GRAINS-Wheat rally stalls, corn and soy also suffer setbacks

* Tough competition in export markets weighs on wheat

* Argentina crop woes underpin corn and soy markets (Adds quotes, updates prices)

LONDON, Feb 14 (Reuters) - Chicago wheat futures suffered a setback on Wednesday after the prior session's advance to a six-month high weighed partly by concerns that the recent run-up had hurt the competitiveness of U.S. supplies on the world market.

The most active wheat contract on the Chicago Board of Trade was down 0.8 percent at $4.57 a bushel by 1219 GMT, well below a six-month peak of $4.67-1/4 set on Tuesday.

The recent run-up has been fuelled partly by concerns about persistent dryness in the U.S. Plains.

"It is dry in the United States and that does pose a threat to production but it looks like the market is unwinding some positions that were developed in recent days on this threat," said Phin Ziebell, an agribusiness economist for National Australia Bank.

Dealers noted there continued to be stiff competition in the global market, a view reinforced by news that FranceAgriMer had lowered its forecast of French soft wheat exports outside the EU in 2017/18 for the fourth month in a row.

The agency has previously attributed the diminished outlook to competition from the Black Sea region.

May wheat futures on Paris-based Euronext were down 0.8 percent at 161.75 euros a tonne.

Chicago corn futures also suffered a slight setback after a recent run-up linked to dry weather in Argentina.

The most active corn futures contract was down 0.4 percent at $3.65-1/4 a bushel after peaking at $3.68-1/2 on Tuesday, its highest since Aug. 16, 2017.

"Argentine weather remains a fundamental concern. Weather forecasters expect some rain over the coming week but it may be too little too late for some Argentine crops," Commonwealth Bank of Australia analyst Tobin Gorey said.

"We would not be surprised if the USDA make another downward revision to Argentinean production if the current weather pattern persists."

The U.S. Department of Agriculture lowered its Argentine corn crop forecast by 3 million tonnes in a monthly report last week.

The most active Chicago soybean futures contract fell 0.2 percent to $10.09-1/2 a bushel, having peaked at $10.13-1/2 on Tuesday, the highest since Dec. 6.

Dealers said the run-up had been fuelled by strength in soymeal prices linked to crop woes in Argentina, the world's top soymeal exporter.

The most active soymeal contract was down 0.6 percent at $363.20 a ton after peaking on Tuesday at $371.60, its highest since July 2016. (Additional reporting by Colin Packham in Sydney; Editing by Sunil Nair and David Evans)