Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Serious injury to the penis and prepuce often follow attempts of the bull to breed a cow across a barbed wire fence or other obstacle. Similar injuries may occur when the bull is running with cows in brushy pastures.
Neal, Fred C.
"Surgical Problems of the Penis and Prepuce of Bulls, Part II,"
Iowa State University Veterinarian: Vol. 22
, Article 5.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol22/iss3/5