0811 ET - U.S. stock futures trade mostly lower as Canada walks away from trade talks with the U.S., allowing 50% tariffs on $20 billion worth of Canadian goods to take effect. Treasury yields slip, with the 10-year at 4.71%, with help from falling oil prices. Focus this week will be split between Nvidia earnings on Wednesday and Fed Chairman Kevin Warsh's speech at Jackson Hole on Friday. The Fed's preferred inflation gauge is also due out Wednesday along with the latest update on GDP growth. S&P futures fall 14 points.