The objective of this study was to evaluate the effects of supplementing a xylanase enzyme (Ronozyme WX) on production performance of dairy cows consuming diets containing corn or sorghum silage as the forage source. Four primiparous (BW 581 ± 47 kg, DIM 47 ± 14 d) and 20 multiparous (BW 707 ± 67 kg, DIM 51 ± 14 d) Holstein cows were assigned to 1 of 4 diets in a replicated 4 × 4 Latin square design with 19-d periods. With a 2 × 2 factorial arrangement (i.e., silage type and xylanase), treatments consisted of (1) corn silage-based diet (CORN-XYL), (2) corn silage-based diet with xylanase (CORN+XYL), (3) sorghum silage-based diet (SORG-XYL), and (4) sorghum silage-based diet with xylanase (SORG+XYL). Diets contained (DM basis) 33% silage (either corn or sorghum), 8% grass hay, and 59% concentrate. Xylanase was included at a rate equal to 1.5 g product/kg DM. Cows were fed once daily by means of a Calan gate system. All variables were analyzed with the MIXED procedure in SAS, and the model included the fixed effects of square, treatment, and square by treatment interaction, and the random effects of period, cow within square, and the random residual error. XYL did not affect any of the variables of production performance (P > 0.09). Xylanase supplementation did not interact with silage type (P > 0.19). Dry matter intake was greater for CORN than for SORG (P < 0.01; 28.8 vs. 25.5 kg/d). Similarly, milk yield was greater for CORN than for SORG (P < 0.01; 51.6 vs. 48.9 kg/d). Milk fat concentration (P < 0.01; 3.84 vs. 3.30%) and milk fat yield (P < 0.01; 1.81 vs. 1.68 kg/d) were greater for SORG than for CORN. Silage type did not affect milk protein (P > 0.59) or milk lactose (P > 0.50) concentrations (2.90 and 4.79%, respectively). CORN yielded more milk protein (P < 0.01; 1.48 vs. 1.36 kg/d) and more milk lactose (P < 0.01; 2.45 vs. 2.29 kg/d) than SORG. CORN resulted in lower milk urea nitrogen than SORG (P < 0.01; 12.3 vs. 14.1 mg/L). Silage type did not affect 3.5% fat-corrected milk (P > 0.24; 50.0 kg/d). In conclusion, xylanase supplementation did not affect production performance when feeding either corn-based or sorghum-based diets to high-producing dairy cows.