Initially, the only way you can obtain a cow is by buying one from Brownie Ranch. They cost 4500 G, and will be a calf when purchased. The calf will take one full season (28 days) to mature, and will not produce any milk before that. You can also breed your own cows, with a Cow Miracle Potion, also purchasable from Brownie Ranch. Each Cow Miracle Potion costs 2100 G, and using it on an adult cow will impregnate it. Your impregnated cow will give birth to a calf in 14 days. It will be fully grown in a season and live about 6 years.