marshall trimble ask the marshall true west magazine

I’ve seen cowboy movies from the 1950s where the actor said he had to shoot his horse because it ate locoweed. What exactly is locoweed? Were they talking about marijuana? If so, why would anyone shoot a valuable horse for eating a weed whose effects are only temporary?

David Duggan
Shelton, Connecticut

No, locoweed isn’t cannabis. It’s any number of plants found throughout the West, especially the genera Astragalus and Oxytropis. They are commonly found on mountains, foothills, plains and semiarid desert regions. Some examples are larkspur, lupine, arrowgrass, chokecherry and milkweed.

oxytropis locoweed true west magazine
Oxytropis, or Early Yellow Field Locoweed.
— Courtesy True West Archives —

It’s relatively palatable to livestock and some other animals, especially in spring and fall, and many will actually seek it out.

Loco is Spanish for “crazy.” Locoed behavior results from locoweed-induced neurologic damage. Most of the time animals become depressed and lethargic.

With chronic locoweed poisoning, livestock become emaciated and wasted as they lose the ability to find and utilize feed. Although some may die of starvation, most die from misbehavior.

Marshall Trimble is Arizona’s official historian and vice president of the Wild West History Association. His latest book is Arizona Outlaws and Lawmen; The History Press, 2015. If you have a question, write: Ask the Marshall, P.O. Box 8008, Cave Creek, AZ 85327 or email him at

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