GRAINS-Corn, wheat extend winning streak as forecast heats up

(Recasts, updates with closing prices)

CHICAGO, July 5 (Reuters) - U.S. wheat futures rose for a sixth day in a row on Wednesday, with technical buyers stepping into the market after prices sank into negative territory, traders said.

Chicago Board of Trade corn and soybean futures also closed higher. Corn, which has risen for seven straight trading sessions, received support from concerns about hot weather stressing the crop as it heads toward pollination and prices closed near session highs.

A U.S. Department of Agriculture report released on Friday that showed soybean stocks and acreage below market expectations continued to underpin prices for the oilseed, which rose for a third straight session.

The benchmark CBOT soft red winter wheat contract and the K.C. hard red winter wheat contract rallied to two-year highs on Wednesday but closed below session peaks.

MGEX spring wheat futures, which notched the smallest gains of the day, hit a four-year high early in the session.

Wheat prices turned negative in mid-morning trade but pushed back into positive territory after holding support near Monday's lows.

Traders said concerns about forecasts for a heat wave that will push temperatures above 100 degrees Fahrenheit (37.8 degrees Celsius) in the northern U.S. Plains by the weekend have mostly been factored into the market.

"You can only kill that crop one time," said Mark Schultz, chief analyst at Northstar Commodity Investment Co.

CBOT September soft red winter wheat settled up 5 cents at $5.60 a bushel and K.C. September hard red winter wheat rose 10 cents to $5.69-1/2 a bushel. MGEX September wheat rose 3-3/4 cents to $8.19-3/4 a bushel.

CBOT December corn rose 4-3/4 cents to $4.04 a bushel while CBOT November soybeans were up 13-1/2 cents at $9.94-1/4 a bushel.

Soybeans notched their seventh gain in the last eight sessions.

Last week, USDA said that farmers planted fewer U.S soybean acres than expected, sparking the biggest one-day rally in soybean futures since May 2016. (Reporting by Mark Weinraub, editing by G Crosse and Diane Craft)