* Corn and soy gain support from Brazil tightening tax credit rules
* Wheat weakens as traders weigh Russian weather concerns against a speedy American harvest
CHICAGO, June 6 (Reuters) - Chicago Board of Trade corn and soybean futures rose on Thursday as news of tighter rules on industry tax credits in Brazil made traders and producers hopeful that it could boost U.S. export business, traders said.
Brazilian food exporter lobbies on Wednesday blasted the government's executive order imposing new rules for use of tax credits, claiming the rules are unconstitutional and will make companies less competitive on global markets.
"That'll spur business going our way," U.S.-based Mark Schultz, analyst at Northstar Commodity, said.
CBOT soybeans added 24-1/4 cents to $12.02-1/4 a bushel, while CBOT corn gained 12-1/4 cents to $4.51-1/2 a bushel.
Wheat ticked lower as brokers weighed concerns over drought and frost damage to the Black Sea crop with a burgeoning U.S. harvest.
The most-active wheat contract on the Chicago Board of Trade (CBOT) was down 1-3/4 cents at $6.45-1/2 a bushel as of 1540 GMT.
The Brazilian executive order prompted a wave of short covering as traders digested the news. Meanwhile, Brazilian exporters hesitated to sell in the aftermath of the new rule.
The new measure was included in an executive order sent to Congress on Tuesday. It takes effect immediately but needs approval from lawmakers within four months to remain valid.
"Exporters say by closing the tax credit loophole it will increase the cost of doing business," Tom Fritz, broker at EFG Group, said. "They're taking offers of table as they reassess profit margins."
Spotty rains in the U.S. corn belt as well as concerns over a mid-June heat wave have also given underlying support to markets.
The U.S. Department of Agriculture reported export sales of old-crop corn and new-crop wheat were at the higher end of trade expectations, while sales of old-crop soy landed on the low end of expectations.
USDA also confirmed sales of 152,000 tonnes of old-crop corn to unknown buyers in a flash sale. (Reporting by Heather Schlitz; editing by David Evans)
