GRAINS-Corn tumbles 3 pct on harvest hopes, dollar rally

* Corn, soy tumble on forecast for better harvest weather

* Dollar rally also presses grains

* Harvest pressure, fund liquidation could hit next week (Updates with U.S trading, new byline, dateline, previously Sydney/Paris)

CHICAGO, Oct 30 - U.S. corn futures tumbled 3 percent on Friday, as forecasts called for clear skies across the Midwest next week that would allow farmers to speed up the harvest, the slowest in about 24 years.

Harvest prospects in the U.S. grain belt combined with pressure from outside markets to take soybean futures down nearly 1 percent. The dollar rose as the stock market fell 2.5 percent.

Traders and analysts said the grain markets will get more direction next week from harvest progress, currency and energy markets and speculative fund flows.

"Outside factors will still be present. We could see good harvest progress and some liquidation from trend-following funds...some defensive play," said analyst Shawn McCambridge of Prudential Bache Commodities.

"Markets are down today because there is going to be a window for farmers next week to harvest their crops," he said.

U.S. farmers are forecast to harvest the biggest ever soybean crop this year along with the second largest corn crop, but incessant rains have slowed the pace of collecting them.

"Corn has gone from heading below $3 per bushel to rallying strongly because of the harvest delays," McCambridge said, adding that the inflow of fund money also fueled the rally.

Chicago Board of Trade corn for December delivery fell 12-1/4 cents, or 3 percent, to $3.67-1/2 at 12:38 p.m. CDT (1738 GMT). For the week, corn is down about 7.6 percent but up 7 percent for the month of October.

November soybeans were down 9-1/2 cents, or nearly 1 percent, at $9.76 per bushel. For the week, the soybeans are down nearly 3 percent but up 6 percent for the month.

HAVE MARKETS TOPPED?

Traders and analysts were growing concerned about U.S. corn export sales, lackluster the past two weeks due to higher prices. A bumper crop in the fields and stiff competition from cheaper feed wheat added to the concern.

Some analysts said the corn and soybean markets have seen their highs.

"These markets are certainly acting like they have all the bullish implications of lousy harvest conditions fully factored into prices," agricultural commodities advisory company Brock Associates wrote in a note to clients on Friday.

"The price action has many the markings of a top," it said.

Sluggish export demand for U.S. wheat has kept prices under pressure as the market continues to digest high stocks after big crops in the northern hemisphere for the second straight year.

"If economic news calls for the market to rebound, (wheat) fundamentals remain heavy as underlined by the new IGC report," French consultancy Agritel said in a note.