Recent breakdowns in the system meant to keep experimental genetically engineered plants from contaminating the hundreds of millions of acres of crops grown in the U.S. has farmers and import markets questioning the purity of U.S. goods.

Mexico, the largest foreign market for U.S. rice, sent tremors through the U.S. sector midmonth when it stopped shipments on the border out of concern the U.S. can't keep its experimental transgenic long-grain rice out of commercial crops.

California's medium-grain rice growers have demanded a statewide moratorium on any biotech field trials to avoid the contamination recently plaguing long-grain growers in the south.

Those contaminations, California Rice Commission spokeswoman Beth Horan said, prompted farmers and millers to say, "Whoa, this isn't as isolated as we thought and really the system isn't working the way that we thought."

California relies on countries such as Japan and South Korea to buy as much as 30% of the state's harvest each year, and producers want to keep the experimental crops as far away from their fields as possible.

That's getting harder, if not impossible, to do with so many field trials going on, said biotechnology experts at nonprofit consumer groups.

The U.S. is the largest producer of biotech crops in the world, with 135 million acres planted last year, according to the International Service for the Acquisition of Agri-biotech Applications.

European Union countries, which were big long-grain buyers, stopped importing when they realized the U.S. couldn't keep biotech rice out of exports.