The objective of this study was to examine the effect of feeding a fibrolytic enzyme preparation (Econase) via a molasses-based liquid feed (MLF) on digestibility, ruminal patterns and milk production of primiparous dairy cows. Thirty-six (n = 18) dairy heifers were blocked by BW, BCS, mature 305 ME and assigned to 2 treatments from d 45 pre- to 56 postpartum: 1) untreated MLF diet (CON); 2) enzyme treated MLF diet (ECO). Treated MLF preparation conferred activity of 3,500 BXU/g of xylanase. An amount of 0.5mL of treated or untreated MLF was mixed to each kg of DM of pre- and postpartum TMR before feeding. Data were analyzed using the MIXED procedure of SAS. Prepartum DMI was similar (14.2 vs. 15.0 kg/d; P = 0.17) between CON and ECO treatments and no differences were observed on ruminal pH (7.1 vs. 7.0; P = 0.64), %NDFd (60.0 vs. 57.9; P = 0.48) or %DMd (64.9 vs. 61.3; P = 0.15) for CON compared with ECO treatment, respectively. However, enzyme supplementation tended (P = 0.09) to increase colostrum yield by 32% (3.9 vs. 5.2 kg) although no changes were observed on total colostrum solids concentration. Postpartum DMI (20.6 vs. 20.9 kg/d; P = 0.78), 3.5% FCM yield (28.5 vs. 29.9 kg/d; P = 0.27) and milk fat (3.6 vs. 3.5%; P = 0.59) were similar for CON and ECO treatments, respectively. Postpartum NEFA and BHBA were also similar (181.0 vs. 161.1µEq/L; P = 0.41) and (5.8 vs. 5.6 mg/dL; P = 0.66), respectively for CON and ECO. Postpartum ruminal fluid pH tended (P = 0.08) to be greater for ECO (6.53 vs. 6.71) compared with CON, but % NDFd was similar (53.7 vs. 52.1; P = 0.58). Although numerical increases in milk yield (28.3 vs. 30.1 kg/d; P = 0.15) for ECO treatment, the design did not have enough statistical power to report a significant effect. Future research is needed to investigate whether different doses and feeding period length of this fibrolytic enzyme may affect performance and growth of first lactation animals.