Wheat rose 1% on short-covering by funds, while soybeans ticked higher.
The most-active corn contract on the Chicago Board of Trade rose 1.2% to $3.74 a bushel by 0720 GMT, after hitting its highest since March 26 at 3.80 a bushel on Wednesday.
Wheat was up 1.3% at $4.54-1/2 a bushel after closing little changed in the last session and soybeans added 0.3% to $8.38 a bushel, having closed up 0.5% on Wednesday. Corn is drawing support as rains forecast across the U.S.
Midwest are likely to further delay planting. If farmers are unable to sow corn, they will switch to soybeans, which has a later planting window.
Still, ample world supplies of the grain kept a lid on prices.
"Corn planting delay is an issue although supplies are pretty ample worldwide," said Phin Ziebell, an agribusiness economist at National Australia Bank.
"We are not in a world where we have lack of corn. This market action is mainly short-covering."
U.S. farmers seeded 30% of the U.S. 2019 corn crop by Sunday, the government said, lagging the five-year average of 66%. The soybean crop was 9% planted, behind the five-year average of 29%.
The National Oilseed Processors Association said its U.S. members crushed 159.99 million bushels of soybeans in April, down from 170.0 million in March and below an average of analyst expectations for 161.6 million.
Commodity funds were net buyers of CBOT corn, soybean, soymeal and soyoil futures contracts on Wednesday and net even in wheat, traders said.
Trader estimates of net fund buying in corn ranged from 9,000 to 25,000 contracts. The following table reflects the average of estimates from trade sources Expectations of higher wheat output in the Black Sea region is limiting gains in prices.
Favourable weather this winter and spring will allow Ukraine to increase its 2019 wheat harvest to 26.2 million tonnes and its barley harvest to 7.9 million, the APK-Inform consultancy said on Wednesday.
CBOT July corn is biased to retrace to $3.66 per bushel, as it failed to break a resistance at $3.80, Wang Tao, a Reuters market analyst for commodities technicals, wrote in a report.