25 percent of all cows are culled because of mastitis?

Yes. Once mastitis has set in, the quarter or quarters in the cow's udder can become infectious to the point where that quarter is no longer functionable to synthesize and secrete milk. Since mastitis is an infection of the udder, pains also have to be taken to take care of the cow and make sure the milk is not combined with cows that do not have mastitis. So, logically, the best way to "treat" cows with mastitis is to cull them and have them in a separate parlor where they can be treated, then once they are better ship them out. It's sad, but its reality.