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.