Cows are killed by what is known as a bolt gun. It is essentialy a little retractable club in a barrel.
From what I understand from visiting Newfoundland is it a humane death for the little critters if the hunter is properly trained in useing his tool/club. Like those that hunt with bows and spears, improper techniques result in the animal suffering.
They use this method of kill because it is quite and does not disturb the rest of the herd. A gun shot would result in panic among the herd.