Cattle are top global livestock emitters
Cattle are the biggest source of greenhouse gases, accounting for more than three-quarters of all emissions from global livestock, a survey shows.
The assessment, described as the most detailed of its kind, identified Europe and the Americas as the world's epicentres of beef production.
Annually, the world produces 586m tonnes of milk, 124m tonnes of poultry and 59m tonnes of beef.
The assessment, funded by the International Livestock Research Institute (ILRI) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO), produced a global dataset of cattle, small ruminants, pigs and poultry, as well as milk, meat and eggs for 28 regions.
Its authors gathered data on what farm animals were eating in different parts of the world, how efficiently the feed was converted into milk, eggs and meat, and the volume of greenhouse gases the livestock produced.
"There has been a lot of research focused on the challenges at the global level but if the problems are global, the solutions are almost all local and very situation-specific," said lead author Mario Herrero.
"Our goal is to provide the data needed so that the debate over the role of livestock in our diets, our environments and the search for solutions to the challenges they present can be informed by the vastly different ways people around the world raise animals."
Concerns have been raised over the rapid rise in global meat consumption. The UN says the "unprecedented" growth in demand has been driven by a number of factors, such as population growth, urbanisation and rising incomes.
This has led to numerous reports to conclude that the rising levels of meat consumption at a global scale is unsustainable.
The UN World Health Organization (WHO) estimated that the number of people fed in a year per hectare ranged from 22 for potatoes and 19 for rice to 1 and 2, respectively, for beef and lamb.
It added that the low energy conversion ratio from feed to meat is another concern, since some of the cereal grain food produced is diverted to livestock production.
The authors of the PNAS paper said there was not a globally uniform picture when it came to the amount of resources required to raise livestock or the environmental impact.
They explained that data showed the inputs and impacts varied dramatically depending on the animal, the type of food it provided, the feed it consumed and the region it lived.
For example, they showed that most of the 1.3bn tonnes of grain consumed by livestock was fed to farm animals - primarily pigs and poultry - in Europe, North America, Eastern China and Latin America.
In contrast, they added, all the livestock in sub-Saharan Africa eat only about 50m tonnes of grain annually.
As for emissions, livestock in South Asia, Latin America, Europe and sub-Saharan Africa had the highest regional totals.
Figures suggested that ruminant animals (cows, sheep and goats) required up to five times more feed to produce a kilo of protein in the form of meat than a kilo of protein in the form of milk.
The researchers found that the quality of an animals diet made a "major difference" to feed efficiency and emissions.
For example, in arid conditions in sub-Saharan Africa, cattle had to consume up to 10 times more feed to produce a kilo of protein than an animal kept in more favourable conditions.
"Our data allows us to see more clearly where we can work with livestock keepers to improve animal diets so they can produce more protein with better feed while simultaneously reducing emissions," explained co-author Petr Havlik, a research scholar at the International Institute for Applied System Analysis (IIASA).