Mexico, the largest buyer of US rice, has halted several shipments at the border pending certification that the grain is free of genetically modified material, a Mexican government official said Wednesday. Chicago Board of Trade rough rice futures took a nose dive Wednesday, falling nearly the 50-cent trading limit on talk of the trade disruption, traders said. US export sales were already lagging about 20 percent from a year ago as business has been hurt since a biotech gene material LLRICE601 was found in the US rice supply last summer.