* Strong winds can topple rice plants
* Storm set to enter Arkansas late Thurs, early Friday
* Farmers had counted on a record rice yield
By Sam Nelson
CHICAGO, Aug 30 (Reuters) - Farmers in the top rice-producing state of Arkansas were harvesting their crop at breakneck speed, industry sources said Thursday, in a bid to limit damage from Hurricane Isaac, which had weakened and made landfall as a tropical storm.
The potential threat to the rice crop from Isaac comes on the heels of the worst drought in half a century, which devastated corn and soybean crops in the Midwest farm belt, but had been beneficial to rice plantings.
"I'm real nervous right now with the storm coming in. I started combining at 8 this morning and I usually don't start until 11," said Arkansas farmer Joe Christian, who was busy harvesting his crop in northeast Arkansas, about 65 miles northwest of Memphis, Tennessee.
"I'm in river bottom right now; if I don't get this out today, I may lose it. The wind would blow it down and floods would wash over it," Christian, who worked 11 hours on Wednesday, told Reuters by cell phone from his rice harvester.
Isaac, which was a Category One hurricane on Wednesday, is likely to become a tropical depression Thursday and move over southern Arkansas by early Friday, the National Hurricane Center said.
"They've been in a marathon of harvesting this week, trying to get ahead of the heavy rains. The biggest worry right now is lodging of crops from the wind and rain," said Mary Hightower, a spokesperson for the University of Arkansas Division of Agriculture.
"Rice is the most vulnerable because its heads are filled out, top heavy and could be blown over easily, cotton is probably the next most vulnerable," she said.
The U.S. Department of Agriculture said on Monday 27 percent of the U.S. rice crop had been harvested. Nearly all of the U.S. rice is grown in the Deep South, which is in the path of Isaac.
Arkansas accounts for about half the rice produced in the United States, which is the world's fifth-biggest exporter of the Asian staple.
Chuck Wilson, director of the Rice Research and Extension Center for the University of Arkansas said the storm was entering southern Arkansas near mid-morning and that about 65 percent of the crop was still in the field.
Wilson said Arkansas farmers had been counting on a possible record rice yield of over 7,200 pounds per acre and above the USDA forecast for 7,196 pounds per acre.
However, a lot of the rice crop was now "vulnerable" to harm from Isaac's winds and rains, he said.
"If we have 5 to 6 inches of rain, it wouldn't be good but not as bad as if we get 40 or 50 mile-per-hour winds along with it," Wilson said.
Heavy winds would lodge or blow over the rice plants, either destroying them or rendering them nearly impossible to harvest.
Andy Karst, meteorologist for World Weather Inc. said the heaviest rainfall from the storm - up to 15 inches or more - was in southeast Louisiana, southern Mississippi and southwest Alabama.
"Two to five inches or more are expected on Thursday in central Mississippi and southeast Arkansas," he said.
The rice crop had benefited from the long days of sunshine during the worst drought in over 50 years in the U.S. crop belt, according to Wilson.
"Arkansas rice is 100 percent irrigated and the sunshine this summer helped the crop," Wilson said. (Editing by Bernadette Baum)