US Republican presidential candidates at the Iowa Straw Poll had to concede defeat to Michele Bachmann [Reuters]

US Congresswoman and Republican presidential candidate Michele Bachmann has won the Iowa straw poll, in an early boost for her campaign for the White House.

Bachmann came out on top as expected after campaigning hard in the heartland state, with 28.5 per cent of the 16,892 votes cast in the early test for Republicans.

Libertarian congressman Ron Paul came in a close second, followed by former Minnesota governor Tim Pawlenty in third.

Saturday’s result comes five months before Iowa holds the caucuses that kick off the Republican nomination season.

It's the first indication of what Iowans think of the Republican presidential field and which candidate has the best get-out-the-vote organization.

Nine candidates were on the ballot. But not all competed.

Mitt Romney, the Republican front-runner in national polls, and former Utah Governor Jon Huntsman sat it out.

And supporters of Texas Governor Rick Perry and former Alaska Governor Sarah Palin waged write-in campaigns.

Source: Agencies