Hard to find unbiased soured of evidence but:
http://www.ciwf.org.uk/farm-animals/cows/dairy-cows/welfare-issues/
http://www.rspca.org.uk/adviceandwelfare/farm/dairy/keyissues
See also:
The most common justification I hear for vegetarianism is “It’s wrong to kill an animal for food.” Of course there are other motivations, such as health, religion, environmentalism, preventing suffering, and trying to score with liberal chicks — but the moral wrongness of killing an animal for food is the probably the most common, at least in my experience.
Consequently, I’ve found it surprising that people so rarely acknowledge that vegetarians do kill millions of animals for food. If you buy eggs or milk or cheese, it’s true in theory that the dairy cows and laying hens don’t have to be killed in order to supply you with those products, but in practice, they are. A modern factory farm isn’t just going to let their animals die of old age; they kill them at whatever point the farm considers to be the most profit-maximizing. For dairy cows, that’s usually at age 3-5, out of a natural 20-25 year lifespan. For egg-laying hens, it’s usually after one or two laying cycles. And since the males of the laying species are useless to the egg farmer, they’re killed right after they hatch.
But surely eating a vegetarian diet must kill far fewer animals than an omnivore diet, right? Well… sort of. I’m sure that a typical vegetarian kills fewer animals than a typical omnivore. But it’s certainly possible to be a vegetarian and kill more animals than an omnivore, and in fact, I’m confident that many vegetarians fall into that category.
The culprit is eggs. While you only need to kill one single steer to get about 450 pounds (405,000 calories) worth of meat, you’d need to kill about 20 chickens to get enough eggs to match that number of calories. So if you’re a vegetarian who eats a lot of omelets, you’re likely responsible for more animal deaths than someone who chows down on burgers and steaks but doesn’t like eggs.
http://measureofdoubt.com/2011/06/22/why-a-vegetarian-might-kill-more-animals-than-an-omnivore/