In NW Montana the long hard winter, and lack of food probably are the two big limiting factors to antler growth in your area. Here in south/central MO we don't have really bad winters, but there is not a lot agriculture. We've planted Alfalfa on our farm and put in several food plots with oats and brassicas and after 5 years we have seen a small increase in antler growth and body weight.
2.5: 110-120
3.5: 120-140
4.5: 130-160
5.5: 140-