There are two key things about US Dairy that Trump never tells you. Nor does the Canadian Govt. - buts that’s only because the Canadians never ‘negotiates’ things in public, in the media, unlike Trump. So here goes ~
Firstly, US dairy herds are pumped full with artificial hormones & antibiotics. Canadian cows are kept free from that stuff. A package of Kraft cheese in my fridge says - "Made with Canadian milk. No artificial hormones & no antibiotics." That stuff is considered a food health hazard. The EU banned the importation of ALL US dairy products years ago for the same reason. You don't hear Trump mentioning that.
(NB: Canada is finalizing a new trade agreement with the EU that does include dairy products. They can only do that because they both follow the same food health standards. The US will never be able to negotiate dairy trade deals with Canada or the EU).
Secondly, US dairy has been heavily subsidized for many years, resulting in massive over-production, many farm failures & high rates of suicide among farming families who have lost their livelihoods. Wisconsin dairy farmers are asking for a 'supply management system' - like in Canada.