I HAVE washed mine in my machine, but it's kinda hard on the washer. Is this a nylon halter? You can use a mild soap, and let it soak, then spray with a nozzle to get All of the soap out. Then, just hang it to dry.
I love to periodically clean up my halters and leads. It's kinda like cleaning and sweeping out your car. You wonder why you don't do it more often! =b