Poultry litter (chicken poop) should not be considered an acceptable source of feed for any animal, especially for cows.  Potentially containing bovine tissue (used in the poultry feed as a source of protein) there is a risk for increased exposure to bovine spongiform encephalopathy (mad cow disease).  Chicken litter is also contaminated with arsenic, antibiotics, and other substances which will be passed on to the cows, creating more of a health hazard for the cows as well as for the consumers of the meat from those cows. 

The FDA has acknowledged that this practice is dangerous when it announced a ban in 2004 as reported in the New York Times [http://www.nytimes.com/2004/01/27/national/27BEEF.html].  The use of poultry litter as cow feed has already been banned in the European Union, Australia, New Zealand, and Canada.  This practice needs to be ended in the United States of America as well.