nein, denn "percentage" enthält ja schon "percent."

I think it does need a percent sign, after any numeral that expresses a percentage, regardless of whether you use the word.

It's comparable to saying, for example, *the cost in dollars was 25. That just looks unclear and unidiomatic to me; I think we would almost always still write $25.

http://mic.sgmjournals.org/cgi/reprint/110/1/...
Results of a fluctuation test showed that the mean percentage of non-mucoid revertants in 12 independent cultures of ~ ~ 0 5 w6a8s 37.8 with a variance of 390-1, whereas for 12 samples taken from a single culture, the mean percentage was 43.9 with a variance of 14-7.

http://ajplegacy.physiology.org/cgi/pdf_extra...
In the anterior pituitaries of the 33 immature control rats, the percent- ages of the eosinophiles ranged from 30.1 to 44.9;

http://www.reproduction-online.org/cgi/reprin...
The mean percentage of the acrosome reactions in twenty-two experiments was 25-0 + 3-2.

dude, again, showing that it exists in occasional examples doesn't necessarily say anything about how common it is. In scientific papers that are stuffed with such data, different disciplines might have their own conventions to save space and avoid repetition; the same might be true of an economics text that was crammed with cost figures in dollars or euros.

I can only say that to me, as a general rule, it doesn't look very good -- and especially not in the example given, where there was another percentage given with a percent sign in the same sentence. The symbol may not always be absolutely required, but there may often be good reasons for retaining it.

the mean percentage in colostrum is 6.7 sounds fairly scientific to me.

I'm not saying that the percentage sign shouldn't be used; I'm just saying it doesn't have to be there (especially, as it seems, in scientific context), and the question, after all, was; muss ein Prozentzeichen hinter 6.7...?

Both ways seem correct to me, too. But not a mixture of both in the same sentence as in the question:

the mean percentage in colostrum is 6.7, compared to 4 % in milk

Good solution: choose which seems more appropiate according to context and audience and stick to it.

RE #6: 4 % in milk

There is no space between the number and the percentage sign in English.

You're raising the dead.