Lactating Holstein cows were assigned to a control diet or a diet containing a blend of supplemental protease enzymes (Rumagentin™, Feed Sources LLC, Alta Loma CA) at the Iowa State University Dairy farm. Study objectives were to examine the effects of proteases on milk yield, milk composition, feed intake and feed efficiency in lactating Holstein cows. Our results indicate that protease enzymes enhanced feed conversion efficiency and nitrogen utilization of dairy cows.