Chicago corn futures slid for a second session on Thursday while soybeans lost more ground as much-needed rains across the U.S. Midwest boosted the outlook for crops.
Wheat dropped to a two-week low on pressure from the rapidly advancing harvest of the U.S. winter crop.
Chicago Board of Trade's most-active corn contract fell 0.8 percent to $4.25-3/4 a bushel by 0130 GMT while soybeans gave up 0.6 percent to $11.48-3/4 a bushel.
Wheat lost 0.4 percent to $4.75-3/4 a bushel after hitting a low of $4.75-1/4 a bushel, weakest since June 2.
Showers across the U.S corn belt have this week brought relief to areas where soil moisture has been waning in the heat, and more rain is expected this weekend.
"Rain fell across much of the Midwest, Delta and Southeast yesterday," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia.
"Meteorologists have also tempered their forecasts for remainder of the June - now expected to be less harsh than previously advertised."
Forecasts are for a return to high temperatures this weekend and into next week, although there is also a high chance of precipitation.
Higher demand for U.S. soybean products amid lower South American output kept a floor under the soybean market. The National Oilseed Processors Association in a monthly report said its members crushed 152.8 million bushels of soybeans in May, more than analysts expected and the most on record for the month.
Wheat is being anchored by good yields in early U.S. harvesting and a backdrop of hefty global supplies.
Russian wheat export prices rose further last week on concerns that recent rains could hit crop quality.
Black Sea prices for Russian wheat with 12.5 percent protein content were at $184 a tonne on a free-on-board basis at the end of last week, up $4 from a week earlier, Russian agricultural consultancy IKAR said in a note.
Commodity funds were net sellers of Chicago Board of Trade corn, soybean and wheat futures contracts on Wednesday.
Trade estimates of fund selling in corn ranged from 9,000 to 14,000 contracts, and in wheat from 3,500 to 6,000 contracts. Funds were seen as net sellers of 8,000 soybean contracts.