Director general of the International Livestock Research Institute, Dr Jimmy Smith, seems to have got it wrong a couple of times when commenting on dietary choices.
Speaking on the Australian Broadcasting Corporation’s Radio National Breakfast program in 2013, he sought to perpetuate the so-called “protein myth” in favour of livestock production, ignoring the fact that soybeans (for example) contain around thirty-five per cent more protein per kilogram than beef, with all the essential amino acids.  
Then in August 2016, he appeared to misstate the findings of an analysis published the previous month in the journal Elementa.   The researchers on that occasion had found that the vegan diet (which excludes all animal products) required the least amount of land (per person fed and in absolute terms) out of ten alternative dietary scenarios. However, Smith claimed in the Guardian that the researchers had found that the vegan diet fell behind certain other diets (including some containing meat) on that measure.
Here’s a chart from the researcher’s paper, showing the vegan diet on the right.
Figure 1: Annual per capita requirements for productive agricultural land by diet scenario and category of land use
Seven of the diets that were analysed included meat. The best-performing of those required nearly twice as much land per person as the vegan diet, while the worst-performing required more than eight times as much. The vegetarian diets, including dairy and/or egg, required nearly eight per cent more land per person than the vegan diet.
While four of the diets were able to feed between 2.3 and 9.7 per cent more people than the vegan diet, the researchers did not analyse consequences in terms of human health, planetary health (along with resultant food production impacts), biodiversity loss or the impact on food production animals themselves. The purpose of the analysis was to compare, firstly, per capita land requirements and secondly, potential carrying capacity as measured by the number of people fed.
Even on the second measure, the vegan diet performed extremely well. The research was based on the US food system, where the authors estimated that a vegan diet could support a population nearly 2.4 times the 2010 figure (although there was no suggestion we should aim for such a population level). The five worst-performing diets based on that measure, all containing meat and other animal products, would feed between 9 and 45 per cent fewer people than the vegan diet.
In the same Guardian article, Smith said that other research had found that medium levels of livestock grazing, rather than none at all, were better for the health, productivity and biodiversity of certain rangelands.  However, the cited paper did not consider the zero livestock grazing option.
The same problem applied to his assertion (citing material from the Food & Agriculture Organization of the United Nations) that such areas sequester large amounts of carbon in their soils when managed well. 
Smith acknowledged livestock-related problems concerning: resource usage (including water); greenhouse gas emissions; human health (including obesity); infectious disease; overuse of antibiotics; and animal welfare. He argued that such problems should not cause us to “turn away from livestock”, noting that the problems are “being addressed today in multi-institutional initiatives”.
However, the problems are so great that the outcome of those “multi-institutional initiatives” might only represent some tinkering around the edges, with little or no meaningful benefit.
In addition to a general transition away from animal products, the solution may include what The Pew Charitable Trusts have referred to in the Australian context as “alternative economic options to pay for stewardship on pastoral properties”, including “the valuing of land management as a vital contribution to a sustainable Australia”. They have reported on the extensive economic and ecological benefits that have been derived from the introduction of indigenous ranger groups and the declaration of indigenous protected areas across huge regions.
They have also highlighted the improved conditions created when pastoralists have transitioned from grazing to ecotourism. Here’s a before-and-after photo showing the “slow but sure regeneration of saltbush and other native vegetation” on the Western Australian property of David Pollock and Frances Jones, Wooleen Station. 
To encourage reduced consumption of animal products, governments could introduce appropriate pricing signals, with related tax revenues being returned to the community through personal income tax reductions and adjustments to welfare payments.
 ABC Radio National Breakfast, “Feeding a hungry world”, 17th September 2013, http://www.abc.net.au/radionational/programs/breakfast/feeding-a-hungry-world/4961802
 USDA National Nutrient Database for Standard Reference at http://www.nal.usda.gov/fnic/foodcomp/search/http://www.nal.usda.gov/fnic/foodcomp/search/ via Nutrition Data at http://www.nutritiondata.com
 Smith, J., “Veganism is not the key to sustainable development – natural resources are vital”, The Guardian, 16th August 2016, https://www.theguardian.com/global-development/2016/aug/16/veganism-not-key-sustainable-development-natural-resources-jimmy-smith
, J., , A.F., , J.L., , T.S., Carrying capacity of U.S. agricultural land: Ten diet scenarios
 Conant, R. T., and K. Paustian, “Potential soil carbon sequestration in overgrazed grassland ecosystems”, Global Biogeochem. Cycles, 16(4), 1143, doi:10.1029/2001GB001661, 2002
 Gerber, P.J., Steinfeld, H., Henderson, B., Mottet, A., Opio, C., Dijkman, J., Falcucci, A. & Tempio, G., 2013, “Tackling climate change through livestock – A global assessment of emissions and mitigation opportunities”, Food and Agriculture Organization of the United Nations (FAO), Rome, http://www.fao.org/ag/againfo/resources/en/publications/tackling_climate_change/index.htm
 Woinarski, J., Traill, B., Booth, C., “The Modern Outback: Nature, people, and the future of remote Australia”, The Pew Charitable Trusts, October 2014, p. 206-212, including image © Sue Vittori (p. 210), http://www.pewtrusts.org/en/research-and-analysis/reports/2014/10/the-modern-outback
International Livestock Research Institute (Paul Karaimu), “Jimmy Smith, director general of ILRI”, Flickr, 2nd June 2011, Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0)
Peters, C.J., et al., op. cit., Figure 2, Creative Commons Attribution License
Wooleen Station © Sue Vittori, from Woinarski, J., et al., “The Modern Outback”, op. cit., p. 210, used with permission
- The words “per person” have been added twice to the first paragraph below Figure 1. [18th February, 2016]
- The third paragraph below Figure 1 has been extended to include a comment on the five worst-performing diets (all animal-based) in terms of the number of people fed. [18th February, 2016]
- The third paragraph of the article has been amended to refer to the amount of land “per person fed and in absolute terms”. [23rd February, 2016]