When should I start using a winter rug on my horse who feels the cold?

At this time of year a 100-150g rug is usually enough, mainly to keep off rain and wind, until it turns colder around the clock. It also depends on what the paddock is like, whether there is a field shelter or trees where the horse can get out of the weather. Ad-lib forage helps too: a horse that can chew through the night holds its warmth far better than one standing without forage for hours.