Best to deck-mow or let stand?

Our 2 1/2 acre hayfield (OG, Meadow Brome & Timothy) didn't grow enough for a late-summer mowing, so its now 12-18 inches tall (mostly closer to 12). Is it best to mow it off now with the deck mower, or leave it stand thru the winter?

 

And does it make much difference which we do?

 

It would be easier to fertilize (now or in spring) if we mow now, but would be better for the field mice (also making the local raptors happy) to leave it stand.

 

Thanks for any thoughts.

Best to deck-mow or let stand?