Seasonal changes in aspen (Populus tremuloides Michx.) resistance to shearing force were described to determine their apparent contribution to seasonal changes in cattle utilization. Shearing resistance of current-year, 1-year-old, and 2-year-old stem-age classes were measured from late spring to early fall. For all stem-age classes, resistance to shearing force increased with time, partially due to increasing stem diameter. Depending on stem-age class, resistance to shearing force increased by 200%–2000% between the first and last measurements of a given year. Increased resistance of aspen stems to shearing force in late summer may reduce their selection by cattle. Grazing management that defers cattle entry until midsummer could reduce damage to regenerating aspen associated with cattle foraging in cut blocks.