Which is the correct grammar? Cows' milk or cows milk or cow's milk?

I think it's cow's milk. Depends on sentence structure, if cow is plural it would be cows' milk.

If it's the milk of one particular cow it would be "the cow's milk". If it's the milk of cows in general (as opposed to ewes or goats, it would be "cow's milk" - non-domesticated milks would be like "aardvard milk" without the possessive 's. If it's the milk of a particular group of cows already referred to by the speaker or writer, it would be "the cows' milk". This could be in reference to yields on a certain farm, or disasters (or marketing opportunities...) like the cows turning green, but their milk turning pink.

"Cow milk" would be OK, but not "cows milk" (no apostrophe) unless one was saying "green cows milk easier in the morning than in the evening", where milk is used as a verb.

All are correct. The use of the apostrophe is determined by quite specific (and controversial) rules. As with all grammar correct usage changes over time, but currently:

1.) Alphonse and Gertrude Cows own the largest dairy in the area. The Cows' milk is considered to be the best in the country.


2.) It's widely known that cows milk is one of the best sources for calcium.

3.) Q. Which cow in your dairy produces the best milk? A. That cow's milk is the best.

It's important to understand that the apostrophe (and its correct usage) has been an important part of English that has allowed the language to evolve in ways that's restricted the development of other, less flexible, languages. I've tried to explain this to my mate's friends but they'll never accept that I've a valid point of view. I'll keep banging my head against the wall 'cause I know I'm right.

http://grammar.about.com/od/punctuationandmechanics/tp/GuideApostrophe.htm

https://en.wikipedia.org/wiki/Apostrophe#Superfluous_apostrophes_.28.22greengrocers.27_apostrophes.22.29

http://theoatmeal.com/comics/apostrophe

If one is simply talking about the kind of milk produced by cows, as opposed to goats, for example, it is perfectly possible in English to treat cow (and goat) as adjectives and refer to 'cow milk' and 'goat milk'. This is standard usage. (See Baloo55th above).