellow scientists and lay persons alike. Yet beef connoisseurs and grass fed beef enthusiasts have been underwhelmed. While verbally varied, their critique most often invokes the (correct) observation that our analysis addresses the totality of U.S. beef production, and thus mostly reflects conventional CAFO based beef, shedding limited light on grass based beef. Fair enough, we concede, as in the above-mentioned analyses we set out to study the U.S. beef as currently grown, not how much less resource intensive it can become if raised differently.

Implied by this critique is the notion that grazing cattle is not only environmentally less destructive, but indeed beneficial. To date, the former of these assertions is backed up only by limited, ambiguous empirical corroborations, and the more stringent notion that grass fed beef is in fact environmentally desirable can only be currently rationally characterized as suggestive.

While these notions are empirically tested by future work, a basic question arises; “suppose sustainability and environmental desirability of grass based beef are ultimately rigorously corroborated; how much are we talking about? Can grazing supply 1% of today’s beef consumption? 10%?! 100%?”. Curiously, this calculation—the crux of our new Nature Ecology & Evolution paper—has never been previously done.

Because we believe land use must rationally balance food production, wildlife conservation and ample supply of clean fresh water, among other societal objectives, we devise our main calculation so as to provide not a single answer to the above question, but instead a continuous beef availability function that spans the full range of pastureland utilization from no grazing to full occupation of the pastureland area U.S beef currently use, ≈275 million ha. In the latter most extreme case, we can have just under half of current beef supply. Amazingly (and somewhat less certainly), cutting used pastureland to half the current area will diminish this amount only trivially.

Finally, is this beef sustainable? Since agricultural sustainability is yet to be generally and cogently defined, the question is ill-defined and currently unanswerable. But we doubt that by the definition we will eventfully rally behind, using 2.7 million square km—about the size of Argentina or Kazakhstan—to produce 16 g protein person^-1 d^-1 (or 13% of the overall per capita daily protein intake of 120 g) while jeopardizing already imperiled wildlife or degrading western hydrology and fluvial geomorphology will prove sustainable. This doubt is what the quotes enclosing the title’s “sustainable” mean.