Soy tumbles on harvest weight, wheat falls for 4th day

US soybeans slid over 1 percent on Wednesday, notching the biggest one-day drop in three weeks as the harvest in the Midwest farm belt got underway and on profit-taking a day after prices set an all-time high near $18 per bushel.

Prices in the cash market have been tumbling on expectations for increased supplies from the harvest, which is expected to yield a smaller crop due to the worst drought in half a century.

Corn eased 2 percent on profit-taking as traders began squaring positions ahead of the US government crop report next Wednesday and on harvest pressure.

Wheat fell over 2 percent, declining for the fourth consecutive session as rain improved prospects for the drought-stricken wheat crop, and on signs of stiff competition for US wheat exports from Black Sea sellers.

"Commercial guys (grain companies) are saying farmers are selling a lot of their (soybean) crop right off the combine at these high prices. That's bringing the basis down," said Dan Cekander, analyst for Newedge USA.

Basis bids to buy soybeans in the cash markets fell as much as 25 cents per bushel on Wednesday due to harvest pressure.

The US Department of Agriculture (USDA) next Wednesday will release its September crop production and supply/demand reports and see saw trade is expected ahead of the release of that data.

"We're just biding our time until we see what USDA says next week," McCambridge said.

Traders said the markets were taking a breather after the worst drought in 56 years slashed crop prospects leading to a summer rally to record highs of nearly $8.50 for corn in late August and almost $18 for soybeans yesterday (Tuesday).

Chicago Board of Trade December wheat was down 21 cents per bushel at $8.67-3/4, December corn was down 14-1/4 at $7.90-3/4 and November soybeans were down 20-3/4 at $17.47-1/2.

Spot soybeans have jumped 33 percent in just two months while spot corn soared nearly 50 percent.

Wheat prices rose more than 50 percent as well but much of the support in wheat came from spillover buying due to the soaring corn and soybean markets.