* Rains moving through Midwest to benefit corn, soy crops
* Some CBOT, K.C. wheat contracts hit lifetime lows
* Stronger dollar also weighs on U.S. crop prices (Updates U.S. market activity to close, adds comments)
CHICAGO, Aug 15 (Reuters) - Chicago Board of Trade corn tumbled to a 10-month low on Tuesday and soybeans fell to a six-week low as rains boosted expectations for bumper U.S. crops, traders said.
Some CBOT wheat and K.C. wheat futures dropped to life-of-contract lows on technical selling and as a stronger dollar threatened to make U.S. crop supplies more expensive for global buyers.
Corn, soybeans and wheat declined to session lows at midday, with most-active CBOT December corn dropping below its support level from Monday of $3.70 before settling down 7-3/4 cents to $3.68-1/2 per bushel.
CBOT November soybean futures finished 14 cents lower at $9.24-1/4 per bushel, lowest since June 30. CBOT September wheat was off 11-1/2 cents at $4.29-1/2, settling above its earlier lifetime low of $4.28.
Investment funds were relatively heavy sellers of 13,000 corn futures contracts, 10,000 soy contracts and 7,500 wheat contracts, traders said.
"It looks like a national down day," independent consultant Sid Love said of declines in many agriculture commodities. "With the dollar up, you put pressure on grains right out of the box."
Prices for U.S. crops had been under pressure from U.S. Department of Agriculture data released late on Monday showing better-than-expected corn crop conditions, and following agency data on Thursday predicting a record-large U.S. soybean crop and the third-biggest corn harvest.
Most analysts expected cuts in production estimates due to relatively hot and dry weather during the growing season.
Rains were falling in parts of Nebraska and Iowa and the storm system was forecast to move east, likely increasing yield potential for corn and soybeans and further weighing on prices.
"Weather forecasters have a fairly corn-friendly outlook for the next week or so," said Tobin Gorey of the Commonwealth Bank of Australia. "And corn crop conditions for last week were a little better, according to the USDA's survey."
The National Oilseed Processors Association's monthly report showed a slightly larger-than-expected U.S. soybean crush in July. However, the data showing stronger domestic demand for soybeans gave little traction to prices.
(Additional reporting by Ana Ionova in London and Naveen Thukral in Singapore; Editing by Dan Grebler and Diane Craft)