 In many climate and circumstantial instances, an equivalent string of pack horses would have to carry more fodder and sacks of high energy grains such as oats, so could carry less cargo.In modern times, strings of sure footed mules have been used to carry riders in dangerous but scenic back country terrain such as excursions into canyons.Pack trains were instrumental in opening up the American West as the sure footed animals could carry up to , survive on rough forage, did not require feed, and could operate in the arid higher elevations of the Rockies, serving as the main cargo means to the west from Missouri during the heyday of the North American fur trade.