Corn eased following its biggest one-day gain since late June on Monday after the United States, Canada and Mexico reached a trade agreement.
The most-active soybean contract on the Chicago Board of Trade had fallen 0.3 percent to $8.55-1/4 a bushel by 0334 GMT, having firmed 1.4 percent on Monday when prices hit their highest since Aug. 23 at $8.64 a bushel.
Corn slipped 0.4 percent to $3.64-1/4 a bushel, having gained 2.7 percent in the previous session. Wheat climbed 0.4 percent to $5.11-3/4 a bushel, after closing little changed on Monday.
"Some wet weather is, and likely will, dog US harvesting for a period," said Tobin Gorey, director of agricultural strategy, Commonwealth Bank of Australia.
"That helps prices to some degree of course. As is the renewed trade agreement between Canada, Mexico and the US"
The US Department of Agriculture said 23 percent of the US soybean crop had been harvested as of Sunday, behind market expectations.
The agency said 26 percent of the corn crop has been harvested, matching analyst forecasts.
"Harvest activity and crop progress were slow for the Crop Watch corn and soybean fields last week as wet and/or cool weather plagued all eight producers (states) in some way or another," Karen Braun, a market analyst for Reuters, wrote in a column.
"That theme is likely to continue in many areas as the forecast for the next two weeks is much wetter than normal across the US Midwest."
Corn and soybeans drew strong support on Monday after the United States, Canada and Mexico announced they had reached a deal on a trilateral pact to replace NAFTA.
Still, expectations of bumper US production continues to weigh on corn and soybean prices.
Commodity brokerage INTL FCStone on Monday raised its estimate of the average US 2018 corn yield to 182.7 bushels per acre (bpa), from 177.7 in its previous monthly report released Aug. 30.
The firm raised its forecast of the US 2018 soybean yield to 54.0 bpa, up from its Aug. 30 figure of 53.8.
Brazil's 2018-19 soybean crop is seen reaching 120.4 million tonnes, a record, according to the average view from 10 analysts and consultants in a Reuters poll.
Commodity funds were net buyers of CBOT corn, wheat, soybean, soymeal and soyoil futures contracts on Monday, traders said.