* USDA pegs U.S. corn, soy harvests below expectations

* USDA cuts grain and soybean stocks

* Wheat futures at 6-year high; corn, soy at 6-1/2 year highs

CHICAGO, Jan 12 (Reuters) - U.S. corn, soybean and wheat futures surged to fresh multi-year highs on Tuesday after the U.S. Department of Agriculture (USDA) said 2020 U.S. corn and soy harvests were smaller than expected and stocks of key crops were thinner that previously thought.

Corn jumped by as much as 5%, while soybeans added nearly 4% and wheat gained almost 5%.

The surge took prices to highs not seen since 2014.

In its eagerly awaited series of January reports, the USDA pegged the 2020 U.S. corn harvest at 14.182 billion bushels based on an average yield of 172.0 bushels per acre and soybean production at 4.135 billion bushels on a yield of 50.2. All were below average trade expectations, particularly corn.

The USDA also forecast end-of-season corn and wheat supplies below trade estimates, and confirmed investor fears of the tightest U.S. soybean stocks in seven years.

"Nobody was expecting corn production to drop like that," said Jack Scoville, analyst with The Price Group. "They (USDA) underestimated the damage from the drought and the derecho and probably overestimated some of the better areas."

Chicago Board of Trade (CBOT) March corn ended up the daily 25-cent limit at $5.17-1/4 a bushel, the highest level for a most-active contract since May 2014. May and June futures were also limit-up, and all three were trading synthetically through options about 8 cents above closing prices.

Corn trading limits will expand to 40 cents on Wednesday following the limit-up close, CME Group said.

March soybeans jumped 45-3/4 cents to $14.18-1/4 a bushel, the highest for a most-active soy contract since June 2014. CBOT March wheat gained 30-1/4 cents to $6.65 a bushel, the highest since December 2014.

Concerns about available supplies from key exporters added support. Argentina has moved to curb corn exports, while top wheat exporter Russia is considering raising a previously announced export tax on the grain. (Additional reporting by Michael Hogan in Hamburg and Naveen Thukral in Singapore Editing by Robert Birsel and Marguerita Choy)