http://www.il-st-acad-sci.org/transactions/PDF/8907.pdf#search='buck%20mortality%20rates'
http://wildlife.tamu.edu/publications/A053.PDF#search='buck%20mortality%20rates'

Here are a couple more pretty good studies. Actually the 1st one indicates that adult male vulnerability to harvest goes down with age but note also that this is not a result but rather an input to the modeling.

A word of caution to IL-Cornfed - These studies contain talk of pre and post season doe:buck ratios as well as talk of several different types of ratios like adult and perceived so you probably won't enjoy them.