© Matthew Staver for The New York TimesConrad Swanson has had bales of hay stolen from his field in Wellington, Colo.
Across the heartland, ranchers, farmers and county sheriffs are grappling with a new scourge: hay rustling.

Months of punishing drought and grass fires have pushed the price of hay, grain and other animal feed to near records, making the golden bales an increasingly irresistible target for thieves. Some steal them for profit. Others are fellow farmers acting out of desperation, their fields too brown to graze animals and their finances too wrecked to afford enough feed for their cattle.

"It's the economics of the times," said Jack McGrath, the undersheriff in Colorado's Weld County, where hay thefts rose to 15 last year from 7 in 2011.

At Mark Reifenrath's farm in northern Colorado, the thieves struck at night. Two men driving a stolen pickup opened an unguarded farm gate by the side of the road, rolled into Mr. Reifenrath's alfalfa field and headed toward their quarry: 800-pound square bundles of freshly cut hay.

They set to work that October night, hefting two bales onto a flatbed trailer. They might have gotten more, but an employee happened by and noticed flashlight beams bouncing around in the darkness. Something was up. He yelled out, and the men disappeared into a patch of cattails, leaving behind a half-loaded trailer.

"Maybe it's not the crime of the century, but it affects us," Mr. Reifenrath said.

Sheriffs in rural counties in Colorado, Oklahoma, Nebraska and Kansas say the spike in hay thefts is part of a broader rise in agricultural crime.

California's farmers have grappled recently with growing thefts of grapes, beehives and avocados, and sheriffs say high prices of scrap metal have made agricultural machinery - whether it works or not - an appealing target. Dubious online merchants are selling feed to farmers but never delivering. On the range, wire fences are being clipped to allow interloping herds to poach grazing land.

Most thieves make off with less than a ton of hay - about $200 to $300 worth, depending on the quality. But on Labor Day in Wellington, Colo., thieves hot-wired a front-end loader and stole enough hay from Conrad T. Swanson's ranch to fill the flatbed trailer of a semi.

"It's not like someone was just driving by and took enough to feed a horse," Mr. Swanson said.

Law enforcement officials said they could do little to prevent the thefts or catch the culprits. Most of the hay is nipped at night along remote roads, from fields and barns hundreds of yards from the nearest home. Because one bundle of hay tends to look like every other one, once a bale is stolen, reclaiming it is harder than finding a needle in a - well, never mind.

To ward off the hay thieves, farmers are padlocking their gates and painting their bales with their brands. Some are splicing their hay with ribbons that mark their ownership.

In Tillman County, Okla., hay thefts became so rampant that Sheriff Bobby Whittington decided to lay a trap. He bugged a bale in a particularly theft-prone field with a GPS unit and set to waiting, sure that thieves would strike. Sure enough, his phone rang one night last March with the news that the tracking device was on the move.

The sheriff hopped into his car and headed toward the signal. When he reached it, he found his culprits, and the bugged bale. Pulling them over, he said, he told the driver, "We need to talk about that hay bale you've got there."

The men were belligerent at first, Mr. Whittington said, until he explained how he had tracked them down. Before being arrested, the driver offered a plea.

"He just hung his old head and said, 'Can I take it back?'" Mr. Whittington said. "And I said, 'No.'"