A cow is personal property (also known as a chattel) and you can not interfere with personal property or deface someone else's property in any way.  Although graffiti is normally associated with real property (like on the side of buildings) it has been done to personal property such as cars, etc. A cow is a new one to me but it would likely fall under nuisance or vandalism statutes in the state in which it was done.  It could also fall under the malicious mischief statutes in the state as well.  Check with an attorney in the state of Texas on this one. Who knows: it may be specifically mentioned.  Good luck.