The deer remain high until winter storms force them to lower places at the park margins. If the fall storms are late, as they were in 2000, they are still within the park during the hunt. In December of 2000, I've made three trips along Zion's western edge. They were foraging on grass in the daytime. This hints at a coming problem.
Mule deer do eat grass, but starve on a diet of only grass.
Also, while full-daylight foraging is not rare for Zion deer, it is not a normal population trait. It suggests they are already experiencing food stress. The mule deer of the West survive within a set of human induced pressures that occur in different combinations and degrees for every population. One common denominator, however, is the drive of these sturdy animals to survive. If you are a writer or photographer and wish to publish your work on this site.