The U.S. government redirected the gift to other hungry African nations after Zimbabwe refused to waive a requirement that imported grain be certified as non-genetically modified organisms. The U.S. government does not segregate GMO grains from conventional crops.

Zimbabwe's reluctance to accept whole-kernel U.S. corn poses an uncomfortable challenge to humanitarian officials planning for an impending food emergency in southern Africa, where 19 million people in six countries face hunger because of drought and mismanagement of food supplies.

The United States, with its awesome agricultural output, is by far the largest donor to the U.N. World Food Program, the agency that distributes food to needy countries. But U.S. grain is not suitable for Zimbabwe, which has potentially the largest food deficit in southern Africa.

Though officials in Zimbabwe say they are aware that the U.S. government has determined these foods to be safe, they are reluctant to accept them without their own testing.

U.N. representatives, African officials, and emissaries from donor countries are scheduled to meet in Johannesburg this week to sort out how food from producing countries will be divvied up in the coming months.

"We don't make judgment calls on GMOs, as long as the food passes safety standards," said Brenda Barton, a spokeswoman for the World Food Program. "We're really trying to stay out of the middle of this debate."

Zimbabwe's neighbors are hoping they will benefit with larger allocations of U.S. maize, as corn is called here.

"If Zimbabwe doesn't want the maize, there are plenty of people here willing to accept it," said Nick Osborne, the country representative of Care International in Malawi, where humanitarian officials expect severe food shortages by September, after a second successive crop failure from drought.

Food imports to Zimbabwe are generally not an issue, because Zimbabwe's farmers usually produce a surplus. But the country of 12 million people has been hit by a triple whammy this year - a serious drought, the devastation of thousands of white commercial farms by government-backed militants, and the government's sell-off of its grain reserves.

The result: Zimbabwe is expected to produce only a quarter of its maize crop this year, and will require imports of 1.6 million metric tons. Corn is the principal staple crop in southern Africa; in poor countries like Malawi, maize accounts for up to 70 percent of a rural resident's diet.

Zimbabwe imposed the restrictions on genetically engineered food two years ago, partly to support its commercial farmers. Zimbabwe exported beef and ostrich to Europe, where consumers demand that meat come from livestock that has not been fed with the engineered grain.

With Zimbabwe's agricultural sector in turmoil because of the government's takeover of many white-owned commercial farms, the African nation last year stopped exporting meat to Europe.

"The Europeans are quite paranoid about GMOs, and we were concerned about keeping our markets," said Paul d'Hotman, chief executive officer of the Zimbabwe Cattle Producers Association. "But it's all academic now."

Zimbabwe's commercial farmers face soaring prices for livestock feed, and have asked the government to relax the restrictions on importing genetically engineered grain. Crops such as corn are typically modified to resist disease or drought.

The government is concerned that donated genetically modified, whole-kernel corn might be replanted in Zimbabwe, starting an irreversible propagation of untested grain.

"It is not so easy to say we're going to suspend our regulations because we have a food emergency," said Abisa Mafa, registrar of the Research Council of Zimbabwe's biosafety board.

Yet the Zimbabwean government is willing to feed genetically engineered grain to its population. This year, it has accepted nearly 43 metric tons of U.S. corn meal and corn-soy milk worth $27.5 million that the Americans could not certify was non-GMO. Zimbabwe says corn meal and corn-soy milk are acceptable because they have already been processed and cannot be planted to grow crops.

But donor nations are unlikely to be able to fill Zimbabwe's vast needs with milled maize, which is more expensive than whole-kernel corn, Mafa said. Zimbabwe might be willing to accept whole U.S. grain as long as it was milled as soon as it entered the country to prevent it from being replanted.

D'Hotman, the livestock producer, said he believed it was unlikely that much of the imported modified grain would get set aside as seed.

"People are hungry, and when they're hungry, they're going to eat the grain and not plant it."