The objective of this study, conducted between April 12 and September 1, 2017, at the Mississippi State University Bearden Dairy Research Center, was to evaluate the effect of Vista Pre-T (AB Vista, Marlborough, UK) on feed efficiency (FE) and milk components during heat stress. Cows (n = 92 Holsteins) were assigned to control (CON) and treatment (TRT) groups based on parity (1 or ≥2), DIM, and milk yield. The same ration was fed except TRT was pre-treated according to manufacturer recommendations (750 mL per ton TMR dry matter). Cows were weighed and milk fat, protein, and lactose were tested weekly. Orts were weighed daily and FE was calculated by dividing energy corrected milk by dry matter intake. Reticulorumen temperature (RT) was obtained every 5 min via bolus (Smaxtec, GmBH, Austria). Temperature humidity index (THI; Tycon Systems Inc., Bluffdale, UT) was obtained hourly. Milk yield was recorded at each milking. The MIXED procedure of SAS was used to determine the relationship between FE, milk components, milk yield, RT, SCC, and THI between groups. Treatment (P < 0.01), yield (P = 0.02), RT (P < 0.01), and THI (P < 0.01) had significant effects on FE. The TRT group was more feed efficient than CON (1.54 ± 0.02 and 1.34 ± 0.05). Dry matter intake did not differ between CON and TRT (69.07 and 68.91 kg/d; P = 0.92). Treatment (P = 0.04), RT (P < 0.01), and THI (P = 0.05) had significant effects on milk fat. The TRT group had higher fat than CON (4.41 ± 0.06 and 4.02 ± 0.14%). Parity group, DIM, milk yield, weight, RT, and treatment (P < 0.01 for all) had significant effects on milk protein. Milk protein was greater in the TRT group compared with CON (2.80 ± 0.01 and 2.68 ± 0.03). The CON group had lower lactose than TRT (4.70 ± 0.03 and 4.85 ± 0.01; P < 0.01). The TRT group had lower SCC than CON (154,170 ± 15,994 and 326,670 ± 33,326 cells/mL). Parity group (P < 0.01) and DIM (P < 0.01) also had significant effects on SCC. Treatment and CON groups did not differ in RT (39.27°C and 39.11°C, P = 0.27). Treatment did not have a significant effect on milk yield (P = 0.55), but THI did (P = 0.02). Vista Pre-T significantly improved FE, milk lactose, protein, fat, and SCC with no measured negative implications.