A dry period (DP) is necessary for cows to attain maximal milk yield in the next lactation and heat stress during this phase compromises mammary gland involution and redevelopment. The objective of this study was to evaluate the effects of nutritional and housing strategies to overcome the effects of heat stress on mammary gland involution and redevelopment of cows during the DP. Before dry-off, all cows were kept in the same environment and exposed to cooling systems, i.e., shade, fans and soakers. For 60 d before dry-off, cows were 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 expected calving and, within nutritional treatment, assigned to cooling (shade, fans and soakers; CL) or heat stress (only shade; HT) pens, which resulted in 4 treatment groups: HT (n = 17), CL (n = 16), HT + OG (HTOG, n = 19) and CL + OG (CLOG, n = 14). Mammary biopsies were collected on d 3, 7, 14, and 25 during the DP from a subset of cows (HT, n = 6; CL, n = 7; HTOG, n = 6 and CLOG, n = 5) for histological evaluation of cell apoptosis and alveolar structures. Mammary tissue was placed in 4% paraformaldehyde overnight at 4°C, dehydrated, paraffin embedded, and sectioned at 5 µm. Mammary alveoli and apoptotic cells were visualized by hematoxylin and eosin staining and TUNEL assay, respectively. Alveoli number and positive apoptotic cells were counted using Image J software. Data were analyzed by mixed models using the MIXED procedure of SAS. There was an interaction of heat stress and dietary treatment (P = 0.08), where the apoptotic rate of CLOG cows was higher versus CL, HT, and HTOG cows (2.2; 1.46; 1.52; 1.47 ± 0.2%, P < 0.05, respectively). Relative to cooling, alveolar number was reduced when cows were exposed to HT (176 vs. 144 ± 12; P = 0.06) and increased when animals received OG versus CON (179 vs. 141 ± 12; P = 0.02). Thus, OG supplementation with CL increased mammary cell apoptotic rate; OG supplementation increased alveoli number and CL increased alveoli number during the DP. OG might improve the capacity of the mammary gland for milk yield after calving.