The Cowboy from Sundown (1940)

The drought-plagued ranchers of Sundown have to market their cattle at a loss in order to meet mortgage payments held by banker Cylus Cuttler. Then, Sheriff Tex Rockett is forced to quarantine all the cattle on the local ranches because of a hoof-and-mouth disease outbreak. Steve Davis herds his cattle to the railhead anyway, and Tex is forced to arrest him. Urged on by the banker's son, Nick Cuttler, the angry ranchers storm the jail, but Steve's sister Bee persuades them to await the trial. Steve, with Nick's help, breaks jail and is told he must kill Tex to aid the ranchers. Meanwhile, government man Bret Stockton and Tex see Nick and his men treating cattle in an unusual way. Tex finally proves that the Cuttlers have been treating the cattle with acid to give a false impression of the hoof-and-mouth disease.