GRAINS-Wheat drops amid record Black Sea supplies, soybeans near 1-week low

* Second straight losing session in view

* Soybeans near lowest since Sept. 5

* Corn falls after 2-day gain

(Adds details, quotes) SINGAPORE, Sept 12 (Reuters) - Chicago wheat futures slid on Tuesday, heading for a second straight losing session with pressure from record supplies from the Black Sea region making it difficult for U.S. exporters to win business. Soybeans dipped for a fourth consecutive session - although losses were marginal - on expectation of a reduction in U.S. yields, while corn fell for the first time in three sessions as markets await key supply-demand reports to be issued by the U.S. Department of Agriculture (USDA) at 1600 GMT. The most-active wheat contract on the Chicago Board Of Trade eased 0.2 percent to $4.34 a bushel by 0257 GMT, having closed down 0.7 percent on Monday, when prices hit a low of $4.31-1/2 a bushel - the weakest since Aug. 31. Soybeans were 0.2 percent lower at $9.58-1/4 a bushel after dropping on Monday to $9.56-3/4 a bushel, the lowest since Sept. 5. Corn gave up 0.3 percent to $3.56-1/2 a bushel. "The market lacks evidence that U.S. (wheat) exports are responding to more competitive U.S. prices," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia. "We still think the evidence will emerge but understand the doubts." Losses in wheat futures came despite Australia lowering its crop production forecast. The Australian government lowered its estimate of the country's 2017/18 wheat crop to 21.64 million tonnes, an eight-year low, from a March forecast of 23.98 million. Russia's wheat production has climbed to an all-time high while Ukraine is harvesting a large crop. These two countries and other exporters in the northern hemisphere are expected to keep global markets well supplied. Meanwhile analysts expect the USDA to trim its U.S. 2017 yield estimates for corn and soybeans. The USDA in a weekly report said the U.S. corn harvest was 5 percent complete, near the five-year average of 6 percent and ahead of the average trade estimate of 4 percent. But crop maturity lagged in key states. In Iowa, the top U.S. corn producer, USDA said 15 percent of the crop was mature, below the five-year average of 28 percent. The corn crop in the No. 2 producer, Illinois, was 26 percent mature, compared to the five-year average of 41 percent.

Grains prices at 0257 GMT

Contract Last Change Pct chg Two-day chg MA 30 RSI CBOT wheat 434.00 -0.75 -0.17% -0.86% 451.47 44 CBOT corn 356.50 -1.00 -0.28% -0.07% 365.01 54 CBOT soy 958.25 -1.75 -0.18% -0.39% 950.81 55 CBOT rice 12.65 -$0.01 -0.04% -0.55% $12.63 47 WTI crude 48.04 -$0.03 -0.06% +1.18% $48.10 51


Euro/dlr $1.196 $0.001 +0.09% -0.59% USD/AUD 0.8011 -0.002 -0.21% -0.58%

Most active contracts Wheat, corn and soy US cents/bushel. Rice: USD per hundredweight RSI 14, exponential

(Reporting by Naveen Thukral; Editing by Kenneth Maxwell)