By Ana Mano

ARLINGTON, Va., Feb 22 (Reuters) - Brazil has spent billions of dollars improving its port infrastructure, but it will be another 20 years before road, rail and waterway bottlenecks that hamper agricultural exports are resolved, a top industry economist said on Thursday.

Brazil is the world's top exporter of soybeans, sugar and beef, and the second largest grains exporter. Improvements in port capacity have ended chronic delays that plagued exports for years.

But the transport networks that feed the ports still need a lot of work, said Daniel Amaral, chief economist of the country's oilseeds association Abiove.

"Brazil still has an inadequate transportation matrix," he said at an industry conference in the United States. "We need a lot more investments in waterways and rails."

Roads in some parts of Brazil that connect producing regions to ports are still not paved, Clay Hamilton, an official with the U.S. Department of Agriculture who works at the U.S. Embassy in Brasilia, said at the same event.

Earlier this month, soy truckers in Brazil posted footage showing they were unable to move on an unpaved stretch of highway in the state of Para connecting the country's agricultural heartland to the northern ports.

Despite the restrictions, exports from Brazil continue to rise and the country's role as an agricultural powerhouse grows. Brazil is on track to eclipse the United States as the world's top corn exporter and is competing with the United States for market share in countries such as Mexico.

(Editing by Simon Webb and Leslie Adler)