* Corn futures have risen for five straight days, gaining 7.1 percent during that time. * Traders noted choppy trading throughout the day, with corn prices briefly turning lower in morning trade due to talk of sales on the cash market. * But prices firmed again as the dollar turned lower following the US Federal Reserve's announcement that it will keep interest rates exceptionally low through late 2014. * Spot basis bids for corn continued to firm at Midwest elevators, river terminals and ethanol plants on Wednesday morning. Supplies in the export pipeline remained thin. * Analysts, on average, were expecting a weekly US Agriculture Department report on Thursday morning to show export sales of corn in a range from 650,000 to 850,000 tonnes compared to 759,900 tonnes last week. * Showers blanketed northern Argentina on Tuesday but MDA EarthSat Weather meteorologist Don Keeney said crop conditions did not improve significantly. Additional light showers expected next week. * Investors who are expecting a potential shortfall in US corn production have placed low-risk bets in the options markets that futures prices will surge 25 to 45 percent by the end of this year.