The generic use of the term "cow" is frequently used colloquially to refer to all cattle, bulls, cows, heifers, calves and steers in a given herd.
"Hey Dusty, how many cows you got on the south quarter?" rolls off the tongue a little easier than "Pardon me Buford, but can you tell me how man head of cattle you are pasturing on your south quarter section of range land?"
"Cowboys" don't just round up the cows, they also herd of the calves, steers, bulls and assorted mavericks.