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The objective of this experiment was to evaluate effects of reducing corn silage particle size on eating behavior, chewing activity, and rumen fermentation in lactating dairy cows. Four cannulated, multiparous cows averaging 110 ± 4 d in milk and weighing 675 ± 70 kg were randomly assigned to a 4 × 4 Latin square. During each of four 14-d periods, animals were offered one of four diets that were chemically similar but varied in corn silage particle size: short (SH), mostly short (MSH), mostly long (MLG), and long (LG), with a geometric mean particle length of 7.4, 7.8, 8.3, and 8.8 mm, respectively. Reducing particle size increased dry matter intake (DMI) linearly (28.0, 26.8, 26.8, and 25.7 kg/d for SH, MSH, MLG, and LG respectively). At 8, 16, and 24 h postfeeding, the neutral detergent fiber (NDF) concentration of feed remaining in the bunk decreased linearly with reduced particle size. Time spent eating or ruminating was not different across treatments; however, total chewing activity (TC; sum of time spent eating and ruminating) exhibited a quadratic response with highest chewing activities observed for diets with shortest and longest particle size. Eating or ruminating time per kilogram of DMI was not affected by corn silage particle size, but TC per kilogram of DMI decreased linearly with decreasing particle size. In comparison, when expressed as minutes per unit of NDF intake (NDFI), ruminating, and TC were linearly reduced as particle size decreased. Rumen pH was not affected by corn silage particle size even though total concentration of volatile fatty acids increased linearly from 89.1 mM/L to 93.6 mM/L as diet particle size decreased. A quadratic effect was observed in molar proportion of acetate and propionate with the highest concentration observed in animals consuming diets of intermediate particle size. Results of this experiment suggest that reducing corn silage particle size may increase DMI, positively affect rumen fermentation, and reduce sorting behavior. Because both chewing activity and sorting tendencies increased when proportion of TMR particles > 19.0mm increased, results suggest that particle size measurement as estimated by the PSPS is useful in understanding some factors that affect feeding behavior.