I can only speak to the NZ dairy industry but, milk is tested... a lot. If if doesn't meet standards set by the dairy company then the farmer doesn't get paid. Even worse, if bad milk causes milk from other farms to get dumped (milk from many farms in one milk tanker) then the dairy company can fine them for that too.

Farms are businesses and the dairy industry is an industry. Food safety is taken seriously, at least where I live.