I don't know if this is the translation for pigweed, but I know Mexican-Spanish very well. Here is goes: the word "weed" translated in spanish is "cizaña" and pig translation is marano or cerdo. In spanish, most of the time, translating your phrase or word are done backwards. For example, pigweed is two words making oneword in English, but in Spanish the word would be weed pig after it is translated.
If you want spanish from Spain the translation would be "cizaña de cerdo" Meaning weed of pig.
If pigweed is actually a herb in Mexican Spanish it would be "heirba de cerdo" or "heirba de marano". If pigweed is just a weed then the translation would be "cizaña de marano" in Mexican Spanish.