Lower commodity prices and weakened stock markets weighed down the Canadian dollar against its US counterpart during foreign exchange trading yesterday (September 21st).Reuters reports that there was a pervasive sense of caution guiding trading yesterday ahead of the opening of the G20 Summit in Pittsburgh and an imminent policy decision meeting to be held by the US Federal Reserve."Going into this week the risks were biased towards the US dollar probably seeing a bit of a reprieve and that is part of what we're seeing today," said David Watt, senior currency strategist, at RBC Capital Markets.Due to meet later this week, G20 leaders are expected to place prominence on developing a concerted strategy to remove fiscal instruments and stimulus introduced in the wake of the financial crisis to shore up the global economy.In statements made to the press ahead of the summit, prime minister Gordon Brown also asserted his confidence in support for measures that will help to rebalance international markets as the economy enters recovery mode.