GRAINS-Corn, soy rise ahead of holiday weekend as traders monitor US weather

By Karl Plume

CHICAGO, July 3 (Reuters) - U.S. corn and soybean futures firmed on Thursday on short covering and positioning ahead of the long U.S. Independence Day holiday weekend and a speech by President Donald Trump in farm-state Iowa later on Thursday.

Gains, however, were limited by hefty South American supplies and strong U.S. harvest prospects amid largely favorable weather across the Midwest.

U.S. wheat futures eased after two days of gains, anchored by an accelerating U.S. winter crop harvest and big crops in the Black Sea region and western Europe.

Traders squared positions ahead of Friday's market holiday in case the U.S. weather outlook shifts over the weekend. U.S. weather has not been threatening to the crop early in the season, and corn is entering its crucial pollination stage with scattered rains in the forecast limiting concerns about pockets of dryness in parts of the Midwest.

"It's going to be a hot weekend, but it's supposed to cool down next week. I believe many other parts of the Midwest will turn cooler, so I don't think the weather's a real super threat," said Jack Scoville, analyst with the Price Group.

"USDA is rating the crops very high, and I don't see that changing anytime real soon," he said.

Trump is due to speak in Iowa later on Thursday after claiming this week that farmers would be "very happy" with his remarks. Market rumors about the speech sent soybeans sharply higher on Wednesday, analysts said.

Chicago Board of Trade August soybeans SQ25 ended up 2 cents at $10.55-1/2 a bushel for a 2.2% weekly gain. New-crop November soybeans SX25 gained 1-1/4 cents on Thursday to $10.49-1/4 a bushel, adding 2.4% in the week.

September corn CU25 rose 2-1/4 cents to $4.20-1/4 a bushel, up 2.1% in the week. The new-crop December contract CZ25 added 3-1/2 cents to close at $4.37, a 2.3% weekly advance.

CBOT September wheat WU25 fell 7-1/4 cents to $5.56-3/4 per bushel, up 3% in the week.