GRAINS-Soybeans ease as commodity setback halts rally

* Soybeans fall after biggest 3-day rally since August 2012

* Drought-busting rain, snow sweep US Plains/Midwest

* Port workers in Brazil plan to strike on Friday

(Adds quotes, updates prices)

LONDON, Feb 21 (Reuters) - U.S. soy prices eased on Thursday as a rally fuelled by dry weather in Argentina and the prospect of a port strike in Brazil was halted by losses in commodity markets linked to a stronger dollar and renewed global economic concerns.

Corn and wheat prices were also lower.

"There is a little bit of weakness on the back of a general sell off in the commodity markets," said Victor Thianpiriya, agriculture strategist at ANZ in Singapore.

Oil and copper hit multi-week lows on Thursday and gold struggled to hold above its weakest level in seven months on concerns the U.S. Federal Reserve might pull the plug on its stimulus program sooner than thought.

Soybeans slipped back after sharp gains earlier this week.

"We've definitely seen a bit of impact from the broader markets. I think we're (also) taking a bit of a breather in the oilseed complex," Rabobank analyst Erin FitzPatrick said.

"We did see pretty strong price moves, especially with soybeans and soymeal, at the beginning of the week because rainfall in South America was less than expected and there's still concerns over the logistics of exporting a record large soybean crop out of Brazil in the coming months," she added.

In Brazil, threats of dockside strikes starting later this week have put the country's agriculture sector on edge as the largest soybean crop ever arrives at its already congested ports.

PORT STRIKE

Port workers nationwide plan to walk off the job on Friday morning and again Tuesday afternoon, according to umbrella union Forca Sindical, and have not ruled out prolonged strikes over planned port regulations reform.

Chicago Board of Trade March soybeans fell 0.1 percent to $14.81-1/2 a bushel by 1226 GMT. The market gained almost 5 percent in the past three trading sessions, the biggest three-day rally since a drought-fuelled jump in August 2012.

Demand from China, the world's largest soybean buyer, also remains brisk, with the U.S. Department of Agriculture (USDA) on Tuesday announcing a sale of 120,000 tonnes of U.S. soy to China.

CBOT corn prices were also slightly lower with March off 0.3 percent at $6.98-1/2 a bushel.

Dealers kept a close watch on conditions in the U.S. with persistent concerns about dry weather, despite some rainfall.

Crop-friendly rain and snow is blanketing much of the drought-stricken U.S. Plains hard red winter wheat region and the dry western Midwest, an agricultural meteorologist said on Wednesday.

"Widespread coverage of the Plains and western Midwest will occur in the next two days, including 10 to 18 inches of snow in much of southern Nebraska, Kansas and northern Missouri," said Commodity Weather Group meteorologist Joel Widenor.

Widenor said a second storm would arrive Sunday and Monday and focus just a bit further north but would bring more beneficial moisture.

"The drought conditions in the U.S., although they've improved in some parts, are still much worse right now than this time of year in 2012," Rabobank's FitzPatrick said.

CBOT March wheat futures were off 0.5 percent at $7.34-1/2 a bushel.

European prices were boosted partly by a decline in the euro to a six-week low against the dollar and March milling wheat in Paris rose 0.6 percent to 245.25 euros a tonne.

Dealers also noted old-crop prices in Paris continued to be supported by short-covering on front-month March and rolling towards May contracts.

(Additional reporting by Naveen Thukral in Singapore and Gus Trompiz in Paris, editing by William Hardy)