Corn fell for a fourth consecutive day to a three-week low on an outlook for favourable weather across the US Midwest as crops approach the critical pollination phase.
The most-active wheat contract on the Chicago Board of Trade had declined 1.0 percent to $4.60 a bushel by 1126 GMT, having closed down 1.7 percent on Wednesday.
The market climbed to its highest since June last year at $4.74-3/4 a bushel on Tuesday.
Corn gave up 0.2 percent to $3.68 a bushel, after earlier in the session marking its weakest since June 1 at $3.67-1/4 a bushel.
Soybeans were unchanged on the day at $9.18-3/4 a bushel, after closing down 1 percent on Wednesday.
"The market has already cut forecasts of high protein spring wheat from Canada and the United States," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia in Sydney.
"The US corn crop faces few major weather issues for now. Producers and traders are thus more likely to sell more as they position themselves for a sizeable crop."
The rally in wheat this month has been fuelled by deteriorating conditions for US spring wheat in the northern Plains, which pushed spring wheat futures to a 2-1/2 year high, and the risk of yield damage from a heat wave in France, which helped Euronext wheat futures to an 11-month top.
However, rainfall was forecast in the coming days in the northern US Plains, while in France the extent of damage unclear and extreme heat was set to ease from Friday.
The prospect of stiff export competition from Russian wheat supplies was keeping a lid on international prices.
"The drop of the rouble is a bearish factor, considering the strong (expected) harvest pressure in Russia, consequence of high carryout," consultancy Agritel said in a note, referring to large estimated wheat inventories in Russia.
A tender being held on Thursday by Egypt, the world's biggest wheat buyer, will give a fresh indication of export competition. Black Sea origins including Russian wheat have dominated the country's tenders in recent months.