Cows producing milk for Parmigiano Reggiano (PR) cheese are prohibited from consuming fermented feeds. Such diets contain finely chopped dry forages and high fermentable carbohydrates. The aim of this study was to: (1) characterize changes in rumen environment, immunity and production in high producing PR Holstein cows subjected to a nutritional and handling stress challenge and, (2) evaluate the effect of feeding OmniGen-AF© (OG, Phibro Animal Health, Teaneck NJ) on immune cell responses to an induced acidosis. Nutritional stress was induced by altering the forage to concentrate (F:C) ratio of the base diet from 46:54 F:C (28% starch) to 25:75 F:C (35% starch), and handling stress initiated by moving cows from free stalls to tie stalls. Cows (n = 24) were blocked by DIM, milk yield and parity and randomly assigned at trial start to control (CON, no OG, n = 12) or treatment (TRT, OG fed at 55 g/h/d, n = 12) group. The study consisted of 3 dietary periods: adaptation (AP: base diet, free stalls, 28d), challenge (CP: acidosis diet, tie stalls, 28d) and recovery (RP: base diet, free stalls, 28d). During the whole trial, rumination time (RT), reticulo-rumen pH, milk yield, milk compositions, blood components and stress markers were recorded. Data were analyzed with MIXED procedure (JMP 14). Period, treatment and interactions were considered as fixed effects, and cow as random. Feeding OG produced the best outcomes in terms of daily pH during CP and RP (6.07, 6.10; respectively, P = 0.05) compared with CON and showed a tendency of less daily minutes of pH <5.8 (249 vs 341 m/d; P = 0.08, CP). TRT showed a numerically higher milk fat % throughout CP than CON (3.07 vs 2.87). OG fed cows tended to have higher leukocytes and neutrophils counts during CP than CON. BHOB level was higher for CON in CP (+0.23mmol/L, P < 0.01) than TRT. FRAP resulted lower in OG during P (P < 0.05) after 3 and 14d (−28.98 and −24.73 µmol/L, P < 0.05 and P < 0.10). Results from this study demonstrated that a combination of stressors could impact the rumen environment and that OG can play a possible positive role to attenuate systemic inflammation in cows experiencing rumen acidosis.