GRAINS-Soy slips ahead of USDA report, wheat, corn also lower

* Soybeans extend losses into third straight session

* USDA expected to cut estimate for soybeans from Argentina (Adds quotes, details)

LONDON, March 8 (Reuters) - U.S. soybean and wheat futures were lower on Thursday weighed down by profit-taking ahead of a closely watched U.S. Department of Agriculture report later in the day.

The most active soybean futures contract on the Chicago Board of Trade was down 0.5 percent at $10.60 a bushel by 1047 GMT.

Soybean prices have risen sharply so far this year boosted by dry weather in Argentina, climbing to $10.82-1/2 last week, the highest level since July 2016.

"The monthly USDA report will be released tonight and has pushed traders to cautiousness," analysts Agritel said in a market note. "Considering current level of prices, some profit takings have weighed on soybeans and wheat."

The market expects the USDA to cut its estimate for Argentina soybean production to 48.36 million tonnes, a Reuters poll showed.

"The market's main focus will be on how much the USDA cuts the estimate of Argentina's soybean crop," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia.

The USDA forecast comes as rains are forecast across Argentina, though traders said the moisture was likely to be too late to save yields.

The loss to world supply is likely to be partially offset by higher production in Brazil.

The most active CBOT wheat futures were down 0.1 percent at $4.96-3/4 a bushel, after closing Wednesday down nearly 2 percent.

CBOT wheat prices have also risen strongly so far this year, climbing to a 7-1/2 month peak of $5.18-1/2 last week, as dry weather in the U.S. Plains hurt the outlook for crops.

"Weather models are projecting a rain event to pass close to the dry region on the shared borders of Texas, Oklahoma, Colorado and Kansas (TOCK)," Gorey said.

"And with that there is always the chance some rain might spill into the dry TOCK borders. Weather forecasters are skeptical. Most forecasters expect dry conditions to persist in those regions," he said.

May wheat futures in Paris were down 0.15 percent at 163.75 euros a tonne.

Corn prices also fell with the most active CBOT contract down 0.3 percent at $3.86-1/4 a bushel.

The USDA is expected to forecast larger U.S. wheat ending stocks, due in part to weak export demand, and slightly smaller domestic stockpiles of corn, according to analyst polls by Reuters. (Additional reporting by Colin Packham in Sydney Editing by Biju Dwarakanath and Edmund Blair)