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The average hourly earnings of all hired farm workers as reported in the USDA Farm Labor Survey have been between 50 and 60 percent of the average hourly earnings of private sector nonfarm workers reported in the BLS Employment and Earnings survey over the past 30 years. These earnings data exclude the cost of benefits provided to employees: in the nonfarm sector, the cost of employee benefits has been rising faster than earnings.
Sources: http://usda.mannlib.cornell.edu/reports/nassr/other/pfl-bb/ and Table B47, Economic Report of the President, 2006