My dad - whose 67th birthday is today, happy birthday, Dad! - visited my uncle and aunt's dairy farm in New Mexico last week. They impregnate their cattle using artificial insemination. My dad said he joked with them that bulls do the same job, and faster.
With artificial insemination you know exactly when the cow is in heat and when she gets pregnant. Using bulls is not as labor-intensive, but you don't know exactly when. Plus, you can buy bull semen from semen companies with desired characteristics.
Speaking of which, did you see the news story about redheads? Apparently there's not a huge demand for redheaded children, so the world's largest sperm bank can "afford to be picky." Just like people can pick a donor with traits they hope to pass along, people can pick bulls with traits they hope the resulting calves will have. I hope next year using a red and white Holstein will change the look of the pasture a little.
Bulls do their job and move on. We bought a bull in July. Today we sold him to a different farm. Next year, we're leasing out a bull to another farm. As long as they're doing their job, and they don't hurt themselves, they don't really go down in value. Whether they're fast, slow, or red, it doesn't much matter as long as they pass on those desireable traits. And continue to sell for (almost) as much as you bought them!