Improvement of digestibility releases more of the nutrients in total mixed rations (TMR) and so increases profitability. The commercial product Optipartum C+ (OC; AB Agri Ltd., Peterborough, UK) is a proprietary blend of enzymes and essential oils, which may increase nutrient digestibility and therefore increase milk production. The purpose of this experiment was to determine the effects of feeding OC without or with supplemental fat (Energy Booster 100, Milk Specialties Global, Eden Prairie, MN) on DMI and production by lactating cows. Primiparous (n = 8) and multiparous (n = 16) Holstein cows [DIM = 170 ± 48.3 d; BW = 665 ± 82.9 kg] were used in 4 × 4 Latin square design with 35-d periods. The 4 treatments were ground corn carrier (CON), 350 g of OC, 350 g of fat (EB), and a combination of 175 g of OC and 175 g of fat (50/50). All treatments were provided as a top-dress on the daily TMR and were a total of 453.6 g/d with the remainder of the weight made up by ground corn. Statistical analysis was conducted using the MIXED procedure of SAS (SAS Institute Inc., Cary, NC). Dry matter intake was greater for EB (22.07 ± 0.45 kg/d) compared with OC (20.40 ± 0.45 kg/d; P < 0.01) and CON (21.12 ± 0.45 kg/d; P = 0.04). Milk production was greater for 50/50 (32.58 ± 0.64 kg/d) compared with CON (30.53 ± 0.64 kg/d; P < 0.01) and greater for EB (31.74 ± 0.64 kg/d) compared with OC (29.95 ± 0.64 kg/d; P < 0.01). Fat yield was greater for 50/50 (1.13 ± 0.04 kg/d) compared with CON (1.05 ± 0.04 kg/d; P < 0.01), OC (1.05 ± 0.04 kg/d; P < 0.01), and EB (1.07 ± 0.04 kg/d; P = 0.03). Energy-corrected milk (ECM) was greater for 50/50 (31.85 ± 0.59 kg/d) compared with CON (29.88 ± 0.59 kg/d; P < 0.01). Efficiency (ECM/DMI) was greater for 50/50 (1.50 ± 0.03 kg/kg) compared with EB (1.41 ± 0.03 kg/kg; P < 0.01). In conclusion, providing a combination of EB and OC resulted in greater yield of ECM and efficiency of production.