It's caused by a strain of bovine papilloma virus, which is contagious and infectious. In the grand scheme of things, it's only a cosmetic issue. It will disappear in time; or you can put her in a chute and snip the wart off with a pair of scissors. That will cause her to bleed and introduce the virus into her bloodstream (the virus is largely isolated to the wart), which creates antibodies and eliminates any other warts on her body relatively fast. Lots of young cattle get them before the age of two years, some are just more noticeable than others.