Heat stress (HT) of cows in the dry period (DP) decreases immune function and lowers milk yield in the next lactation compared with cooled dry cows. The objective of this study was to evaluate the effects of a dietary treatment (OmniGen-AF) fed to HT cows before, during and after the DP on immune function, hematology and immune related gene expression. Sixty days before dry-off, cows were cooled (i.e., shade, fans and soakers) and divided into 2 groups: control (fed 56 g/d of AB20; CON) and OmniGen-AF (fed 56 g/d of OmniGen-AF; OG). Cows were dried-off 45 d before parturition and further split into cooling (shade, fans and soakers; CL) or HT (only shade) pens, which resulted in 4 treatments: HT (n = 17), CL (n = 16), HT + OG (HTOG, n = 19) and CL + OG (CLOG, n = 14). In the DP, rectal temperature (RT; °C), respiration rate (RR; breaths per min) and temperature humidity index (THI) were recorded to evaluate heat strain. Blood samples were collected before dry-off, during the DP and lactation from a subset of cows (HT, n = 12; CL, n = 12; HTOG, n = 11 and CLOG, n = 9) to evaluate L-selectin (CD62L, copies per ng of total mRNA) and CXCR2 mRNA (a.k.a. IL8-R) gene expression in immune cells. Other samples were used before dry-off and in the DP to evaluate neutrophil function and blood hematology (HT, n = 8; CL, n = 7; HTOG, n = 8 and CLOG, n = 6). HT increased RR (45.2 vs. 77.2 ± 1.6 bpm) and RT (38.9 vs. 39.3 ± 0.05 °C) versus CL (P < 0.01). OG increased L-selectin expression versus CON before dry-off (10229 vs. 5893 ± 2353; P = 0.09). L-selectin expression did not differ during the DP, but after calving there was an interaction of DP heat stress and dietary treatment (P = 0.05); CLOG cows had increased L-selectin expression versus CL cows (24,951 vs. 7,198 ± 5,061). Expression of CXCR2 and neutrophil function did not differ among groups. OG tended to increase neutrophil (10³/µL) count versus CON (3.6 vs. 3.3 ± 0.17; P = 0.13) and HT cows had lower hematocrit % versus CL (29.4 vs. 31.6 ± 0.6; P = 0.12). OG supplementation increased L-selectin expression before dry-off, and that may be related to improved immune status of cows during the DP and in the next lactation.