Horn growth and shedding is directly related to testosterone levels in bucks. When testosterone levels rise in late summer the velvet begins to shed and after the rut, when all available does are bred testosterone levels begin to drop, and so do the antlers. Generally the dominant bucks will be the last to shed as they are still seeking any doe that comes into heat.
A few years ago my wife and I were in Cades Cove, in the Great Smoky Mountains in late march. All but two dominant bucks had shed there antlers and the dominant bucks were still chasing a few hot does.
The photo period probably has an effect because antler growth and the rut are effected by the amount of daylight hours.