* Recent rains benefit Argentina's core farmland
* CBOT soymeal retreats from contract high
* Exporters sell U.S. soybeans to China
CHICAGO, Oct 25 (Reuters) - U.S. grain and soybean futures slumped on Wednesday as the markets monitored beneficial rains in South American and U.S. crop belts.
Traders are watching the weather after drought has hurt crop production in Argentina and the U.S. southern Plains and slowed soybean plantings in Brazil.
Heavy rains last weekend in Argentina improved prospects for corn planting as well as the upcoming wheat harvest. In Brazil, light showers in dry central zones this week may help soybean planting, though soggy conditions in the southeast are raising risks for the wheat harvest.
"Weather is in focus," said Terry Reilly, senior agricultural strategist for Marex. "Brazil's center-west may see rain during the second week of the forecast and Argentina could see scattered showers over the next 7-10 days."
In the U.S., rains are set to delay the final stages of corn and soybean harvesting in the Midwest but benefited recently planted winter wheat in the Plains, analysts said.
Precipitation across almost the entire territory of Ukraine also significantly improved conditions for winter crops, the APK-Inform consultancy said.
The most-active corn contract on the Chicago Board of Trade (CBOT) set a three-week low and was down 4 cents at $4.80 a bushel by 11:20 a.m. CDT (1620 GMT).
Wheat fell 9-1/4 cents to $5.71-1/4 a bushel and touched its lowest price since Oct. 12 at the CBOT. Soybeans dropped 8-1/2 cents to $13.06 a bushel and hit its lowest price since Oct. 13.
CBOT soymeal was also lower, with December futures trading down $5.4 at $428.8 per ton after climbing earlier to a contract high of $439.40 per ton. The setback in soymeal pressured soybean futures, traders said.
The U.S. Department of Agriculture, in a daily reporting system, said exporters sold 126,000 metric tons of soybeans to China.
Traders are watching for the USDA to confirm further U.S. soybean export sales, after an industry group on Tuesday said that a Chinese delegation of commodity importers signed agreements to buy billions of dollars' worth of agricultural goods. (Reporting by Tom Polansek in Chicago. Additioanl reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Janane Venkatraman, Mark Potter and Jonathan Oatis)
