U.S. corn futures dropped for a third straight session on Tuesday because of sluggish demand and expectations for wetter weather in world No. 3 exporter Argentina.
Soybean prices rebounded from an earlier profit-taking slump on technical buying and some concerns that the still-developing South American crop may shrink further if the weather worsens.
Wheat dropped for a fourth consecutive day and hit a 3-1/2-week low on forecasts for rain in the drought-parched U.S. Plains.
Some investors were beginning to take positions ahead of Friday's U.S. Department of Agriculture monthly supply and demand reports, which were expected to show tighter U.S. and global soybean stocks and a larger U.S. corn supply.
"The closer we get to Friday's USDA report I think we'll see some more positioning and profit-taking and settle into a little more of a range," said Dax Wedemeyer, a broker for U.S. Commodities.
Wedemeyer said that the Argentina forecast looks a little wetter in the next six to 15 days, "but confidence is kind of low because rains in these areas have been less than expected in the last two weeks."
Argentina is the world's No. 3 soybean and corn exporter after the United States and Brazil. Consumer nations are hoping it can provide ample supplies of both crops to bolster world food stocks.
Wedemeyer said that for soybeans "there's still a lot of unknowns as far as how much of the crop in Argentina has been affected by the recent dry weather."
Chicago Board of Trade (CBOT) March soybeans rose 6-3/4 cents, or 0.5 per cent, to $14.95-1/2 per bushel after touching a seven-week high of $14.98 on Monday but failing to breach chart resistance at $15/bu.
CBOT March corn fell 5-1/4 cents, or 0.7 per cent, to $7.29/bu. The contract found underlying support at the 50-day moving average of $7.23-1/4 and overhead resistance at the 100-day moving average of $7.35-1/4.
Spot corn has also struggled to breach the $7.50/bu. mark as high prices have pared demand from both exporters and ethanol makers. Several ethanol plants have been shuttered in recent weeks, most recently an Ag Processing Inc. plant at Hastings, Nebraska.
USDA was expected to raise its U.S. corn ending stocks forecast in a Friday report due to lower demand from the ethanol industry and weak corn exports, which are forecast to fall to a 41-year low in the current marketing year.
However, soybean stocks, both in the United States and globally, were expected to show a decline in Friday's report due to reduced South American production and resilient demand for U.S. soybeans.
Rain in Plains
Some minor relief from the drought in the U.S. Plains hard red winter wheat region was expected by the weekend with rain and snow likely over a broad swath of the crop belt, an agricultural meteorologist said Tuesday.
Commodity Weather Group meteorologist Joel Widenor said a weekend storm will bring from 0.25 to one inch of moisture from northeast Colorado and northern Kansas into Iowa and Wisconsin.
An unexpected drop in year-on-year Canadian wheat stocks in a Statistics Canada report on Tuesday helped to limit the market's decline.
News that Russia's government backs a plan to suspend a grain import duty had little impact on the market. Operators were waiting to see when the measure would be implemented and what import volumes it could trigger.
Potential suppliers could include Europe, South America and the United States.
CBOT March wheat fell 5-1/2 cents, or 0.7 per cent, to $7.57-1/2 a bushel.
Commodity funds bought an estimated net 4,000 soybean contracts on the day and sold a net 6,000 corn and 3,000 wheat contracts, trade sources said.
-- Karl Plume writes for Reuters from Chicago. Additional reporting for Reuters by Sam Nelson.