CHICAGO, June 3 (Reuters) - Spot basis bids for corn and soybeans shipped by barge to U.S. Gulf Coast export terminals weakened on Wednesday, reflecting softer demand from shippers following a flurry of farmer grain sales last month, traders said.

• Grain sales by producers stalled in recent days as Chicago Board of Trade corn CN26 and soybean SN26 futures fell sharply on improving U.S. crop production prospects and aggressive speculative selling. GRA/

• But some exporters were adequately supplied following active grain movement earlier this spring as farmers took advantage of surging commodity prices.

• At the U.S. Gulf, CIF corn barges loaded in June traded at 80 cents over CBOT July CN26 futures and were re-bid at 78 cents over futures, down 4 cents from Tuesday.

• CIF July corn barges traded at 88 cents over futures, down 1 cent from Tuesday's trades.

• FOB export premiums for Gulf corn vessels loaded in July were steady at around 102 cents over futures.

• For soybeans, CIF barges loaded in June were bid at 80 cents over CBOT July SN26 futures, down 2 cents from Tuesday.

• July-loaded soybean barges traded at 94 cents over futures.

• FOB export premiums for soybean vessels loaded at the Gulf in July held steady at around 108 cents over futures.

• Ahead of Thursday's weekly export sales report from the U.S. Department of Agriculture, traders expected the government to report export sales of U.S. old-crop corn in the week ended May 28 at 900,000 to 1,500,000 metric tons and old-crop soybean sales of 100,000 to 500,000 tons.

• Traders estimated weekly sales of old-crop wheat as high as 100,000 tons and new-crop wheat sales at 250,000 to 600,000 tons.

• Under its daily reporting rules, the USDA confirmed private sales of 136,000 tons of U.S. new-crop corn to South Korea.

For displays of CIF basis, click on the codes in brackets:

U.S. CIF Gulf soybeans GRYM

U.S. CIF Gulf corn GRYN

U.S. CIF Gulf SRW wheat GRYO

U.S. CIF Gulf HRW wheat GRYP

For displays of FOB basis, please click on the following codes in brackets:

U.S. FOB Gulf corn GRZC

U.S. FOB Gulf soybeans GRZD

U.S. FOB Gulf SRW wheat GRZE

U.S. FOB Gulf HRW wheat GRZF

LINKS

Corn prices page: cpurl://apps.cp./cms/?pageid=corn-cash-market

Wheat prices page: cpurl://apps.cp./cms/?pageid=wheat-cash-market

U.S. barge freight BG/US

U.S. export sales estimates USDA/EST

U.S. grain export summary GRA/U

Brazil soybean export prices SYBS

Brazil corn export prices SYBX

Argentina grain prices GRA/ARG

Russia grain prices GRA/RU