With the extreme drought occurring in most of the Midwest, and non-hay crops getting baled just to have some stored forage, the likelihood of accidental poisoning of cattle increases dramatically. Here are some resources on the dangers of haying and grazing drought stressed forages:
http://www.uaex.edu/Other_Areas/publications/PDF/FSA-3024.pdf
http://www.southernlivestock.com/articles/herd_health/nitrate_poisoning_in_cattle.6032.sls
http://www.ext.colostate.edu/PUBS/LIVESTK/01612.html
http://today.agrilife.org/2012/06/26/experts-texas-cattle-deaths-due-to-prussic-acid-isolated-incident/