The overall top 10 U.S. export markets for corn.
U.S. Department of Agriculture: Corn - http://www.ers.usda.gov/Briefing/Corn/trade.htm
Corn is by far the largest component of global coarse-grain trade, accounting for about three-quarters of total volume in recent years.
USDA-Marketing and Trade - http://www.usda.gov/wps/portal/!ut/p/_s.7_0_A/7_0_1OB?navtype=SU&navid=MARKETING_TRADE
The USDA has helpful tools to understand the marketing and trade of the agriculture business both domestic and international.