It’s midnight, and I’m hitching a ride with a four-truck convoy hauling 250 cows up to South Dakota. We’re huddled around a broken-down 18-wheeler at an Oklahoma truck stop, ringleader Skipper Shepherd trying to repair an air dryer’s pop-off valve, when someone notices a black SUV slinking between rows of idling rigs.

“DOT police,” a trucker in sandals says. “Dickheads.”

“He better watch out or he’s going to get a free shower,” one driver says, referring to the cattle’s tendency to shit and piss through the trailers’ open vents.

Although Shepherd might understandably worry about the Department of Transpor­tation checking on his rigs, he’s got more pressing concerns. Moving live animals across state lines is a time-sensitive operation, so when one of the trucks breaks down around midnight — moving the cattle at night ensures they won’t get overheated — the entire convoy pulls over to find a solution.

“We’re broke down,” Shepherd says in voice-mail message after message, trying to find a replacement. “Need a truck to go to South Dakota. Can you go? Call me back.”

After an hour, Shepherd finds a replacement rig, and we’re back on the road. With drivers required by law to be at the wheel no more than 11 hours a day, each truck carries two passengers so they can trade off the driving. Behind a curtain at our seat backs, Shepherd’s 26-year-old son sleeps.

I’m starting to nod off myself when the faint sound of high-pitched screaming cuts through the diesel drone. “Some ol’ gal just flashed me!” Shepherd says, crumpling another can of Rockstar and tossing it out the window. Having driven 18-wheelers most of his life, Shepherd is accustomed to the mystical draw 80,000 pounds of speeding metal can command: deer exploding on impact, kids pulling arms in “honk honk” pantomime, the occasional flash of boob. Passing through some High Plains town just after last call, he isn’t surprised by a woman lifting her shirt.

The cattle, however, are freaked. The truck dips and ratchets back up before rolling side to side as 40,000 pounds of beef bawl and shuffle in the rig’s perforated-steel trailer. Ignoring the turbulence, Shepherd is more concerned about making up lost hours.

“I’ve been all over the world in this thing, but never stopped,” he says, pressing his boot down until the 550-horsepower engine sings and the needle passes 80. “Time is money.”

Shepherd will pull in $3,500 for the trip. At the moment, he’s averaging 10,000 miles a month, trying to stay busy before all the herds have been moved and demand plummets.

He has been awake for what has to be approaching 24 hours straight as he navigates a dirt road just past Buffalo Gap, South Dakota. Rolling hills of grass give way to the angular Black Hills. A lone rider lopes his horse toward us, a spotted border collie at its hooves. Unlike the West Texas cowboys, who often curl their hats like taco shells, the young man’s brim is flat. Tassels hang from his cream-colored chaps, a red scarf around his neck.

“A buckaroo,” Shepherd says, our convoy winding toward the pens where the Sixes cattle will be unloaded. Shepherd backs his rig to a ramp, walks to the trailer’s rear, and raises the door. Cattle race down the plank to the waiting feed. Troughs of water go untouched, the dazed cattle sniffing at their reflections but not yet drinking.

“It’ll take ’em a little bit to get used to the water here,” Sam Prill says. He wears lace-up boots and a salt-and-pepper mustache. In lieu of a horse, his Polaris ATV is parked near the pens. A former stockbroker, Prill oversees 8,000 acres of nutrient-rich South Dakota grassland. While the Sixes prides itself on top-quality beef bred through the generations, Prill maintains no fixed herd. He occasionally buys “total starved-out junk” calves that no one else wants, fattening them on his abundant grass before selling them off.

“Ninety-nine percent of what I sell will be made into hamburger,” Prill says of his game plan to buy low and sell high. “Americans may not be buying T-bones, but they’re still eating ground chuck.”

Prill will winter 1,000 of the Texas cattle on his property, feeding and monitoring them until April, when they’ll move to a larger lease in Montana. In the coming weeks, Leathers and the other cowboys will arrive to brand the newborn calves, the first glimpse many of them will have of their ­future work environs. Miss Anne will drop by soon thereafter to check in on the herd. And though the pastures east of the Black Hills get much less snowfall than those on the west, it’s a far cry from Texas. For the Sixes cowboys, seeing their life’s work north of the Red River brings an unaccustomed uncertainty.

“It’s a world of hurt,” Leathers says. “We could burn off tomorrow. Just depends on the wind.”