Canada only has a little more than 400,000 sq. km of arable land. The US has four times as much land for farming.
The US's most conservative anti internationalist will end up losing their battle because the population growth is decreasing. As women become more and more educated the less the number of children they have as a whole group. The same goes for Canadian women.
Therefore, Canada will need to grow their population just to ensure young adult workers to support aging elderly who receive pensions.