Welcome to the forums. What I would suggest is to call the Wyoming DOW and see if they have a record of when it may of been. I would suppose that it would be about the same time that they started regular hunting seasons in the state.
Winter is tough on whitetails, that is an accepted fact.
You can help the quality of your herd by providing winter plots - that are not necessarily designed as kill plots.
Having what I call green plots - that the deer can browse in when most of the other food is gone, can greatly benefit the deer herd when they need it most.
Come December - at least in our area of the south - most of the mast is gone, all of the crops were long harvested and this green browse can really make the difference for...