Many grass fed cattle  are 'finished' on grain for a few months until slaughtered.
This practice  causes the ruminants to become very ill or die., so naturally they start pumping remedies into them.
 
Always look into how your source finishes the beasties.
 
This is a good article on the subject 
 http://www.natural-beef.net/grassfed.html
 
Further down the page it refers  to  The Weston A. Price Foundation that is a "nonprofit, tax-exempt charity founded in 1999 to disseminate the research of nutrition pioneer 
 
Dr. Weston Price, whose studies of isolated non industrialized peoples established the parameters of human health and determined the optimum characteristics of human diets. 
 
Dr. Price's research demonstrated that humans achieve perfect physical form and perfect health generation after generation only when they consume nutrient-dense whole foods and the vital fat-soluble activators found exclusively in animal fats.