Argentine farmers have started harvesting soybeans but heavy rains that arrived after months of drought have slowed efforts to gather the parched crops, the Buenos Aires Grains Exchange said on Thursday.

Downpours have been bogging down harvesting machines as they try to move across fields in the Pampas farm belt that were bone dry only two months ago during the dog days of the Southern Hemisphere summer.

The hot, dry weather hit soy and was especially hard on corn, which needs moisture during its shorter and more delicate flowering period.

"Rain over the last seven days interrupted harvesting in much of the central soy belt," the exchange said in its weekly crop report. While soybeans were just starting to be collected, 12 percent of this season's corn crop is already in.

Argentina is the world's No. 1 exporter of soyoil, used for cooking and in the booming international biofuels sector. The country is also a major supplier of soybeans and corn, and loads an average 200,000 tonnes of farm products per day.

Last week the exchange cut its estimate for Argentina's 2011/12 corn harvest to 20.8 million tonnes, from a previous 21.3 million tonnes, due to the drought.

Soy, which has a longer flowering period that allows more time to absorb moisture, was less hard hit by the dryness. The exchange sees this season's soy harvest at 46.2 million tonnes.

Soybeans, plus the vegetable oil and livestock feed made from them, account for about half of Argentina's export tax collection.

The U.S. Department of Agriculture chopped its 2011/12 Argentine soy estimate to 46.5 million tonnes from 48 million previously because of the dry spell. Argentina's government sees the oilseed's output at 43.5 million to 45 million tonnes.

The USDA held its estimate for Argentina's 2011/12 corn production at 22 million tonnes after slashing it in February.

Argentina's government sees corn output of between 21 million and 22 million tonnes, and plans to set 8 million tonnes of that aside for domestic use.

The world will count on Argentina's 60 million hectare (148.3 million acres) Pampas farm belt to help meet global demand for food that the United Nations expects to double as global population grows to an estimated 9 billion by 2050

Farmers and economists have complained that government interference in Argentina has stunted investment in the farm sector. Business has chided President Cristina Fernandez for increasing the state's role in the economy.

But the 59-year-old leader was easily re-elected last year, bolstered by strong economic growth and popular social spending. (Additional reporting by Maximiliano Rizzi; Editing by David Gregorio)