Cows are the best source of animal income. Having four adult cows producing milk create more money then four chickens can produce. Cows are even a better source of income then Sheep.

The cow barn will already be on the farm in the beginning. It is the first barn directly east of the farmhouse. You can buy cows from Chet at the animal shop in the village, but he'll only sell you a cow after you have planted at least 8 bags of grass back behind the cow barn in the pasture area. The grass also must be sprouting and not sitting in seed-form. A cow from Chet costs 5000G. It's a good idea to also purchase the Brush as well. It costs 800G. Chet also has Cow Feed for sale in case you run out of grass. He will have 1 bag for sale and won't have another bag of feed for sale until you use up the one you have.