Dairy farming is environmentally damaging and unsustainable; causing pollution and global warming, excessive water use, overuse of antibiotics, deforestation (forests destroyed in order to grow crops used for animal feed).
Intensive dairy farming isn’t just bad for cows but also has a negative effect on the environment. Concentrating the populations of cows in one place by having excessively large herds means millions of litres of slurry to dispose of, enormous amounts of greenhouse gases contributing to climate change, risking water pollution, diseases spreading and ammonia damage to wildlife. There will also be increased traffic to and from the farm for milk, feed, cattle and slurry. And producing the feed itself for the animals is also very detrimental:
“The sheer size of the [dairy] industry has also placed great strains on the environment, mainly through the destruction of forests and permanent pastures to create vast tracts of maize and soy bean grown for livestock feed.”
D’Silva, J. & Webster, J. (eds), 2010. The Meat Crisis: Developing More Sustainable Production and Consumption. Earthscan
Humans have not been consuming dairy for very long in terms of evolutionary history, read more about the origins of dairy farming.