I actually dove into a document and some examples. I'm not saying that this is an exhaustive list of invasive agricultural weeds in Canada, but there's the Canada thistle, and yes, it's called the Canada thistle, but it actually comes from Europe. There's also something called the ox-eye daisy, as well as leafy spurge, which you will have heard of, spotted knapweed, which my colleague from Parks Canada mentioned already, and quackgrass, wild oats, and green foxtail. These are some examples of species that are having a significant economic impact.
On October 4th, 2011. See this statement in context.