As you drive north, entering Napa Valley along Highway 29, look to your left. You'll see the Belted Galloway / Dutch Belted cows and quickly understand how the locals came to nickname them 'The Oreo Cows.' This breed is black (or all brown) on the ends with a creamy white belt through the middle.
Paul & Ailene Tarap are the current owners and caretakers of The Stewart Ranch, now in it's fourth generation of cattle operation. (Yes, some of the cattle are for beef, which you can buy at the local Brown's Valley Market.)
The cows may be curious enough to walk up to the fence, but for the most part they are more interested in food than in people. I had the chance to visit one of their other grazing properties in a far flung corner of Napa. I watched Paul try and catch a juvenile that had been lead rope trained. Even though the cow could wear a halter it was less than certain about going on a parade!
The ranch and it's oreo inhabitants have been featured on Good Morning America and in many magazine articles. In 2008, while ambitiously grazing, they accidently took out a fence and wandered out onto the highway. No one (cow nor car) was hurt, but of course it was news in the small local paper.
If you go, bring a camera. *Do NOT drive up to the house or harass the owners. You can park at the nearby Stanly Lane marketplace and walk or bike on Golden Gate Drive to get close to the ranch property. You'll see many locals joggin and walking the road past the ranch.