Not sure why that applies to cows ~ that's why Jews eschew the consumption of cheeseburgers.
It's the mixing of milchig and fleishig.
Milk represents birth and life sustenance. The flesh stands for corruption and death.
McDonald's founder Ray Kroc mentions running into a Florida health ordinance that prohibited milkshakes and hamburgers from being prepared in the same room. In order to pass inspection, they ended up having to put the Mixmaster in a plexiglas 'booth'.
That upset Ray's carefully planned 'service ballet' that he was so proud of.