Heavy rains since August have swamped Argentina, the world's No. 3 soybean exporter and No. 2 corn supplier. Topsoils have been flooded in the farm belt, which has just barely recovered from a December-January drought that decimated 2011/12 crops.

"Given all the hail, rain, waterlogging and flooding we've seen, some corn fields will be lost. Others can still be replanted, but with uncertain results," said Buenos Aires-based economist and agricultural consultant Manuel Alvarado Ledesma.

"You can expect a drop in corn production of about 20 percent, to 22.4 million tonnes," he said.

"There has also been a delay in planting soy, which at this point looks like it will reduce the harvest by 10 percent to 50 million tonnes."

Consumer nations hope South American breadbaskets Argentina and Brazil will produce enough grain to make up for some of the shortfall in Russia and the United States, where droughts decimated crops and pushed prices sky-high.

The US Department of Agriculture expects Argentina to harvest 55 million tonnes of soy and 28 million tonnes of corn in the 2012/13 crop year.

Martin Fraguio, who heads local corn industry chamber Maizar, said up to 15 percent of Argentina's corn area might be lost to the rains this year. Still, he said, 2012/13 output will be above the 21 million tonnes produced last season.

"Most of the corn belt has received, from September until today, from 50 percent to 100 percent of the rain they would get in a normal year," Fraguio said.

"We should have planted 50 to 70 percent by this point in the season but we are only at 35 to 40 percent," he added. "But I think it is still reasonable to expect 24 to 26 million tonnes of production, or even more."

The flow of grains from Argentina is important to exporters such as Cargill, Bunge Ltd and Noble Group Ltd , which operate grains terminals along the Parana River, leading to the shipping lanes of the South Atlantic.