Prospects of South America replenishing world corn supplies are taking a dent from excessive rains which are delaying plantings, and mean that the crop has got off to a "below average start".

The Buenos Aires grains exchange on Thursday said that the pace of corn sowings in Argentina, the second-ranked exporter, had slowed to 6.9% of area in the last week, leaving 32% of corn planted.

That is 11 points, equivalent to some 370,000 hectares, behind the average pace.

"Planting of commercial corn has slowed due to continuous rainfall that has been recorded over much of the agricultural area," the exchange said, flagging amounts of up to 300mm (12 inches) in some areas.

'Below-average start'

The data follow setbacks from excessive rains, and even hail and frost, in parts of southern Brazil, where some growers will be forced to replant their crops.

"South American corn has got off to a below-average start," Michael Cordonnier, at Soybean and Corn Advisor, said, flagging in particular the setbacks in Argentina.

"They have not lost any crop yet, but it is starting to get a little bit concerning," he told Agrimoney.com.

Many buyers are relying on Argentina in particular to fill the void left in world export supplies by the poor US harvest.

The US Department of Agriculture is predicting Argentine corn shipments hitting a record 18.5m tonnes in 2012-13, 2.0m tonnes clear of the current high set three seasons ago.

Corn vs soybeans

However, corn's loss may be soybeans' gain, with farmers potentially using sowing setbacks to swap to the oilseed, which has a shorter development period, and requires less fertilizer to grow.

In Argentina, soybeans are planted from the start of October until early January, while corn is typically planted in September or October, or sometimes December.

November is typically avoided as a planting month because it leaves crops open to hitting pollination during the peak of summer heat, Dr Cordonnier said.

"We could see a switch out of corn in Brazil too into soybeans – a little but less corn and more beans, in acreage terms," he said.