SAO PAULO- Port premiums for Brazilian soybeans have turned negative as rains disrupt harvesting and transportation of the oilseeds in key growing states such as Mato Grosso, analyst Luiz Roque from agribusiness consultancy Safras & Mercado said on Tuesday.

In some parts of Brazil, trucks are waiting two days in line to unload the freshly reaped grains, he added, underscoring the challenges in shipping a record soy harvest delayed by rains.

"Logistical problems hit premiums as buyers don't like disruptions," Roque said. "We had not seen negative premiums in years."

Brazilian farmers had harvested an estimated 35% of the planted soybean area by Thursday, the slowest pace in a decade due to the heavy rains.

Delays getting soybeans from farms to ports reduced exports in February and increased expectations for shipments this month, when Brazil is forecast to ship a monthly record of 15.5 million tonnes of soybeans.

With Chinese demand still strong and Brazil's real currency reeling from political and economic woes, exports of soybeans are forecast to reach 83 million tonnes in 2021, Roque said.

Domestic farmers will harvest a record soy crop of at least 130 million tonnes in 2021, Safras & Mercado projects. Around 62% of the output has been sold compared with 49% in the previous season, Roque said.

In January, Safras & Mercado forecast Brazil's soy production at 133.1 million tonnes, an estimate that should be lowered in coming days as rains during the harvest spoil quality and hurt yields in midwestern states, he said.

(Reporting by Ana Mano; editing by Jason Neely and David Evans) ((ana.mano@thomsonreuters.com; Tel: +55-11-5644-7704; Mob: +55-119-4470-4529; Reuters Messaging: ana.mano.thomsonreuters.com@reuters.net))