To take over the world using only cows, one must first possess cows. The first country to take would be India, with its nuclear capabilities and religious beliefs.
Hinduism is the most popular religion in India. It turns out that in the Hindu religion, it is taboo to shoot cows. With that in mind, let's strap a machine gun or two to a cow. You take over New Delhi using the cow for cover. The vast nuclear arsenal of India is now at your disposal.
With the vast nuclear arsenal of India, let's take over the surrounding countries with threats of nuclear holocaust. As the surrounding countries surrender, you are sure to attract some attention from the international press.
With the press covering your hostile take-over, they will surely exaggerate the actual problem. With this hearsay, the world will readily surrender. With your new position as Supreme, Omnipotent, Ruler of Earth, you can choose an either good or evil path. A good path is recommended as the people will revolt if an evil path is followed. HAVE FUN!
Special thanks to a friend of mine, who asked for anonymity, for helping me with the development of this idea.