The first US wheat purchase by Egypt's state buyer since April fueled early gains, but prices turned lower by midday in a profit taking setback and amid concerns that US wheat would still face stiff competition in the global marketplace.
Corn turned lower on profit taking and pressure from sluggish exports.
A weaker US dollar offered some underlying support to grains markets in general. T he dollar index dipped to a one-month low as signs of progress in tackling Greece's debt lifted the euro, making dollar-denominated grains more attractive to overseas buyers.
Soybeans advanced because of traders' concerns that overly wet weather in No. 3 exporter Argentina and persistently dry conditions in southern areas of Brazil, the world's largest supplier after the United States, would trim production at a time when global supplies were uncomfortably tight.
"The weather in South America is now front and center. There's been a less-than-ideal start to planting and the market is building back in a weather premium after that really sharp selloff we saw in soybean futures in November," said Brian Basting, commodity research analyst with Advance Trading.
Brazil was still expected to harvest a record-large crop next year amid ideal weather in central Brazil, but dryness in the south could limit the crop's potential, analysts said on Monday.
Rains in Argentina this week may sustain flooding that has swamped some areas of the grain belt and delayed planting.
STRONG DEMAND
Meanwhile, robust demand from top importer China and from domestic US processors, who have seen rising demand for soymeal and soyoil in recent weeks, further supported futures.
Karl Setzer, a commodity trading advisor and market analyst with MaxYield Cooperative in West Bend, Iowa, said: "Crush is running high and our exports are high and the United States is the only source right now in the global market."
Chicago Board of Trade January soybeans rose 10 cents, or 0.7 percent, to $14.48-3/4 per bushel by 12:43 p.m. CST (1843 GMT), b r iefly rising above its 200-day moving average of $14.62-1/4 before slipping back.
Corn futures retreated as sluggish exports limited buying interest. CBOT March futures fell 1 cent to $7.51-3/4 a bushel.
The US Department of Agriculture on Monday said only 9.6 million bushels of corn were inspected for export last week, just a quarter of the total in the same week a year earlier and season-to-date shipments were only half of last year's pace.
"In the first quarter of the marketing year, (corn) shipments were barely 200 million bushels. In a good year we get that in three or four weeks," said Basting, the Advance Trading analyst.
WHEAT TURNS LOWER
CBOT March wheat fell 3cents, or 0.4 percent, to $8.60-1/2 per bushel after shedding 2.5 percent on Friday.
Wheat fell for the third straight session despite US wheat claiming the bulk of a 400,000 tonne purchase by Egyptian government buyer GASC in a weekend tender.
The market has anticipated for some time an upturn in US wheat exports after weather-related supply snags in rival exporters like Russia and Argentina, as well as tight stocks in the European Union. Offers in the Egyptian tender showed the US was now matching EU prices, even including higher shipping costs.
Still, US wheat exports to date are well behind the normal pace and the world's top wheat exporter will face competition from a large Australian crop in the coming months.