A third of Argentina's soy farms remain swamped after early April storms, with crop loss estimates at 5 million tonnes as harvesting starts in areas dry enough to support the 30-tonne carbines used to bring in the beans, experts said on Wednesday.

Flooding on the Pampas grain belt could end up benefiting U.S. farmers. The weather is improving, but big importers like China are already looking to the Midwest to make up for a likely drop in supply from the South American soybean powerhouse.

"In the first 20 days of the month, 700 millimeters of rain fell in the worst-hit areas," said Rosario grain exchange agronomist Cristian Russo, who estimated flood-related soybean losses at 5 million tonnes or more.

"We've never seen so much rain in such a short period in the middle of harvesting season," he said.

The exchange estimates this year's crop at 59 million tonnes, a forecast that is likely to fall in the weeks ahead as the sun comes out and farmers measure flood-related losses.

Last week the Buenos Aires Grains Exchange chopped its harvest forecast to 56 million tonnes from 60 million.

"Of Argentina's 21 million hectares of soy land, 7 million are fine; 7 million are wet but not swamped; and in the remaining 7 million, growers have to wait for flooding to recede before they can start trying to harvest," said Eduardo Sierra, the exchange's weather adviser.

"All the fields that are not waterlogged are now being harvested," Sierra said. "Those that are still waterlogged will be partially lost."

The areas hardest hit by the storms were in Cordoba, Buenos Aires, Entre Rios, Santa Fe and Chaco provinces.

The first cold front of the Southern Hemisphere autumn hit Argentina on Sunday, ending the rains. Mostly fair weather is expected in May, which should allow harvesting machines to get to farms left isolated when floods washed out the poorly maintained roads that crisscross the Pampas.

President Mauricio Macri was elected in November on promises of increasing investment in the farm sector. He has slashed grain export taxes and a deal he recently negotiated to get the country out of default is expected to open credit lines needed for farming provinces to improve transportation routes.

Argentina is the world's top exporter of soymeal livestock feed and its No. 3 supplier of raw soybeans.