Wheat was little changed, having tracked corn higher on Tuesday, while soybeans eased.
The most-active corn contract on the Chicago Board of Trade (CBOT) was down 0.4pc at $4.26-1/4 a bushel by 1124 GMT, having risen 2.9pc in the previous session when it touched a one-week high.
The US Department of Agriculture (USDA) slashed its corn yield projection in a monthly report on Tuesday by 10 bushels an acre, or 5.7pc, to 166 bushels. Analysts had on average expected a smaller decline to 172.4 bushels.
The drop, along with a 3pc decrease in the USDA's acreage estimate, left the government's production outlook at 13.68 billion bushels, which would be the smallest harvest in four years.
"Our estimates now have the US corn loss at 40-50 million tonnes, so the USDA will show more reduction in the next few months," said Ole Houe, director of advisory services at brokerage IKON Commodities.
A soggy spring in the US Midwest has led to the slowest corn planting pace on record, raising the prospect that some acreage will be lost while planted crop will lose yield potential.
Drier conditions last week helped planting accelerate and traders are watching to see if more heavy rain forecast in parts of the Midwest in the week ahead adds a further threat to planting and yield prospects.
"In the coming few months, CBOT traders will hotly debate the weekly crop conditions and implications for yield potential above or below 170 bushels per acre," Rabobank analysts said in a note.
"The market will also grapple with current high inventories and the potential that these can buffer a supply-side shock."
CBOT wheat was up a quarter of a cent at $5.18-1/4 a bushel, after ending Tuesday up 2.1pc, and soybeans were down 0.3pc at $8.56-1/2 a bushel.
The USDA cut its projection of US wheat stocks at the end of 2019/20, anticipating some feed demand would shift to wheat given reduced corn supply.
At the same time, it raised its forecast for US wheat production while also increasing output estimates for major wheat exporters Russia and Ukraine.
The agency on Tuesday kept its US production and yield estimates for soybeans unchanged, noting soybean planting could continue for several weeks, but this surprised analysts who were expecting declines.
Adjustments to the soybean crop outlook can be expected in the USDA's July report, its chief economist told Reuters at an industry event in London.