Dairy cows eat after being milked on July 20, 2012 near Whiteland, Indiana

Greenhouse gases emitted by agriculture are on track to rise by 30 percent by mid-century, driven especially by livestock and use of fertiliser, a UN agency reported on Friday.

"Global agricultural emissions are projected to increase in 2030 and 2050 by 18 percent and 30 percent respectively," the Food and Agriculture Organisation (FAO) said in a report issued in Rome.

Annual emissions by mid-century will be more than 6.3 billion tonnes of equivalent (CO2eq), a standardised measure of carbon gas, it said.

Looking at the period from 2001 to 2011, emissions from crop and livestock production rose by 14 percent, the FAO said.

The biggest source in this sector was "enteric fermentation"—methane emitted in flatulence and belching by livestock—which accounted for 39 percent of the sector's total output of greenhouse gases. These emissions rose by 11 percent between 2001 and 2011.

Another big contributor was man-made fertiliser, which accounted for 13 percent of agricultural emissions in 2011—a rise of more than a third since 2001.

Asia accounted for 44 percent of agricultural emissions in 2011, followed by the Americas (25 percent), Africa (15 percent), Europe (12 percent) and Oceania (four percent).

Rice paddies, by generating methane through biological processes, account for 10 percent of total agricultural emissions, while the burning of savannah grasslands accounts for five percent.

Over the decade, emissions of greenhouse gases from land use change and deforestation fell by nearly 10 percent.

Around a seventh of of come from the farm sector, previous estimates have said.