Ok, nevermind.
I just googled a little bit and found out that I was right. A cow that freshens gives birth and is called "fresh" afterwards. Maybe we could add this to the dictionary, although it's probably not that important :-)
And I know, google first, then write an entry.
I'll do the next time. I promise. :-)
Bye,
Lucy

There are these nice buttons with the i next to the entries in the dictionary. They show the MW entries for the word and help to answer questions like yours:

e: having the milk flow recently established *a fresh cow*
http://www.merriam-webster.com/dictionary/fresh

If you want to propose a new entry, you need to clarify if the definition is usually based on the milk flow or the birth (though both are undoubtedly linked).

But I would'nt have a clue as to the german term.

OED online:
"c. Of a cow: yielding a renewed or greatly increased supply of milk; coming into milk. U.S.

1884 Vermont Agric. Rep. VIII. 29 The cows will go dry for a time during the hot weather in summer and be fresh in fall. 1896 Ibid. XV. 67 This [inoculating of cream] may be done by using a ‘starter’ made from cream of the skim-milk of a fresh cow. 1971 Independent (Deerfield, Wis.) 23 Sept. 22/4 (Advt.), Fresh, springing, bred back cows and heifers."

"fresh" in that sense seems to be an AE term.