From the LIFE magazine article, Horse Roping in Nevada is a Picturesque Feat; 21 June 1937.
Photo 3 of 3 - Getting a Wild Horse ready for Hoof Trimming. The caption: "Securely hog-tied, the horse is now ready to have its hooves trimmed. Placing a sack under its head is Mrs. Doris Case, trick rider, who assists her cowboy husband."