Article is based on an ignorant and biased view of our trade with Mexico. I can't really speak to our trade with Canada.

Mexican cattle arrive in the US around 300-400lbs. They are well known for excellent health, as they have been tested and inspected several times before they cross the border.

Afterward, they are pastured and/or fed by US farmers and feeders, fed US corn and other grains, slaughtered by US packers, hauled by US truckers, and marketed by US companies.

We imported around 1 million head of live cattle from Mexico in 2018 (number is expected to decline as Mexico is rapidly developing their own feedlot industry) and exported around $374,000,000 in value added boxed beef and also exported around 50,000 head of higher value breeding stock. In the process, the US employed tens of thousands of people who might otherwise be without a job.  The trade also allows marketers to increase value, as many of the cuts popular in Mexico have less value in US (they get the skirt and flank steak and tripe and we sell the ribeyes in the US).

http://beef2live.com/story-beef-exports-country-year-date-0-109756

Our free trade agreement also allows US farmers access to the lucrative Mexican grain markets, and they are rapidly building infrastructure to import and feed more grain from the US.  In 2017, we had some interruptions due to trade negotiations and still exported 2.7 billion USD in grain and 936 million in total fresh/frozen meat.

https://www.export.gov/article?id=Mexico-Agriculture