Most of the forage-finished beef cattle produced in Argentina are fed on pastures of cultivated sorghum or alfalfa and are supplemented with corn.  They are a lot leaner and eat a lot less corn than the beef cattle from feedlots in the United States, but they would not meet the strict American Grassfed Association definition of "grassfed".  Beef from Uraguay is probably finished the same way, supplemented with varying amounts of corn.  Corn has been a staple food in the pre-Columbian Americas for close to 10,000 years.  I doubt anyone in South America is concerned, or is checking to see whether their beef has been fed corn, or not.

Look at the beef.  Grassfed beef will be a darker color, more burgandy than red, and the fat will be yellow, not white.  If the beef has a lot of marbling, odds are it was finished with corn.  The only way to be sure that your beef is grassfed is to ask the rancher if they are raising their beef according to American Grassfed Association standards for "grassfed".
 
http://mbfc.s3.amazonaws.com/2007_projects_&_technical_info_page/argentina_may_31_09_low_res_8x11.pdf