The objective was to determine if feeding lactating Holstein dairy cows with OmniGen-AF (OG; Phibro Animal Health Corp.) would have an effect on energy metabolism cortisol secretion, and immunity during overcrowding. Four groups of 15 cows, balanced by parity, DIM, and 305ME milk, were exposed to 2 stocking densities (SD) [100% = 1 lying stall/cow and 0.67 m of feed bunk (FB) space/cow; 200% = 0.5 stalls/cow and 0.34 m of FB space/cow], and one of 2 feed treatments [Control = No OG; and OG = OG fed at 56 g/cow/d]. Each group assigned to a feed treatment (2 groups/feed treatment) were exposed to both 14-d SD treatments, separated by a 10-d washout period. OG was fed 45 d before the start of the SD treatment. During the SD treatment blood and fecal samples were collected every 2 d. Total RNA from blood was analyzed for CD80 gene expression and fecal samples were used to measure fecal cortisol metabolites [11,17-dioxoandrostanes (11,17-DOA)]. At the end of each SD treatment, 8 cows from each pen were given an intravenous glucose tolerance test (GTT) and glucose and NEFA were measured. Data were analyzed using the mixed procedure of SAS. 11,17-DOA was greater during the 200% vs. 100% SD periods (3.74 ± 0.26 vs. 3.04 ± 0.26 pg/10uL; P = 0.04); there was no effect of feed treatment on 11,17-DOA. Overstocked cows had slower glucose clearance from circulation as evidenced by greater area under the curve (AUC) for the glucose response curves (7689 ± 806 vs. 4649 ± 709 mg/dL × 180 min; P = 0.01). Overcrowded cows fed OG had a less AUC than overcrowded cows not fed OG (P = 0.05). NEFA did not differ between treatments during the GTT (P > 0.39). Overcrowded cows expressed less CD80 in circulating immune cells (P = 0.04), indicating overcrowded cows may have reduced T-Cell signaling capacity; there was no effect of feed treatment on CD80. These results provide evidence that overcrowding has negative consequences for immunity and energy metabolism, changes that may influence by increased cortisol secretion. OG may mitigate some of these physiological changes during overcrowding.