U.S. grain futures tumbled on Tuesday as the dollar rose and equities fell on renewed concerns about the global economy, with soybeans dropping nearly 1 percent, the biggest one-day loss in a week.

Wheat slid more than 1 percent, reversing four consecutive days of gains, and corn also eased on signs of continued tepid export sales of U.S. corn due to historic high prices. Corn also was under pressure from hints that U.S. cattle feeders were scaling back operations because of continued poor profits due to high feed costs.

"Risk-off is in control at the moment, as the weaker equity markets are casting a long shadow on the markets this morning," said Sterling Smith, futures specialist for Citigroup.

At 10:18 a.m. CDT (1518 GMT), Chicago Board of Trade November soybean futures were down 0.84 percent, or 13 cents, at $15.33-1/2 per bushel, December corn was down 1.44 percent, or 11 cents, at $7.50-1/4 per bushel, and December wheat was down 1.88 percent, or 16-1/2 cents, at $8.61-3/4 per bushel.

"I would suggest that the outside markets' firming is a return to risk-off trade in favor once again of buying the safe-haven assets of the dollar and bonds," said Mike Zuzolo, analyst for Global Commodity Analytics.

U.S. stocks slid more than 1 percent as poor earnings from major multinationals heightened fears about the weak global economy and as Moody's downgraded credit ratings for five regions in Spain.

The Thomson Reuters Jefferies Commodity Research Bureau index was down 1.43 percent at 299.1669.

At 10:20 a.m. CDT (1520 GMT), the dollar index was up 0.45 percent, New York crude oil was down nearly $2.50 per barrel, gold was down 0.83 percent, or $14.40 per ounce , and the Dow Jones stock index was down 1.78 percent. "It could be that some of this weakness in the outside markets is a post-debate sell-off on fears of potentially seeing increased trade tensions with China develop ... regardless of who wins the U.S. presidential election," Zuzolo said.

In their third and final debate on Monday, President Barack Obama and Republican challenger Mitt Romney vowed to get tough on China's trade policies. The U.S. presidential election will be held Nov. 6.

POTENTIAL BIG SOUTH AMERICAN CROP LENDS PRESSURE

Prospects for a bumper soybean harvest in South America early in 2013 also contributed pressure to the soy complex, although losses were slowed by current tight stocks of soy, firm cash soybean markets, slow farmer selling, and waning harvest of the 2012 U.S. soybean crop.

Brazilian analyst Safras e Mercado raised its estimate of the country's soybean production to a record 82.5 million tonnes from its July estimate of 82.3 million due to farmers' expanding the area planted with the oilseed.

Corn found lingering pressure from the release on Friday of a U.S. government cattle-on-feed report that showed the number of cattle placed in America's feedlots in September was well below analysts' average estimate and was the lowest September figure on record. Fewer mouths to feed in upcoming months would mean lower demand for corn, which remains priced at historically high levels.

And on Tuesday, the U.S. Department of Agriculture (USDA) said private exporters switched to non-U.S. corn for a 270,000-tonne sale to Mexico, further evidence of a marked slowdown of exports of U.S. corn.

WHEAT LOWER BUT FINDS UNDERPINNING

Wheat prices were lower as the market fell back slightly after rising for four consecutive sessions. But prices remained underpinned by concerns about crops in Australia and the United States and the prospect of an export ban in Ukraine.

Chicago wheat futures rallied nearly 2 percent last week after setting a two-month low early in the week. The rally was helped by diminishing prospects for the Australian crop and expectations that Ukraine could ban exports from Nov. 15.

"The world balance sheet for wheat is tightening with major exporters, but the United States is still missing out on export business," said Brett Cooper, a senior markets manager at FCStone Australia.

Traders said on Friday that Ukraine's agriculture ministry would ban exports of wheat from Nov. 15 after bad weather hit the harvest.

Commerzbank, in a market note, pointed to a Ukrainian ban as a factor supporting European wheat.

"This improves in particular the export prospects for the EU, which is already making European wheat more expensive - its price has meanwhile overtaken that of U.S. wheat," Commerzbank said.

November milling wheat in Paris was down 0.10 percent, or 0.25 euro, at 262.75 euros per tonne. Prices at 10:23 a.m. CDT (1523 GMT)