SÃO PAULO - The soybean harvest, which kicked off this week in Brazil's leading production area, is expected to accelerate after Christmas, which may help drive the world's largest soybean exporter to supply the market well ahead of previous seasons, farming groups said.

The harvest in Mato Grosso state normally starts in mid-January, but favorable weather prompted some farmers to plant soybeans earlier than usual.

"Some farmers started to harvest this week. They are those who risked an early planting in September and were lucky to get sufficient rain," said Marcos da Rosa, head of Brazil's soybean producers association Aprosoja.

Mato Grosso's production is closely watched by global markets. It harvests about as much as Iowa and Illinois combined.

"Next week, harvest will start at a larger scale. Christmas and New Year's period will be very busy here," said Nery Ribas, technical director at Mato Grosso's farmers association.

Both officials received reports that small areas are already being harvested in northeastern Mato Grosso, including the municipalities of Vila Rica and Confresa.

"So far, crop ratings are excellent," Ribas said.

By the end of January, the state will have 25 percent of its soybean area harvested, dumping more than 7 million tonnes of the oilseed in the market, according to private consultant AgRural.

Customarily, the percentage of soybeans collected by end-January in Mato Grosso is 10 percent to 15 percent, though the figure was 16 percent for the 2009-2010 season.

Mato Grosso is usually the first state in Brazil to begin the soybean harvest.

Paraná, Brazil's second largest soybean producer, will start harvesting in mid-January, local institute Deral said.

Elsewhere in Brazil, the harvest generally begins in early February.

The U.S. Department of Agriculture forecasts Brazil will post a record soybean crop of 102 million tonnes in the 2016-17 season, second only to the United States.

One imponderable, however, is the amount of rain that will fall in coming weeks in Mato Grosso.

Heavier-than-normal rainfall could a major setback for soybeans.

When the soil is too wet, tractors and combines may not enter the fields because they risk getting stuck in the mud and damaging the crop.

Cumulative rainfall through Dec. 31 will be just above normal, according to weather forecasts from U.S. government agencies.