Wheat prices slid after jumping to a six-month high, while soybeans were little changed, having risen for the last four sessions.
The most-active corn contract on the Chicago Board Of Trade climbed to its highest since July 2015 at $4.45-1/4, extending gains into a fourth session.
Corn was up 6.6pc for the week after closing down 2.6pc last week.
Wheat rose to its highest since December at $5.37-1/2 a bushel and has added nearly 6pc for the week in what would be its fifth straight weekly gain.
The corn market got a boost this week after the US Department of Agriculture (USDA) cut its forecast of the US 2019 corn yield in a monthly report and slashed its forecast of 2019/20 ending stocks, citing "unprecedented planting delays."
"It is a serious matter now, it will further reduce yields and there are expectations that the USDA will further cut production estimates in the next report," said one Singapore-based trader.
Soybeans have gained ground for the last four sessions as persistent rain in the Midwest crop belt raised concerns that later-planted beans would be hit too.
The USDA said the US soybean crop was 60pc seeded by June 9, compared to the five-year average of 88pc.
Soybeans have gained nearly 4pc this week, rebounding from last week's 2.4pc loss.
A worsening of European soft wheat's competitiveness on world markets led crop consultancy Strategie Grains to cut by more than 1 million tonnes its forecast for EU soft wheat exports to third countries for the 2019/20 season on Thursday.
Strategie Grains now expects the European Union to export 22.1 million tonnes of soft wheat in the season starting on July 1, down from 23.2 million estimated last month.
Commodity funds were net buyers of CBOT corn, soybean, wheat, soyoil and soymeal futures contracts on Thursday, traders said.
Trade estimates of net fund buying in corn ranged widely from 18,000 to 50,000 contracts.