Fifty lactating Holstein dairy cows (679 ± 75 kg of BW; 164 ± 84 DIM) were enrolled in a study following a randomized design to assess the effects of a fermentation product derived from Trichoderma reesei strains RF8055 and RF 5427 (VistaPre-T, ABVista, UK) on efficiency of milk production. Cows were blocked by parity (26 primiparous and 24 multiparous) and allocated to either a Control ration (no supplementation) or ration (VIST) containing 750 mL/ton (DM basis) of VistaPre-T for 2 mo. The ration provided 15.4% CP, 33.8% NDF, 2.8% fat, and 1.56 Mcal of NEl/kg (DM basis). Individual feed intake, milk production and fat and protein content, and BW were monitored daily. Data were summarized by week and analyzed using a mixed-effects model for repeated measures accounting for the fixed effects of treatment, week, parity and their 2- and 3-way interactions. Multiparous (24.8 ± 0.68 kg/d) cows consumed more (P < 0.05) feed than primiparous cows (21.6 ± 0.65 kg/d) but there were no differences in DMI between treatments (23.2 ± 0.39 kg/d). There was interaction between treatment and week (P < 0.05), treatment and parity (P < 0.05), and among treatment, week, and parity (P < 0.01). Primiparous cows on VIST produced more milk (33.5 ± 1.46 kg/d) than primiparous cows on Control (30.6 ± 1.34 kg/d) after about 3 wk of exposure to treatments. Milk fat (3.71 ± 0.08%) and protein (3.31 ± 0.03%) contents were unaffected by treatment, but fat was greater (P < 0.05) in multiparous cows after 4 wk on VIT than in multiparous cows on Control, with no differences in primiparous cows. Body weight was also affected by an interaction among treatment, week, and parity (P < 0.05), with primiparous cows on VIST gaining more weight than primiparous cows on Control. Feed efficiency (kg of milk/kg of DMI) increased in VIST cows also after about 3 wk of exposure, and the increase was more (P < 0.01) pronounced in primiparous (+0.21 increase; P < 0.001) than in multiparous (+0.02 increase; not significant) cows. In conclusion, supplementation of fungal-derived extracts from Trichoderma reesei improves milk yield and feed efficiency in first lactation animals in mid lactation and milk fat content in multiparous cows.