Wheat also extended gains as flooding similarly threatened to hamper the start of spring wheat sowing.
Soybeans ticked lower as the oilseed market weighed the potential for US farmers to shift some area from corn to later-planted soybeans.
Favourable South American harvest prospects, uncertainty over the outcome of US-China trade negotiations and ample global grain supplies were acting as a curb on prices.
The most active corn futures on the Chicago Board Of Trade were up 0.7 percent at 3.79 a bushel by 1228 GMT after earlier reaching their highest since Feb. 26 at $3.79-3/4.
"Parts of the US Midwest are looking at planting delays because of heavy rain. And that might shunt a few corn acres into beans," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia.
The US National Oceanic and Atmospheric Administration on Thursday said that the extensive flooding in Nebraska and Iowa would persist in the wider region through May.
"Planting begins in April and it is uncertain whether the water will have drained by then, leaving soils sufficiently dry," Commerzbank analysts said of corn.
The flooding has also shut down about 13 percent of the nation's production capacity for corn-based ethanol fuel.
Price gains in corn and grain markets generally were being capped by the fact that spring planting had yet to start in the heart of the Midwest, while large stockpiles have made traders less nervous about emerging weather threats.
The most active CBOT soybean futures were down 0.3 percent at $9.08-1/4 a bushel.
Analysts highlighted uncertainty over the US-China trade talks weighing on soybeans, the most valuable US farm export to China.
Lower than expected weekly US soybean export sales, including minimal reported sales to China, added to caution about the trade negotiations.
The most active CBOT wheat futures were up 0.3 percent at $4.68 a bushel, having earlier touched their highest in more than three weeks at $4.70-3/4.
Heavy snowfall and expected flooding in the US northern Plains spring wheat belt, particularly in top producer North Dakota, have also raised the risk of delays in planting the 2019 crop. Seeding normally begins in April.