PDF's from UW. I mostly use the Nutrient Management Fast Facts as it has all the reference data I need on it. As it states you can also take a 50lb/ac credit for the 2nd cropping year following alfalfa (from the slow release decomposition).  We have typically leaned towards over applying based on the credits from UW (manure and alfalfa) but this year we are putting out a bit less N and will see how accurate their guidlines are.

http://ipcm.wisc.edu/downloads/nutrient-managment/