US leads the world in farm productivity per labour force attributable to agriculture. Labour force % by occupation for each country is available at http://www.cia.gov/cia/publications/factbook/fields/2048.html Multiply this by labour force of that country which is available at http://www.cia.gov/cia/publications/factbook/rankorder/2095rank.html The result is the portion of labour force attributable to agriculture. Now divide the agricultural output by this number to get the $ value of farm productivity per labour per year. Among the top five nations based on agricultural output, US is way ahead on top... 
$122,189 - US
$21,089 - Japan
$3,349 - Brazil
$680 - China
$508 - India

Smaller economies may have higher productivity than US but they are insignificant on a global scale plus in practicality, small agri projects with unusually high yields have had mixed or no success when scaled up to large commercial markets.