Rumen-protected dry grape extract supplementation enhances milk production, behavior traits, and immunometabolism of mid-lactating Fleckvieh cows under naturally occurring heat stress.

Heat stress (HS) in dairy cows disrupts homeostasis and thermoregulation, negatively affecting milk production, health status, and metabolism. Dietary supplementation with phytoextracts such as polyphenols may offer an effective dietary strategy by stimulating the immune system, reducing oxidative stress, and ultimately enhancing welfare, metabolic function, and milk performance. This study aimed to evaluate the effects of supplementing mid-lactating Fleckvieh cows with rumen-protected grape extract (Nor-Grape BP-O, Nor-Feed) on milk performance, rectal temperature (RT), behavior, immunometabolism, rumen fermentation, and innate immune response under naturally occurring HS conditions (temperature-humidity index >72). Thirty cows were balanced by days in milk, milk yield, and parity, and randomly assigned to receive either 470 mg/cow per day of Nor-Grape BP-O supplementation (NG-BPO, n = 15) or a control diet without supplementation (CTR, n = 15) and monitored for 35 d. Both groups were cooled daily using forced ventilation and fans. Temperature-humidity index, milk yield, heavy breathing (HB), rumination, and eating time were recorded daily. Milk, blood, and rumen fluid samples were collected at different time points, and RT was measured. The NG-BPO cows exhibited greater milk production, fat-corrected milk, energy-corrected milk, and yields of fat and protein, alongside lower SCC compared with CTR cows. The NG-BPO cows exhibited improved thermoregulatory responses, reflected by significantly lower RT and reduced HB. Supplementation with Nor-Grape BP-O lowered inflammatory status and enhanced innate immune responses, as indicated by higher plasma zinc and myeloperoxidase levels, lower haptoglobin concentrations, and increased phagocytic activity of neutrophils and monocytes compared with CTR cows. These findings were supported by higher levels of proinflammatory cytokines TNF-α and IL-1 β during peak HS periods, compared with CTR cows. In addition, the NG-BPO group had lower circulating platelet and eosinophil counts compared with the CTR group. In conclusion, supplementation with a low dose of rumen-protected dry grape extract-rich in low-molecular-weight polyphenols (oligomeric and monomeric flavanols, anthocyanins)-effectively supported milk production and quality in heat-stressed dairy cows. Moreover, supplementation improved thermotolerance during heat waves and enhanced innate immune functions while reducing inflammatory responses. These findings suggest Nor-Grape BP-O as an effective dietary strategy to mitigate the negative effects of heat stress by enhancing cow performance and welfare during periods of elevated environmental temperatures.