Artificial selection to obtain cows yielding higher milk output represents:
Stabilizing selection as it stabilizes this character in the population.
Directional as it pushes the mean of the character in one direction.
Disruptive as it splits the population into two, one yielding higher output and the other lower output.
Stabilizing followed by disruptive as it stabilizes the population to produce higher yielding cows.
There are three types of selections:
Stabilizing
Directional
Disruptive
The extremity of the trait distribution that is being selected against is known as directional selection. So we use directional selection for high yield of milk form cows.