GRAINS-Soy at 3-week low on big S.America crops; wheat, corn rise

* Soybeans retreat from early advance * Corn firm on technicals, expectations of drop in U.S. acreage * Wheat firm on light pickup in global export business * Traders adjust positions ahead of annual

USDA

forum (New throughout; updates prices, adds quotes, changes byline, dateline, previous HAMBURG) By

Julie Ingwersen

CHICAGO,

Feb

22 (

Reuters

) - U.S. soybean futures fell to a three-week low on Wednesday, on track for a fourth straight decline on strong South American harvest prospects and a lack of supportive news, analysts said. Corn and wheat futures were modestly higher as traders awaited direction from a U.S.

Department of Agriculture

forum this week in which the government will offer forecasts on what U.S. farmers will plant this spring. As of 12:18 p.m. CST (1818 GMT),

Chicago Board

of Trade March soybeans were down 2-1/2 cents at $10.23-3/4 per bushel after dipping to $10.23, the contract's lowest since Feb. 1. CBOT March corn was up 2-1/4 cents at $3.71-1/2 a bushel and March wheat was up 5 cents at $4.41 a bushel. Soybeans firmed in early moves but turned lower as traders mulled huge new soybean crops expected in South America. The harvest in

Brazil

was about 25 percent complete as of Friday, while

Argentina

's crop is still developing. "Rises today are being limited by the expected large soybean crops in

South America

," said

Graydon Chong

, senior commodity analyst with

Rabobank

. "The market is closely watching the weather in Brazil and Argentina," Chong said. Also, U.S. farmers are expected to plant a record number of acres to soybeans this spring, the USDA has said. The government is expected to release an updated forecast at its Outlook Forum this week. Expectations of a year-on-year decline in U.S. corn plantings helped to underpin corn futures. "Corn is trying to buy back some acres here," said

Terry Reilly

, analyst with Futures International. Strength in CBOT corn was limited in part by news from the USDA that private exporters in the last day cancelled sales of 136,000 tonnes of optional-origin corn to

South Korea

. Wheat rose on bargain-buying after a three-session decline, drawing additional support from a pickup in global export business. Egypt's

General Authority for Supply Commodities

(GASC) bought 300,000 tonnes of Russian wheat and 60,000 tonnes of Ukrainian wheat at an international tender. The deal followed a Feb. 17

GASC

purchase of another 360,000 tonnes of wheat. "Having

Egypt

in twice in one week is a bit supportive,"

Reilly

said. CBOT prices as of 12:18 p.m. CST (1818 GMT): Net Pct Volume Last change change CBOT wheat WH7 440.25 4.25 1.0 33267 CBOT corn CH7 371.50 2.25 0.6 88718 CBOT soybeans SH7 1023.25 -3.00 -0.3 52311 CBOT soymeal SMH7 335.60 -1.80 -0.5 36898 CBOT soyoil BOH7 32.74 0.07 0.2 48015 NOTE: CBOT March wheat, corn and soybeans shown in cents per bushel, soymeal in dollars per short ton and soyoil in cents per lb. (Additional reporting by Michael Hogan in

Hamburg

and

Colin Packham

in Sydney; Editing by

Alexander Smith

and

James Dalgleish

)