We no longer shecht dairy cows, since most of them turn out to be treifas, as R. Schachter explained in a YU Audio shiur, which I have partly transcribed below.
R. Hershel Schachter
Nov 22, 2008
"Kashrus in the Home"
"Dairy cows have a very high incidence of treifos -- it's taka a
question how we're still allowed to drink the milk if there's such a
high incidence of treifos....In America they milk the cow 3 times a
day, which is abnormal -- years ago they never did that, and because
they milk them for all they're worth, that's why they develop the
sicknesses, and then when they are not producing enough milk, they
kill them; and these dairy cows don't taste so good -- the fleish
doesn't taste so good -- so usually in the industry they use this
flesh to make hotdogs or salami, since with hotdogs or salami you put
in so many spices, you don't taste the flavor of the fleish anyway;
this fleish has a funny taste -- it doesn't taste bad, but it has an
irregular taste...They told me that years ago they used to schecht
these dairy cows and in America they found 80% incidence of treifos
and in South Africa when they schechted the cows they had 95%
incidence of sirchos (scabs) on the lungs..."
He then goes on to explain that, based on this, the milk should also be non-kosher, if we know that the cows are non-kosher, and that he is not sure how we are allowed to drink milk nowadays.