CEDAR RAPIDS — Despite warnings from Canada that the future of NAFTA is in jeopardy unless the Trump administration backs off its most controversial proposals, Sen. Chuck Grassley remains optimistic negotiations will preserve the trade pact critical to Iowa export markets.Grassley, who has some concerns about the impact of changes in NAFTA being negotiated with Mexico and Canada, has been making the case for continuing the trilateral agreement which, according to the U.S.