Fresh or aged? If fresh, don't keep it bagged, instead, make a compost pile of it and turn it over now and then to get oxygen into the pile. It will take a month or two to mature. The temperature inside the pile will be very hot so cold weather will not be much of a factor. If aged, keep it bagged and use next spring.

Best composted with straw, so hopefully, if its fresh, its already mixed in with straw from the stable bedding. If its already composted (recommended time in the UK is around a year, sometimes less depending how fast it rots down) then, as Loosestrife says, just keep it bagged, it'll be fine for ages.